Garcia, Loraine Marianne L.October 2, 2015MT1425

Constellations Constellations are formed of bright stars which appear close to each other on the sky, but are really far apart in space. The shapes you see all depend on your point of view. Many societies saw patterns among the stars with gods and goddesses or stories from their culture.Most of the constellations with which we are familiar come from ancient Greece. But other civilizations created their own patterns in the sky based on stories and people that were important to them.Many peoples noticed that the planets, the moon, and comets moved through the sky in a different way than the stars.Because of the rotation of the Earth and its orbit around the Sun, we divide the constellations into two groups. Some constellations never rise nor are set, and they called circumpolar. All the rest are divided into seasonal constellations. Which constellations will be circumpolar and which seasonal depends on your latitude.Common ConstellationsAquariusLatin for water carrier or cup carrierBest viewed in: OctoberWhile one of the biggest, most famous, and oldest named constellations, Aquarius is faint and often hard to find/see. In Greek mythology, Aquarius represented Ganymede, a very handsome young man. Zeus recognized the lads good looks, and invited Ganymede to Mt. Olympus to be the cupbearer of the gods. For his service he was granted eternal youth, as well as a place in the night sky.

Despite its prominent position and large size, you can see that Aquarius doesnt really have defining features, nor does it contain any bright stars. The protruding line to the right is Aquariuss right arm, with the large downward shape being a combination of the water flowing down out of the vase and his right leg. While not the entire constellation, whats drawn above is what youre most likely to see in the night sky. You wont see this one in the city; youll need a dark sky to find the cupbearer.Aquila was the eagle that in Greek mythology actually bore Ganymede (Aquarius) up to Mt. Olympus. The eagle was also the thunderbolt carrier for Zeus.This constellation lies in the Milky Way band, and its most prominent star is Altair, which is actually one of the closest naked eye stars to the earth. The top portion of Aquila forms a shallow inverted V, with Altair nearly the point. This represents the head and wings of the eagle. A line then descends from Altair, which forms the body of the eagle.Look towards the southern sky in the late summer, near the Milky Way band, for Aquila.AriesLatin for ramBest viewed in: DecemberWhile many constellations have gone through various iterations of mythological stories, Aries has always been the ram. This constellation is one of 12 constellations that form the zodiac the constellations that straddle the suns path across the sky (known in scienctific terms as the ecliptic). In ancient times, that gave the constellations of the zodiac special significance.In Greek mythology, Aries is the ram whose fleece became the Golden Fleece. The Golden Fleece is a symbol of kingship and authority, and plays a significant rolein the tale of Jason and the Argonauts. Jason is sent to find the fleece in order to rightfully claim his throne as king, and with some help from Medea (his future wife), finds his prize. Its one of the oldest stories in antiquity, and was current in Homers time.Aries is formed by just 4 (sometimes 5) visible stars, which createa line from the rams head (the lowest point in the image above) and down its back. Hamal is the largest and most visible star, and is classified as anorange giant.Canis MajorLatin for greater dogBest viewed in: FebruaryCanis Major represents the famed Greek dog Laelaps. There are a few origin stories, but the common theme is that he was so fast he was elevated to the skies by Zeus. Laelaps is also considered to be one of Orions hunting dogs, trailing behind him in the night sky in pursuit of Taurus the bull.Canis Major is notable because it contains the brightest star in the night sky, Sirius. Tradition notes that the first appearance of Canis Major in the dawn sky comes in late summer, ushering in the dog days of the season. In the night sky, it almost looks a stick figure, with Sirius at the head, and another bright star, Adhara, at its rear end.

CassiopeiaNo Latin meaning, its the name of a queen in Greek mythologyBest viewed in: NovemberCassiopeia, in Greek mythology, was a vain queen who often boasted about her beauty. She was the mother of Princess Andromeda, and in contrast to other figures being placed in the sky in honor, Cassiopeia was forced to the heavenly realms as punishment. As the story goes, she boasted that her beauty (or her daughters, depending on the story) was greater than that of the sea nymphs. This was quite an offense, and she was banned to the sky for all to gawk at.With its distinctive W shape formed by five bright stars, Cassiopeia is one of the most easily recognizable constellations in the night sky come fall and early winter. And because of that, the vain queen is one of the most oft-mentioned in pop culture and one of the earliest constellations that young children come to recognize in the sky.

Cygnus (also known as the Northern Cross)Latinized Greek for swanBest viewed in: SeptemberMultiple personas take on the form of the swan in Greek mythology.At one point Zeus morphed into a swan to seduce Leda, mother of both Gemini and Helen of Troy. Another tale says that Orpheus was murdered and then placed into the sky as a swan next to his lyre (the constellation Lyra, also in the drawing above).The constellation may also have gotten its name from the tale of Phaethon and Cycnus. Phaethon was the son of Helios (the sun god), and took his fathers sun chariot for a ride one day. Phaethon couldnt control the reins, however, and Zeus had to shoot down the chariot with Phaethon in it, killing him. Phaethons brother, Cycnus (now spelled Cygnus), spent many days grieving and collecting the bones, which so touched the gods that they turned him into a swan and gave him a place in the sky.

The Northern Cross is really just an asterism (recognizable pattern of stars) within Cygnus the swan. Deneb, the swans tail (or top point of the cross), is one of the brightest stars in the night sky. Youll find Cygnus within the Milky Way, which is why youll sometimes see the constellation referred to as the backbone of the galaxy. In the night sky, the goose is looking down with its wings spread out parallel to the horizon.GeminiLatin for twinsBest viewed in: FebruaryGemini represents the twins Castor and Pollux. While the twins mother was Leda, Castors father was the mortal king of Sparta, while Polluxs father was King Zeus (He seduced Leda in the form of a swan, remember? These stories tend to all tie together!). When Castor was killed, the immortal Pollux begged Zeus to give Castor immortality, which he did by placing the brothers in the night sky for all time.Castor and Pollux also happen to be the names of the brightest stars in the constellation, and represent the heads of the twins. Each star then has a line forming their bodies, giving the constellation a rough U shape. The twins sit next to Orion, making them fairly easy to find in winter.LeoLeo Minor is a recognized constellation, but is so small and faint that Ptolemy didnt include it in his original list. To this day, Leo Major is solely regarded as Leo.Latin for lionBest viewed in: AprilLeo has been a great lion in the night sky across almost all mythological traditions. In Greek lore, Leo is the monstrous lion that was killed by Hercules as part of his twelve labors. The lion could not be killed by mortal weapons, as its fur was impervious to attack, and its claws sharper than any human sword. Eventually Hercules tracked him down and strangled the great beast, albeit losing a finger in the process.

The large, orange star underneath Leo is actually the planet Jupiter.Because Leo actually lookssomewhat like its namesake, itis the easiest constellation in the zodiac to find. A distinctive backwards question mark forms the head and chest, then moves to the left to form a triangle and the lions rear end. Regulus is Leos brightest star, and sits in the bottom right of the constellation, representing the lions front right leg.

LyraLatin for lyreBest viewed in: AugustLyra is associated with the myth of Orpheus the great musician (remember him from earlier?). Orpheus was given the harp by Apollo, and its said that his music was more beautiful than that of any mortal man. His music could soothe anger and bring joy to weary hearts. Wandering the land in depression after his wife died, he was killed and his lyre (harp) was thrown into a river. Zeus sent an eagle to retrieve the lyre, and it was then placed in the night sky.

Lyra sort of forms a lopsided square with a tail to its brightest star, Vega, which is one of the brightest stars in the sky. It is small, and almost directly overhead in the summer months, but the bright Vega makes it fairly easy to find.OrionThe lions head being held by Orion is also sometimes visualized as a shield.Named for Orion, the mythological Greek hunterBest viewed in: JanuaryOrion is one of the largest and most recognizable of the constellations. It is viewable around the world, and has been mentioned by Homer, Virgil, and even the Bible, making it perhaps the most famous constellation.Orion was a massive, supernaturally gifted hunter who was the son of Poseidon. It was said he regularly hunted with Artemis (Goddess of the Hunt) on the island of Crete, and that he was killed either by her bow, or by the sting of the great scorpion who later became the constellation Scorpius.

Orions belt of three stars is the easiest asterism to find, with Rigel (bottom right) and Betelgeuse (top left) being the brightest two individual stars. The two other corners form a rough quadrangle, with his head and bow also sometimes visible. Orion is also unique in that you can use himto find a variety of other constellations in the winter sky.PiscesLatin for fish (plural)Best viewed in: NovemberThe two fish of the sky represent Aphrodite and her son Eros, who turned themselves into fish and tied themselves together with rope in order to escape Typhon, the largest and most vile monster in all of Greek mythology.

Its not likely youll find Pisces in the middle of a city, as none of its individual stars are really worth noting or particularly bright. It forms a large V with the right fish forming a small O on the end, and the left fish forming a small triangle on the end (the image above doesnt connect the dots in the upper left to make it a triangle).

ScorpiusScorpius is sometimes also known as just Scorpio.Latin for scorpionBest viewed in: JulyThere are a variety of myths associated with the scorpion, almost all of them involving Orion the hunter. Orion once boasted that he could kill all the animals on the earth. He encountered the scorpion, and after a long, fiercefight, Orion was defeated. It was such a hard-fought battlethat it caught the eye of Zeus, and the scorpion was raised to the night sky for all eternity.

With many bright stars, Scorpius is fairly easy to find in the night sky. Antares, the brightest star in the constellation, is said to be the heart of the scorpion. That will be the easiest star to locate, but is sometimes confused with Mars because of its red-orange coloring. To the right of the heart are 3-5 stars that form the head. To the left are a long line of stars that curve into a sideways or upside-down question mark.TaurusLatin for bullBest viewed in: JanuaryTaurus is a large and prominent fixture in the winter sky. As one of the oldest recognized constellations, it has mythologies dating back to the early Bronze Age. There are several Greek myths involving Taurus. Two of them include Zeus, who either disguised himself as a bull or disguised his mistress as a bull in multiple escapades of infidelity. Another myth has the bull being the 7th labor of Hercules after the beast wreaked havoc in the countryside.

The constellation is fairly easy to find as its most recognizable asterism forms a very prominent V, which represent the head and horns of the bull. The brightest star in the constellation is Aldebaran, which forms the bulls right eye. Five stars, fairly close together to the naked eye, form an almost perfect small V, with the V extending up quite a ways to two more final stars that are the points of the horns.

Ursa Major

Latin for larger bearBest viewed in: AprilThe Big Dipper is popularly thought of as a constellation itself, but is in fact an asterism within the constellation of Ursa Major. It is said to be the most universally recognized star pattern, partially because its always visible in the northern hemisphere. It has great significance in the mythologies of multiple cultures around the world.The Greek myth of Ursa Major also tells the story of Ursa Minor (below). Zeus was smitten for a young nymph named Callisto. Hera, Zeuss wife, was jealous, and transformed Callisto into a bear. While in animal form, Callisto encountered her son Arcas. Being the man that he was, he was inclined to shoot the bear, but Zeus wouldnt let that happen, and so turned Arcas into a bear as well, and placed mother (Ursa Major) and son (Ursa Minor) permanently in the night sky.

The seven stars of the Big Dipper are easily recognized and almost always visible. They form part of the backside and tail of the large bear. While the rest of the bear definitely takes the shape of its namesake, its not often visible in light polluted areas. The Big Dipper is more than just a pretty shape; the outer edge of its bowl will always lead you to the North Star, aiding in navigation for centuries past. Simply make a line with the two stars of the Big Dippers outer edge, extend that line up into the sky, and at about five times the distance between the two stars you started with, youll find the North Star.Ursa MinorLatin for smaller bearBest viewed in: JuneUrsa Minor is famous for containing Polaris, the North Star. Many people erroneously think that the North Star is directly over their heads, but thats only true at the North Pole. For most people in the Northern Hemisphere, it will be dipped into the night sky.

You can see the Big Dipper sitting prominently below Ursa Minor. This also gives a great visualization of how to use the Big Dipper to find the North Star.Ursa Minor is better known as the Little Dipper. Its visualized as a baby bear, with an unusually long tail. It can be distinguished from the Big Dipper not only by size, but by the emphasized curvature of the tail. When youve found the North Star at the end of the bears tail using the Big Dipper, its then easy to identify the rest of the constellation.

PlanetsA Planet is a celestial body distinguished from the fixed stars by having an apparent motion of its own (including the moon and sun), especially with reference to its supposed influence on people and events.The planetsBelow is a brief overview of the eight primary planets in oursolar system, in order from the inner solar system outward:

MercuryThe closest planet to the sun, Mercury is only a bit larger than Earth's moon. Its day side is scorched by the sun and can reach840 degrees Fahrenheit(450 Celsius), but on the night side, temperatures drop to hundreds of degrees below freezing. Mercury has virtually no atmosphere to absorb meteor impacts, so its surface is pockmarked with craters, just like the moon. Over its four-year mission, NASA'sMESSENGER spacecrafthas revealed views of the planet that have challenged astronomers' expectations.1. Discovery:Known to the ancients and visible to the naked eye1. Named for:Messenger of the Roman gods1. Diameter:3,031 miles (4,878 km)1. Orbit:88 Earth days1. Day:58.6 Earth daysVenusThe second planet from the sun, Venus is terribly hot, even hotter than Mercury. The atmosphere is toxic. The pressure at the surface would crush and kill you. Scientists describe Venus situation as a runaway greenhouse effect. Its size and structure are similar to Earth, Venus' thick, toxic atmosphere traps heat in a runaway "greenhouse effect." Oddly, Venus spins slowly in the opposite direction of most planets.The Greeks believed Venus was two different objects one in the morning sky and another in the evening. Because it is often brighter than any other object in the sky except for the sun and moon Venus has generated many UFO reports.1. Discovery:Known to the ancients and visible to the naked eye1. Named for:Roman goddess of love and beauty1. Diameter:7,521 miles (12,104 km)1. Orbit:225 Earth days1. Day:241 Earth days

EarthThe third planet from the sun, Earth is a waterworld, with two-thirds of the planet covered by ocean. Its the only world known to harbor life. Earths atmosphere is rich in life-sustaining nitrogen and oxygen. Earth's surface rotates about its axis at 1,532 feet per second (467 meters per second) slightly more than 1,000 mph (1,600 kph) at the equator. The planet zips around the sun at more than 18 miles per second (29 km per second).1. Diameter:7,926 miles (12,760 km)1. Orbit:365.24 days1. Day:23 hours, 56 minutes

MarsThe fourth planet from the sun, is a cold, dusty place. The dust, an iron oxide, gives the planet its reddish cast. Mars shares similarities with Earth: It is rocky, has mountains and valleys, and storm systems ranging from localized tornado-like dust devils to planet-engulfing dust storms. It snows on Mars. And Mars harbors water ice. Scientists think it was once wet and warm, though today its cold and desert-like.Mars' atmosphereis too thin for liquid water to exist on the surface for any length of time. Scientists think ancient Mars would have had the conditions to support life, and there is hope that signs of past life possibly even present biology may exist on the Red Planet.1. Discovery:Known to the ancients and visible to the naked eye1. Named for:Roman god of war1. Diameter:4,217 miles (6,787 km)1. Orbit:687 Earth days1. Day:Just more than one Earth day (24 hours, 37 minutes)

JupiterThe fifth planet from the sun, Jupiter is huge and is the most massive planet in our solar system. Its a mostly gaseous world, mostly hydrogen and helium. Its swirling clouds are colorful due to different types of trace gases. A big feature is the Great Red Spot, a giant storm which has raged for hundreds of years. Jupiter has a strong magnetic field, and with dozens of moons, it looks a bit like a miniature solar system.1. Discovery:Known to the ancients and visible to the naked eye1. Named for:Ruler of the Roman gods1. Diameter:88,730 miles (428,400 km)1. Orbit:11.9 Earth years1. Day:9.8 Earth hours

SaturnThe sixth planet from the sun is known most for itsrings. WhenGalileo Galileifirst studied Saturn in the early 1600s, he thought it was an object with three parts. Not knowing he was seeing a planet with rings, the stumped astronomer entered a small drawing a symbol with one large circle and two smaller ones in his notebook, as a noun in a sentence describing his discovery. More than 40 years later,Christiaan Huygensproposed that they were rings. The rings are made of ice and rock. Scientists are not yet sure how they formed. The gaseous planet is mostly hydrogen and helium. It hasnumerous moons.1. Discovery:Known to the ancients and visible to the naked eye1. Named for:Roman god of agriculture1. Diameter:74,900 miles (120,500 km)1. Orbit:29.5 Earth years1. Day:About 10.5 Earth hours

UranusThe seventh planet from the sun, Uranus is an oddball. Its the only giant planet whose equator is nearly at right angles to its orbit it basically orbits on its side. Astronomers think the planet collided with some other planet-size object long ago, causing the tilt. The tilt causes extreme seasons that last 20-plus years, and the sun beats down on one pole or the other for 84 Earth-years. Uranus is about the same size as Neptune. Methane in the atmosphere gives Uranus its blue-green tint. It hasnumerous moonsand faint rings.1. Discovery:1781 byWilliam Herschel(was thought previously to be a star)1. Named for:Personification of heaven in ancient myth1. Diameter:31,763 miles (51,120 km)1. Orbit:84 Earth years1. Day:18 Earth hoursNeptuneThe eighth planet from the sun, Neptune is known for strong winds sometimes faster than the speed of sound. Neptune is far out and cold. The planet is more than 30 times as far from the sun as Earth. It has a rocky core. Neptune was the first planet to be predicted to exist by using math, before it was detected. Irregularities in the orbit of Uranus led French astronomer Alexis Bouvard to suggest some other might be exerting a gravitational tug. German astronomer Johann Galle used calculations to help find Neptune in a telescope. Neptune is about 17 times as massive as Earth.1. Discovery:18461. Named for:Roman god of water1. Diameter:30,775 miles (49,530 km)1. Orbit:165 Earth years1. Day:19 Earth hoursPluto (Dwarf Planet)Once the ninth planet from the sun, Pluto is unlike other planets in many respects. It is smaller than Earth's moon. Its orbit carries it inside the orbit of Neptune and then way out beyond that orbit. From 1979 until early 1999, Pluto had actually been the eighth planet from the sun. Then, on Feb. 11, 1999, it crossed Neptune's path and once again became the solar system's most distant planet until it was demoted to dwarf planet status. Pluto will stay beyond Neptune for 228 years. Plutos orbit is tilted to the main plane of the solar system where the other planets orbit by 17.1 degrees. Its a cold, rocky world with only a very ephemeral atmosphere. NASA's New Horizons mission performed history's first flyby of the Pluto system on July 14, 2015.[Related:New Horizons' Pluto Flyby: Latest News, Images and Video]1. Discovery: 1930 by Clyde Tombaugh1. Named for: Roman god of the underworld, Hades1. Diameter: 1,430 miles (2,301 km)1. Orbit: 248 Earth years1. Day:6.4 Earth day

What is retrograde motion?

Sometimes the planets appear to change direction in the sky. This retrograde motion is entirely an illusion caused by the Earth passing the slower moving outer planets.Sometimes, the planets seem to move backwards!Typically, the planets shift slightly eastward from night to night, drifting slowly against the backdrop of stars. From time to time, however, they change direction. For a few months, theyll head west before turning back around and resuming their easterly course. This is retrograde motion. Though it baffled ancient astronomers, we know now that retrograde planets are an illusion caused by the motion of Earth.

You can test this the next time you pass a car on the highway. As you approach the slower car, it is clearly moving in the same direction you are. Right as you pull along side and pass it, however, the car appears to move backwards for just a moment. As you continue pulling away, the car resumes its forward motion.The same thing happens as Earth passes the slower moving outer planets. When we pass Jupiter, for example, the gas giant appears to reverse course in the sky for a couple of months.This makes the planets appear to move very strangely. Ancient astronomers went to complicated lengths to try and explain these motions. They envisioned each planet not only orbiting the center of the solar system (which to them was Earth) but also spinning around a moving point on their orbit. Imagine whipping a ball on a length of string around your hand while you rotated in place. Astronomers like Copernicus and Kepler finally set us all straight when they realized Earth orbited the sun.Suddenly, the retrograde motion made a lot more sense!

A schematic of how astronomers envisioned the motion of the planets before Copernicus. The Earth sat near the center of the universe. The planets moved around a small circle (the epicycle) which in turn moved along a larger circle (the deferent). The deferent was centered on a point (X) midway between the Earth and another spot called the equant. This complicated setup was needed to explain the complex motions of the planets. Credit: Wikipedia user Fastfission.Retrograde illusions on other planets can lead to very strange phenomena. On Mercury, for example, the sun sometimes moves in retrograde! As the planet speeds through its closest approach with the sun, its orbital speed overtakes its rotational speed. An astronaut on the surface would see the sun partially rise, then dip back below the horizon, then rise again before resuming its east-to-west trek across the sky. Once a year, Mercury gets two sunrises on the same day!But retrograde movement isnt always an illusion.There are real retrograde motions in the solar system. Venus rotates in the opposite direction from every other planet! If the clouds ever parted, the Venusians would see the sun rise in the west and set in the east.Some moons also have retrograde orbits around their planets. Most of the large moons orbit in the same direction their planet spins. But not Triton, the largest moon of Neptune. And among the smaller asteroid-like moons that swarm about the giant planets, many have retrograde orbits.

A photomosaic from Voyager 2 of Neptunes largest moon, Triton. The moon orbits Neptune opposite the direction that the planet rotates. Does this mean that Triton came from the Kuiper Belt and was eventually captured by the ice giant? Credit: NASA / Jet Propulsion Lab / U.S. Geological SurveyA retrograde orbit most likely means the moon was captured after the planet formed. Triton probably came out of the Kuiper Belt, the region of icy debris beyond Neptune where Pluto lives. Perhaps a collision in the belt sent Triton careening inward toward the sun. A close encounter with Neptune could have slowed it down and forced it to settle into a backwards orbit around the distant planet.In the past decade, astronomers have also discovered planets in other solar systems with retrograde orbits. These exoplanets orbit their suns in the opposite direction from how the star rotates. This is puzzling because planets form out of debris disks that orbit young stars, disks which share the stars rotation. The only way to get a planet orbiting backwards is either by a near-collision with another planet or if another star once passed too close to the system. Close encounters tend to disrupt orbits.Retrograde motion refers to the occasional backwards motion of the planets. It is entirely an illusion caused by the moving Earth passing the outer planets in their orbits. Real retrograde motionsof planet rotation, orbiting moons, and planets in other solar systemsare a sign of long forgotten collisions and captures. They are one way that astronomers piece together the history of our solar system, and the systems of other stars in our galaxy!

Geocentric TheoryThe geocentric model, also known as the Ptolemaic system, is a theory that was developed by philosophers in Ancient Greece and was named after the philosopher Claudius Ptolemy who lived circa 90 to 168 A.D. It was developed to explain how the planets, the Sun, and even the stars orbit around the Earth. The geocentric theory has existed even before Ptolemy though. This model has been described in various early Greek manuscripts, and as early as the 4th century B.C. Plato and Aristotle were writing about the geocentric model.

As the Greeks noticed discrepancies between the way planets moved and the basic geocentric model, they began adjusting the model and creating variations on the original. In these models, planets and other celestial bodies move in circles that have been superimposed onto circular orbits around the Earth.The Ptolemaic system, the most well-known versions of the geocentric model, was a complex interaction of circles. Ptolemy believed that each planet orbited around a circle, which was termed an epicycle, and the epicycle orbits on a bigger circlethe deferentaround the Earth. The center of the deferent is not the Earth, but a point near the midpoint of the distance between Earth and the equant. The equant was Ptolemys solution to some of the discrepancies that the geocentric model could not explain. The equant can be defined as the point at which an epicycles center always seems to move at the same speed. When an epicenter was at a different point on its deferent, then the planet moved at a different speed. To further complicate matters, each planet had a different equant. A diagram of the Ptolemaic system looks like a mess of overlapping circles. Despite its difficulties and complexity, the geocentric model survived well into the 16th century. One reason why the geocentric model held sway for so many years was because it fit in with a number of observations that the Greeks made. Those observations included the fact that things fall toward Earth and that according to Venus brightness it stays roughly the same distance away from the Earth. As theories evolved and more evidence was uncovered though, the geocentric model was slowly replaced by models developed by Copernicus and other astronomers.Copernicus proposed a heliocentric model of the solar system a model where everything orbited around the Sun. Today, with advancements in science and technology, the geocentric model seems preposterous. Simple tools, such as the telescope which helped convince Galileo that the Earth was not thecenter of the universe can prove that ancient theory incorrect.The celestial sphere is an imaginary sphere of gigantic radius with the earth located at its center. The poles of the celestial sphere are aligned with the poles of the Earth. The celestial equator lies along the celestial sphere in the same plane that includes the Earth's equator.An astronomer can only see half the sky at a time, that is, only half the sky is above the horizon at any time. But the sky keeps moving as the earth rotates. Just as the sun rises and sets every day, so does every star in the sky each night. The celestial sphere is a large sphere surrounding the earth and with it we can keep references to where celestial bodies lie in the sky. North Celestial Pole (NCP) and the South Celestial Pole (SCP)- these are just the north and south poles extended into space. Celestial Equator- The earth's equator, but at a much greater radius. If the earth's equator was a rubber band, then the celestial equator is the same rubber band just stretched away from the earth. Horizon- The horizon changes depending on your position on earth. Zenith- The point on the celestial sphere directly overhead. Meridian- The line that extends from the north point on the horizon upwards through the zenith and then downward to the south point on the horizon.

We can locate any object on the celestial sphere by giving it two coordinates, called theRight Ascensionand theDeclination. These are calledcelestial coordinates. Analogous to the longitude on Earth, theRight Ascensionof an object on the celestial sphere is measured along the celestial equator, as the angular distance to some fiducial direction for with R.A. = 0 degrees. By convention, this fiducial direction is the point on the celestial where the Sun is found on the first day of spring (the vernal equinox). Analogous to the latitude on Earth, theDeclinationof an object on the celestial sphere is measured northward or southward from the plane containing the equator. The declination of the equator is 0 degrees, the North Celestial Pole, +90 degrees, the South Celestial Pole, -90 degrees.Stars and galaxies have (almost) fixed positions in Right Ascension and Declination. The Sun and planets, on the other hand, move among the distant stars so that their coordinates change throughout the year. Because of the Earth's yearly orbital motion, the Sun appears to circle theecliptic.

Parallaxis a displacement ordifferencein theapparent positionof an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines.The term is derived from theGreekword (parallaxis), meaning "alteration". Nearby objects have a larger parallax than more distant objects when observed from different positions, so parallax can be used to determine distances.Astronomersuse the principle of parallax to measure distances to the closerstars. Here, the term "parallax" is the semi-angle of inclination between two sight-lines to the star, as observed when theEarthis on opposite sides of the Sun in its orbit. These distances form the lowest rung of what is called "thecosmic distance ladder", the first in a succession of methods by which astronomers determine the distances to celestial objects, serving as a basis for other distance measurements in astronomy forming the higher rungs of the ladder.Parallax also affects optical instruments such as rifle scopes,binoculars,microscopes, andtwin-lens reflex camerasthat view objects from slightly different angles. Many animals, including humans, have twoeyeswith overlappingvisual fieldsthat use parallax to gaindepth perception; this process is known asstereopsis. Incomputer visionthe effect is used forcomputer stereo vision, and there is a device called aparallax rangefinderthat uses it to find range, and in some variations also altitude to a target.A simple everyday example of parallax can be seen in the dashboard of motor vehicles that use a needle-style speedometer gauge. When viewed from directly in front, the speed may show exactly 60; but when viewed from the passenger seat the needle may appear to show a slightly different speed, due to the angle of viewing.HeliocentrismCopernicusThe idea of placing the sun at the center of the universe was not a particularly new one. But few either saw advantage to it and many considered it physically impossible (it was inconsistent with Aristotelian physics). That started to change with a polish physician, lawyer, artist and astronomer Nicholas Copernicus (1473 - 1543).Copernicus was aware of earlier writings which suggested a moving earth. He, however, seemed to be motivated by two main suggestions. One, he believed that the earth was not a particularly fit object to be the center of the universe but that the sun was a more divine object and thus more fit for the center. Second, Copernicus very much disliked the concept the equant. He thought it an abomination and a betrayal of the concept of circles. He appears to have been aware of works by Arabic mathematicians who, in an attempt to reconcile the Quran (which suggested the earth moved) with the Ptolemaic system removed the equant in favor of additional epicycles. Copernicus, likewise, used epicycles in his calculations but no equant.What really set Copernicuss heliocentric model apart was its simplicity. It did no better than Ptolemys model at predicting the planets but it was easier to use and handle. While few actually read Copernicuss deathbed publication of his workDe Revolutionibus Orbium Coelestium(On the Revolutions of the Heavenly Spheres), it did gain a few fans (such as Galileo).Observations ExplainedCopernicuss model handled the basic observations: Like the geocentric model, the earth was believed to be round. The earth rotated, and thus the stars, sun, and planets appeared to move around the earth Mercury and Venus were closer to the sun than the earth and so always appeared near the sun. As the earth passed Mars, Jupiter, and Saturn during opposition, the planets would appear to undergo retrograde motion.Inertia and MassNewton's first law of motionstates that "An object at rest stays at rest and an object in motion stays in motion with the same speed and in the same direction unless acted upon by an unbalanced force." Objects tend to "keep on doing what they're doing." In fact, it is the natural tendency of objects to resist changes in their state of motion. This tendency to resist changes in their state of motion is described asinertia.Inertia: the resistance an object has to a change in itsstate of motion.Newton's conception of inertia stood in direct opposition to more popular conceptions about motion. The dominant thought prior to Newton's day was that it was the natural tendency of objects to come to a rest position. Moving objects, so it was believed, would eventually stop moving; a force was necessary to keep an object moving. But if left to itself, a moving object would eventually come to rest and an object at rest would stay at rest; thus, the idea that dominated people's thinking for nearly 2000 years prior to Newton was that it was the natural tendency of all objects to assume a rest position.MAREMare (plural maria) means "sea," but maria on the moon are plains on the moon. They are called maria because very early astronomers thought that these areas on the moon were great seas. The first moon landing was in the Mare Tranquillitatis (the Sea of Tranquility). Maria are concentrated on the side of the moon that faces the Earth; the far side has very few of these plains. Scientists don't know why this is so.

Arotationis a circular movement of an object around a center (or point) of rotation . A three-dimensional object always rotates around an imaginary line called a rotation axis. If the axis passes through the body's center of mass, the body is said to rotate upon itself, or spin. A rotation about an external point, e.g. the Earth about the Sun, is called a revolution or orbital revolution, typically when it is produced by gravity.Revolutiona.Orbitalmotionaboutapoint,especiallyasdistinguishedfromaxialrotation:theplanetaryrevolutionaboutthesun.b.Aturningorrotationalmotionaboutanaxis.c.Asinglecompletecycleofsuchorbitaloraxialmotion.PrecessionIn astronomy, precession refers to any of several gravity-induced, slow and continuous changes in an astronomical body's rotational axis or orbital path. Precession of the equinoxes, perihelion precession, changes in thetilt of Earth's axisto its orbit, and theeccentricityof its orbit over tens of thousands of years are all important parts of the astronomical theory ofice ages.

Moon Phases

It's probably easiest to understand the moon cycle in this order: new moon and full moon, first quarter and third quarter, and the phases in between.As shown in the above diagram, thenew moonoccurs when the moon is positionedbetweenthe earth and sun. The three objects are in approximate alignment (why "approximate" is explained below). The entire illuminated portion of the moon is on the back side of the moon, the half that we cannot see.At afull moon, the earth, moon, and sun are in approximate alignment, just as the new moon, but the moon is on the opposite side of the earth, so the entire sunlit part of the moon is facing us. The shadowed portion is entirely hidden from view.Thefirst quarterandthird quartermoons (both often called a "half moon"), happen when the moon is at a 90 degree angle with respect to the earth and sun. So we are seeing exactly half of the moon illuminated and half in shadow.Once you understand those four key moon phases, the phases between should be fairly easy to visualize, as the illuminated portion gradually transitions between them.An easy way to remember and understand those "between" lunar phase names is by breaking out and defining 4 words: crescent, gibbous, waxing, and waning. The wordcrescentrefers to the phases where the moon islessthan half illuminated. The wordgibbousrefers to phases where the moon ismore than half illuminated.Waxingessentially means "growing" or expanding in illumination, andwaning means "shrinking" or decreasing in illumination.Thus you can simply combine the two words to create the phase name, as follows:After the new moon, the sunlit portion is increasing, but less than half, so it iswaxing crescent. After the first quarter, the sunlit portion is still increasing, but now it ismorethan half, so it iswaxing gibbous. After the full moon (maximum illumination), the light continually decreases. So thewaning gibbousphase occurs next. Following the third quarter is thewaning crescent, which wanes until the light is completely gone -- a new moon.At special times during the year, the earth, moon, and sun do in fact "line up". When the moon blocks the sun or a part of it, it's called asolar eclipse, and it can only happen during the new moon phase. When the earth casts a shadow on the moon, it's called alunar eclipse, and can only happen during the full moon phase. Roughly 4 to 7 eclipses happen in any given year, but most of them minor or "partial" eclipses. Major lunar or solar eclipses are relatively uncommon.A corona (Latin, 'crown') is an aura of plasma that surrounds the sun and other celestial bodies. The Sun's corona extends millions of kilometres into space and is most easily seen during a total solar eclipse, but it is also observable with a coronagraph. The word "corona" is a Latin word meaning "crown", from the Ancient Greek (korn, garland, wreath).

The high temperature of the Sun's corona gives it unusual spectral features, which led some in the 19th century to suggest that it contained a previously unknown element, "coronium". Instead, these spectral features have since been explained by highly ionized iron (Fe-XIV). Bengt Edln, following the work of Grotrian (1939), first identified the coronal lines in 1940 (observed since 1869) as transitions from low-lying metastable levels of the ground configuration of highly ionised metals (the green Fe-XIV line at 5303 , but also the red line Fe-X at 6374 ). These high stages of ionisation indicate a plasma temperature in excess of 1,000,000 kelvin, much hotter than the surface of the sun.

Light from the corona comes from three primary sources, which are called by different names although all of them share the same volume of space. The K-corona (K for kontinuierlich, "continuous" in German) is created by sunlight scattering off free electrons; Doppler broadening of the reflected photospheric absorption lines completely obscures them, giving the spectral appearance of a continuum with no absorption lines. The F-corona (F for Fraunhofer) is created by sunlight bouncing off dust particles, and is observable because its light contains the Fraunhofer absorption lines that are seen in raw sunlight; the F-corona extends to very high elongation angles from the Sun, where it is called the zodiacal light. The E-corona (E for emission) is due to spectral emission lines produced by ions that are present in the coronal plasma; it may be observed in broad or forbidden or hot spectral emission lines and is the main source of information about the corona's composition.UmbraThe umbra (Latin for "shadow") is the innermost and darkest part of a shadow, where the light source is completely blocked by the occluding body.An observer in the umbra experiences a total eclipse. The umbra of a round body occluding a round light source forms a right circular cone; to a viewer at the cone's apex, the two bodies are equal in apparent size. The distance from the Moon to the apex of its umbra is roughly equal to that between the Moon and Earth. Because the Earth is 3.7 times wider than the Moon, its umbra extends correspondingly farther, roughly 1.4 million kilometers. PenumbraThe penumbra (from the Latinpaene"almost, nearly" andumbra"shadow") is the region in which only a portion of the light source is obscured by the occluding body. An observer in the penumbra experiences apartial eclipse. An alternative definition is that the penumbra is the region wheresome or allof the light source is obscured (i.e., the umbra is a subset of the penumbra). For example,NASA's Navigation and Ancillary Information Facility defines that a body in the umbra is also within the penumbra. Inradiation oncology, thepenumbrais the space in the periphery of the main target ofradiation therapy, and has been defined as the volume receiving between 80% and 20% of isodose. The angular resolving power (or resolution) of a telescope is the smallest angle between close objects that can be seen clearly to be separate. Resolution is limited by the wave nature of light. For a telescope having an objective lens or mirror with diameterDand operating at wavelength , theangular resolution(in radians) can be approximately described by the ratio...Arefractingorrefractor telescopeis a type ofoptical telescopethat uses alensas itsobjectiveto form an image (also referred to adioptrictelescope). The refracting telescope design was originally used in spy glasses andastronomicaltelescopes but is also used forlong focuscamera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes the refracting telescope has been superseded by thereflecting telescopewhich allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece. Refracting telescopes contain two main lenses: the objective lens and the eyepiece. Theobjective lens refracts or bends light, causing parallel light rays to converge (or meet) at a particular point. Theeyepiece positions that image in such a way that you have both a good field of view, and so that the image is positioned at infinity, allowing your eye to comfortably view the image for long periods of time.

The reflector telescope uses a mirror to gather and focus light. All celestial objects (including those in our solar system) are so far away that all of the light rays coming from them reach the Earth as parallel rays. Because the light rays are parallel to each other, the reflector telescope's mirror has a parabolic shape. The parabolic-shaped mirror focuses the parallel lights rays to a single point. All modern research telescopes and large amateur ones are of the reflector type because of its advantages over the refractor telescope.Advantages1. Reflector telescopes do not suffer from chromatic aberration because all wavelengths will reflect off the mirror in the same way.2. Support for the objective mirror is all along the back side so they can be made very BIG!3. Reflector telescopes are cheaper to make than refractors of the same size.4. Because light is reflecting off the objective, rather than passing through it, only one side of the reflector telescope's objective needs to be perfect.

Light pollutionwhat is it and why is it important to know?Light pollution is excessive and inappropriate artificial light. The four components of light pollution are often combined and may overlap: Urban Sky Glowthe brightening of the night sky over inhabited areas. Light Trespasslight falling where it is not intended, wanted, or needed. Glareexcessive brightness which causes visual discomfort. High levels of glare can decrease visibility. Clutterbright, confusing, and excessive groupings of light sources, commonly found in over-lit urban areas. The proliferation of clutter contributes to urban sky glow, trespass, and glare.For most of Earths history, our spectacular universe of stars and galaxies has been visible in the darkness of the night sky. From our earliest beginnings, the vast spectacle arrayed across the dark sky has inspired questions about our universe and our relation to it. The history of scientific discovery, art, literature, astronomy, navigation, exploration, philosophy, and even human curiosity itself would be diminished without our view of the stars. But today, the increasing number of people living on earth and the corresponding increase in inappropriate and unshielded outdoor lighting has resulted in light pollutiona brightening night sky that has obliterated the stars for much of the worlds population. Most people must travel far from home, away from the glow of artificial lighting, to experience the awe-inspiring expanse of the Milky Way as our ancestors once knew it.The negative effects of the loss of this inspirational natural resource might seem intangible. But a growing body of evidence links the brightening night sky directly to measurable negative impacts on human health and immune function, on adverse behavioral changes in insect and animal populations, and on a decrease of both ambient quality and safety in our nighttime environment. Astronomers were among the first to record the negative impacts of wasted lighting on scientific research, but for all of us, the adverse economic and environmental impacts of wasted energy are apparent in everything from the monthly electric bill to global warming.In refreshing contrast to some of todays complex and lingering environmental problems, many existing solutions to light pollution are simple, cost-effective, and instantaneous. Recognizing when outdoor lighting no longer serves its function and becomes a pollutant is the first step toward choosing appropriate solutions.Increasedurban sky glowis responsible for the disappearance of the Milky Way from our night skies. For professional astronomers, the increasing distance to prime observing sites, well away from sources of air pollution and urban sky glow, becomes more problematic as economic and environmental energy costs continue to rise. Amateur astronomers, meanwhile, find prime observing spots eradicated by commercial and residential development and must travel farther from home for a clear view of the skies. Increasingly, the most important equipment needed to enjoy the wonders of the night sky is an automobile with a full tank of gas and a map.

The adverse effects of light pollution extend well beyond astronomy. New research suggests that light at night may interfere with normal circadian rhythmsthe 24-hour cycle of day and night that humans have used to maintain health and regulate their activities for thousands of years.Light trespass, occurring when streetlights or a neighbors security light directs unwanted lighting onto our property or into our homes, contributes to a loss of natural darkness. Wildlife, too, is harmed by the unnecessary brightening of the night. From newly hatched sea turtles to migrating birds, fish, frogs, salamanders, and lightning bugs, artificial night lighting disrupts the cycles of nocturnal creatures in potentially devastating ways. While research is still ongoing, it is becoming apparent that both bright days and dark nights are necessary to maintain healthy hormone production, cell function, and brain activity, as well as normal feeding, mating, and migratory behavior for many species, including humans.Paradoxically, in addition to wasting resources, a nighttime environment that is over-lit results in lowered visibility: directglarefrom improperly shielded fixtures is often blinding. Light spilling into the sky does not light the ground where we need it. The redundant lighting found in many urban centers results in aclutterof lights that contribute to sky glow, trespass, and glare while destroying the ambiance of our nighttime environment. Our eyes, when dark-adapted, have good natural capacity in lowlight situations. But when nightscapes are over-lit, eyes never have a chance to become dark-adapted, and areas adjacent to brightly lit areas become impenetrable, reducing safety. Some communities have experienced a decrease in crime by reducing or eliminating nighttime lighting in appropriate areas.Light pollution wastes money and energy. Billions of dollars are spent on unnecessary lighting every year in the United States alone, with an estimated $1.7 billion going directly into the nighttime sky via unshielded outdoor lights. Wasted lighting in the US releases 38 million tons of carbon dioxide into the atmosphere annually; unshielded outdoor lights are directly responsible for 1.2 million tons of carbon dioxide waste. Simply reducing and removing unnecessary lighting saves money and energy, often at minimal expense. Over-lighting the night neither improves visibility nor increases nighttime safety, utility, security, or ambiance.Light pollution affects every citizen. It is a serious environmental concern that wastes money and resources while jeopardizing wildlife, our environment, health, and human heritage. Each of us can implement practical solutions to combat light pollution locally, nationally, and internationally.Aspectrum(pluralspectraorspectrums) is a condition that is not limited to a specific set of values but can vary infinitely within acontinuum. The word was first used scientifically within the field ofopticsto describe therainbowof colors invisible lightwhen separated using aprism. As scientific understanding of light advanced, it came to apply to the entireelectromagnetic spectrum.Spectrum has since been applied by analogy to topics outside of optics. Thus, one might talk about thespectrum ofpolitical opinion, or the spectrum of activityof a drug, or theautism spectrum. In these uses, values within a spectrum may not be associated with precisely quantifiable numbers or definitions. Such uses imply a broad range of conditions or behaviors grouped together and studied under a single title for ease of discussion.In most modern usages ofspectrumthere is a unifying theme between extremes at either end. Some older usages of the word did not have a unifying theme, but they led to modern ones through a sequence of events set out below. Modern usages in mathematics did evolve from a unifying theme, but this may be difficult to recognize.Anabsorption spectrumoccurs when light passes through a cold, dilute gas and atoms in the gas absorb at characteristic frequencies; since the re-emitted light is unlikely to be emitted in the same direction as theabsorbed photon, this gives rise to dark lines (absence of light) in thespectrum.Theemission spectrumof achemical elementorchemical compoundis the spectrum offrequenciesofelectromagnetic radiationemitted due to anatomor molecule making atransitionfrom a high energy state to a lower energy state. The energy of the emittedphotonis equal to the energy difference between the two states. There are many possible electron transitions for each atom, and each transition has a specific energy difference. This collection of different transitions, leading to different radiatedwavelengths, make up an emission spectrum. Each element's emission spectrum is unique. Therefore, spectroscopycan be used to identify the elements in matter of unknown composition. Similarly, the emission spectra of molecules can be used in chemical analysis of substances.What is the Doppler Effect?The Doppler effect is observed whenever the source of waves is moving with respect to an observer. The Doppler effectcan be described as the effect produced by a moving source of waves in which there is an apparent upward shift in frequency for observers towards whom the source is approaching and an apparent downward shift in frequency for observers from whom the source is receding. It is important to note that the effect does not result because of anactualchange in the frequency of the source. Using the example above, the bug is still producing disturbances at a rate of 2 disturbances per second; it just appears to the observer whom the bug is approaching that the disturbances are being produced at a frequency greater than 2 disturbances/second. The effect is only observed because the distance between observer B and the bug is decreasing and the distance between observer A and the bug is increasing.The Doppler effect can be observed for any type of wave - water wave, sound wave, light wave, etc. We are most familiar with the Doppler effect because of our experiences with sound waves. Perhaps you recall an instance in which a police car or emergency vehicle was traveling towards you on the highway. As the car approached with its siren blasting, the pitch of the siren sound (a measure of the siren's frequency) was high; and then suddenly after the car passed by, the pitch of the siren sound was low. That was the Doppler effect - an apparent shift in frequency for a sound wave produced by a moving source.

The Doppler Effect in AstronomyThe Doppler effect is of intense interest to astronomers who use the information about the shift in frequency of electromagnetic waves produced by moving stars in our galaxy and beyond in order to derive information about those stars and galaxies. The belief that the universe is expanding is based in part upon observations of electromagnetic waves emitted by stars in distant galaxies. Furthermore, specific information about stars within galaxies can be determined by application of the Doppler effect. Galaxies are clusters of stars that typically rotate about some center of mass point. Electromagnetic radiation emitted by such stars in a distant galaxy would appear to be shifted downward in frequency (ared shift) if the star is rotating in its cluster in a direction that is away from the Earth. On the other hand, there is an upward shift in frequency (ablue shift) of such observed radiation if the star is rotating in a direction that is towards the Earth.RedshiftAstronomers often use the term redshift when describing how far away a distant object is. To understand what a redshift is, think of how the sound of a siren changes as it moves toward and then away from you. As the sound waves from the siren move toward you, they are compressed into higher frequency sound waves. As the siren moves away from you, its sound waves are stretched into lower frequencies. This shifting of frequencies is called the Doppler effect.

A similar thing happens to light waves. When an object in space moves toward us it light waves are compressed into higher frequencies or shorterwavelengths, and we say that the light is blueshifted. When an object moves away from us, its light waves are stretched into lower frequencies or longer wavelengths, and we say that the light is redshifted.

In the visible portion of theelectromagnetic spectrum, blue light has the highest frequency and red light has the lowest. The term blueshift is used when visible light is shifted toward higher frequencies or toward the blue end of the spectrum, and the term redshift is used when light is shifted toward lower frequencies or toward the red end of the spectrum. Today, we can observe light in many other parts of the electromagnetic spectrum such as radio, infrared, ultraviolet, X-rays and gamma rays. However, the terms redshift and blueshift are still used to describe a Doppler shift in any part of the spectrum. For example, if radio waves are shifted into the ultraviolet part of the spectrum, we still say that the light is redshifted - shifted toward lower frequencies.

The light from most objects in the Universe is redshifted as seen from the Earth. Only a few objects, mainly local objects like planets and some nearby stars, are blueshifted. This is because our Universe is expanding. The redshift of an object can be measured by examining theabsorption or emission linesin its spectrum. These sets of lines are unique for each atomic element and always have the same spacing. When an object in space moves toward or away from us, the absorption or emission lines will be found at different wavelengths than where they would be if the object was not moving (relative to us).

The change in wavelength of these lines is used to calculate the objects redshift. Redshift is defined as the change in the wavelength of the light divided by the wavelength that the light would have if its source was not moving (called the rest wavelength).