celestial bodies and names planets, stars, the sun, and the moon were the subject of mystical powers...

Celestial Bodies and Names

Planets, stars, the Sun, and the Moon were the subject of mystical powers well before the development of the foundations of science appeared in the ancient Mediterranean cultures

Ancient Egyptian, Chinese, and Indian mythology identified the powers of their Gods with the imaginary forces of the heavenly bodies

Our western culture inherited the Egyptian, Mesopotamian, and Greek mythological identities and names of the planets and many of the bright stars

The names we use for the planets and many of the moons are the Roman names adopted from the Greek names


Planet Greek Name Roman Name

Mercury Hermes - The Messenger GodMercury - The Messenger God

Venus Aphrodite - Goddess of Love Venus - Goddess of Love

Earth

Gaea or Terra - Represented the Earth. Created the Universe and gave birth to the first race of Gods (the Titans), and to the first humans

Terra - Goddess of the Earth

Mars Aires - God of War Mars - God of War

JupiterZeus - King of the Gods and son of Gaea

Jupiter - King of the Gods

Saturn Cronus - God of Agriculture Saturn - God of Agriculture

Uranus Uranus - God of the Sky Uranus - God of the Sky

NeptunePoseidon - God of the Sea; brother of Jupiter

Neptune - God of the Sea

Pluto Hades - God of the UnderworldPluto - God of the Underworld


Later discoveries of planets using telescopes followed the convention of Roman/Greek mythological gods

Moons are generally derived from the Greek and Roman mythological characters The moons of Uranus are named after Shakespearean

literary figures (e.g., Cordelia, Ophelia, and Bianca)

Asteroids (minor planets) were first named after mythological and literary characters With the discovery of tens of thousands of asteroids came a

more practical naming scheme that includes the year and month of discovery, and at times, the name of the discoverer

Comets are named after the discoverer(s) and the sequence of discovery


Stars

Stars and star names originated in the ancient cultures, with the western world introduced to the names and positions of stars coming from the Mesopotamians and Greeks

Groups of stars called constellations came from the Babylonians and/or Sumerians and were copied by the early Greeks

These early constellations is recorded in Ptolemy's Almagest, along with more than a thousand stars, and a summary of all of Greek astronomical knowledge


Stars

Stars were first classified by their parent constellation and their relative brightness within that constellation using the Greek alphabet

Alpha Ursa Majoris for example is the brightest star in the constellation Ursa Major

With the advent of powerful telescopes and the ability to resolve billions of stars and galaxies, the convention for identifying stars is their right ascension and declination (latitude and longitude) in the sky


Stars

Twelve constellations that surround the equatorial belt of the celestial sphere are called the constellations of the zodiac Zodiacal constellations circle the sky in the Sun’s

apparent path through the stars Known also as the 12 signs of the Zodiac Zodiac is Greek for “circle of animals” Origin of the longitude on the celestial sphere is the

vernal equinox


Stars

Although no more or less important than the other constellations, these have become commonplace because of their position in relation to the Sun's path through the equatorial belt Commonly associated with astrology, a pseudo-

science that was separated from astronomy some 500 years ago

Calendars and Time

Time and Calendars

Calendars are based on the seasonal differences in the year and the lunar cycle

Egyptian calendar was based on twelve months per year with 30 days in each month, totaling 360 days in a year which was more than five days short of the actual solar year

To correct for the difference five days were inserted into the calendar as a festival during the harvest season This festival called heb set was a part of the

Egyptian's 365 day calendar

Egyptian calendar was 12 months/year x 30 days/month +5 days harvest festival (heb set) = 365 days/year

Time and Calendars

Egyptian calendar was off by about ¼ day per year but used by the Greeks and the Romans

A calendar correction was made by inserting one-quarter day per year, but by using a full day every four years called leap year Called the Julian Calendar, after the Roman ruler

Julius Caesar who commissioned the change

Julian Calendar – similar to Egyptian calendar but added 1 day every four years (leap year) = 365.25 days/year

Time and Calendars

The ¼ day/year correction was also inaccurate, although less so than without the leap year added

The actual value of 365.24219... days per year could be approximated by adjusting the number of leap years in a longer period

Over a century, the difference between 365.25 and 365.24219.. was less than a day and the accumulated difference of one day was vary close to four centuries

As a solution, the Roman Catholic Church adopted a method that would resolve the problem of drift in the calendar and reconcile the vernal equinox and Easter

Time and Calendars

That calendar correction omitted leap years every century except those divisible by 400

This was called the Gregorian calendar that was named for Pope Gregory and the calendar in general use today

Gregorian Calendar – similar to Julian calendar but leap year is removed on century dates except those divisible by 400

Time and Calendars

Gregorian calendar

Ten days were removed in October, 1582, to reconcile the accumulated difference in time from the earlier Julian calendar. For this reason, astronomers use dates based on the original Julian day calendar - called the Julian date - for continuous time and date reference

The last day of the Julian calendar was Thursday 4 October, 1582, which was followed by the first day of the Gregorian calendar Friday, 15 October, 1582

Time and Calendars

A second type of calendar is based on the Moon's orbital period around the Earth

The lunar calendar is not fixed with reference to the solar calendar base which results in a progressive shift of the Islamic calendar's cardinal dates on the Gregorian calendar

A third type of calendar called the lunisolar calendar is based on the lunar cycle, but reconciled (intercalated) with the tropical year every few years This creates a discontinuity in both of the lunisolar

calendars that use this dual time base - the Chinese and the Hebrew calendars

Hence, the Chinese New Year is not on the same day on the Gregorian calendar from year to year

Time and Calendars

Time is ultimately based on the Earth's orbit around the Sun

This orbital period defines dynamical time which must account for the irregular motion of the Earth

Other definitions of the Earth's orbit period around the Sun include the period measured from one vernal equinox to the next Average is 365 days, 5 hours, 48 minutes,

45.51 seconds Called the vernal year

Time and Calendars

The average over all seasonal positions is called the mean tropical year

Time with respect to the background stars called the sidereal year is slightly different than solar and vernal year because of the Earth's changing rotational axis orientation called precession Period of approximately

26,000 years

This difference due to this precession is approximately 20.40 minutes per year

Time and Calendars

The two most common time references which define the year are called solar time and sidereal time

Solar time incorporates the Sun as the primary reference

Sidereal time uses the background (fixed) stars as the primary reference Background stars are also useful as an inertial

(fixed) reference for navigation and astronomy

Solar time - Sun used as primary reference

Sidereal time - Background stars are used as primary reference

Time and Calendars

The Earth’s day is defined roughly as one rotation of the Earth on its axis every 24 hours

Since the Earth also orbits the Sun, there is a difference between a complete rotation with reference to a fixed background versus the moving solar background

As the Earth rotates and orbits the Sun, it most rotate slightly farther to make up for the 0.99o orbital motion per day (360o in 365.242 days)

This produces a 3.94 minute per day difference between solar (orbit) time and sidereal (star) time

Because the Earth's orbit also produces one rotation, there is a difference of one extra day per year

Time and Calendars

The difference between solar and sidereal time is calculated using:

1/365.242 days/year x 24 hr/day x 60 min/hr = 3.943 min/day

This calculation is also the time difference representing the apparent motion of stars in the night sky of 3.943 minutes per day, or, 24 hours per year

Using 30 day months, the star motion is approximately 2 hours per month (30 days x 4 minutes/day = 120 minutes/month) or 1,440 minutes per year = 24 hr

Sidereal time is shorter than solar time by:

4 min per day 2 hours per month 24 hours per year

Time and Calendars

Prague Astronomical Clock

Time and Calendars

Development of Science


3,000 BC Constellations named by the Minoan, Sumerian and Babylonian cultures

2,000 - 3,000 BC The Egyptian culture made extensive celestial observations that were used for their calendars, for characterizing their mythological deities, and for agricultural and ceremonial purposes

300 BC The first scientists in the Classical Greece period used logic to develop the foundation of the sciences by using knowledge, measurements, and reasoning to help understand their surroundings


The cornerstones of science were later dismantled by a number of the well-known Pythagorean philosophers who proposed that data was unnecessary in modeling the physical world, and that a science like astronomy was a futile attempt in trying to discover the secrets of the universe

True science was replaced with a divine practice using logic that pursued the same questions but with ratios

“So if we mean to study astronomy in a way which makes proper use of the soul's inborn intellect, we shall proceed as we do in geometry -- working at mathematical problems -- and not waste time observing the heavens (Plato's' Republic)”


Planetary motion was also characterized in Aristotle’s teachings as follows:

Solid crystalline spheres in the sky carried the five visible planets, the Sun, and the Moon

The outermost sphere held the stars

The Earth was at the center of the universe and did not move

Sublunar world was imperfect, consisting of everything under the Moon, with a natural motion towards Earth

Superlunar world was perfect, consisting of everything beyond the Moon (stars and planets) with circular orbital motion around the Earth

Aristotle’s Universe


250 BC Aristarchus of Samos developed the first sun-centered (heliocentric) concept of the "universe" which consisted of the solar system and visible stars

140 A.D. Ptolemy refined the Greek view of the geocentric (Earth-centered) universe with eccentric, epicycle, and equant geometries to describe the motions of the Sun, Earth, planets, and the Moon


These complicated refinements remained as the accepted explanation of the unusual motion of the planets (from planetes, or "wanderers" in Greek) until the Copernican Revolution in the 1500s The incorrect theory of the geocentric solar

system is often referred to as the Ptolemaic universe because of its increased refinement by the astronomer and mathematician Claudius Ptolemaeus (90-168 AD)


1543 Nicolaus Copernicus published On the Revolutions of the Celestial Spheres which established the Sun as the center of the solar system

His theory was far more convincing than Ptolemy's geocentric model, but the new concept was inconvenient to adopt for scientists and the church There were also inaccuracies in the

Copernican theory since his circular orbit model for the planets still did not account for the irregular planetary motion, including the retrograde motion of Mars.


1570-1601 Tycho Brahe made careful measurements of the stars and planets which would be used for the more refined heliocentric theory of Kepler

Although Tycho was meticulous in his observations, he believed in the Ptolemaic geocentric solar system

Because the model could not be reconciled with his own observations, Tycho constructed his own theory using a modified Ptolemaic geocentric solar system that had all planets except for the Earth and the Moon orbiting the Sun, with that collection orbiting the Earth

Tycho Brahae’s Model


1600 Johannes Kepler begins work for Tycho Brahe and applies mathematics to the observations to arrive at his three laws, and an accurate physical model for the solar system

Kepler's laws

1. The planets orbit the Sun in elliptical orbits, with the Sun at one focus of the ellipse

2. Equal areas are swept out in equal times by the orbiting planets

3. The period of orbit for a planet squared is equal to its semi-major axis cubed


1600-1640s Galileo Galilei established a much more accurate definition than Aristotle for force and motion which would later become the fundamentals of mechanics

His demonstration of the uniformity of falling bodies due to the force of gravity was only part of the relationship established between forces, velocity and velocity changes, and inertia

Galileo also demonstrated the first basic understanding of gravity


Galileo applied the optical telescope for the first time to make accurate astronomical observations and found: The inner planets have

phases similar to the Moon (proving heliocentric theory)

There are four large moons of Jupiter (Galilean moons)

Numerous stars make up the diffuse Milky Way galaxy

Lunar mountains, valleys and seas looked similar to those on Earth


1660-1720s Isaac Newton developed precise mathematical and physical laws of gravity and the laws of physical motion

Newton also constructed the basic theories of light, and created one of the most important mathematical tools - calculus – at the same time as and independent of Gottfreid Leibniz

Newton would also develop analytical tools and approximation methods useful for two-body and n-body orbit calculations


Newton's three laws of motion

1. Newton's first law (also known as the law of inertia)An object at rest will stay at rest and an object in motion will stay in motion unless acted upon by an external force.

2. Newton's second lawAn applied force equals the rate of change of (the derivative of) momentum. This is a statement that force equals mass times acceleration, or F=ma.

3. Newton's third lawFor every action there is an equal and opposite reaction. This is the basic principle of rocket propulsion.


Joseph Lagrange (1736-1813) - In addition to developing a number of mathematical tools for physics and astronomy applications, Lagrange was intrigued with planetary motion and orbital motion in general

Lagrange also developed a powerful technique for approximating the motion of small objects throughout the solar system using the simplified three-body problem

His work led to the stability and motion characterization of the solar system's members, from the largest planets to the smallest particles


1900 - Max Planck theorized that electromagnetic energy (light) is quantized in units called photons

This gave rise to the beginning of the theories that constitute quantum mechanics, a powerful tool in understanding the physical laws of the atomic world


1905 - Albert Einstein postulated the photoelectric effect which corroborates the quantum mechanical view of the atomic world Although Einstein was uncomfortable with the later

developments of quantum mechanics and its implications, he received the Nobel prize in physics in 1912 for his early work on the quantum photoelectric effect

1910-1920s - Albert Einstein developed the special and general theories of relativity which greatly advanced our understanding of gravity (mass), motion, time and space These theories are still used for understanding

observations of the distant universe, galaxies, and even nearby stars


1920s Edwin Hubble and Harlow Shapely establish the concept and scale of our galaxy, the Milky Way (also known as The Galaxy)

His pioneering work established the expanding universe and later the Big Bang theory

Hubble's groundbreaking work that measured the receding galaxies continues today with much greater accuracy


As more distant and more precise measurements are made of distant galaxies, the Big Bang theory has become much more complex

Our understanding of the structures in the universe and the foundation of the Big Bang theory have evolved from these early observations

Questions?

celestial bodies and names planets, stars, the sun, and the moon were the subject of mystical powers...

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