Chapter 2

A User’s Guide to the Sky

Guidepost

• In this chapter, you will consider Earth as a rotating planet, moving in an orbit

• How are stars and constellations named?

• How is the brightness of stars measured and compared?

• How does the sky appear to change and move in daily & annual cycles?

• What causes seasons?

• How do astronomical cycles affect Earth’s climate?

2-1 Stars and Constellations

• In ancient times• Constellations = brightest stars that appeared to form groups

• Represented great heroes and mythological figures

• Position in the sky told stories handed down from generation to generation over thousands of years

Constellations – An Ancient HeritageThe constellations are an ancient

heritage handed down for thousands

of years as celebrations of

mythical heroes and monsters. Here,

Sagittarius and Scorpio appear

above the southern horizon.

In antiquity, constellation boundaries

were poorly defined, as shown on

this map by the curving dotted lines

separating Pegasus from

Andromeda.

Constellations – Today • A constellation is a stellar pattern usually identified by name, usually of mythological

gods, people, animals, or objects; also the region of the sky containing that star pattern.

• Constellations = well-defined regions on the sky• Irrespective of presence or absence of bright stars in those regions

• Constellation stars • Only appear to be close to one another• Projection effect• Stars may be located at very different distances from Earth

An asterism is a named group of stars not identified as a constellation, for example the Big Dipper.

International Astronomical Union (IAU) is an international society of astronomers that, among other activities, decides definitions and naming conventions for celestial objects and surface features. The IAU defined the constellation boundaries by establishing 88 official constellations in 1928 (or 1930 depending on where you get your info) and reclassified Pluto as a dwarf planet in 2006.

Projection of Stars in the Sky

Star Names

• Named by a Greek letter according to their relative brightness within constellation

• Betelgeuse = a Orionis; Rigel = bOrionis

• The stars in Orion do not quite follow the rule for assigning Greek letters in order of decreasing brightness. In Orion, (Beta) is brighter than (Alpha), and (Kappa) is brighter than (Eta). Fainter stars do not have Greek letters or names, but if they are located inside the constellation boundaries, they are part of the constellation. Alpha Orionis is also known as Betelgeuse; Orionis is also known as Rigel. Stars in a constellation can be identified by Greek letters and, in many cases, also by names derived from Arabic. The spikes on the star images in the photograph were produced by an optical effect in the telescope.

Easily Recognizable Constellations and Their Brightest Stars

Favorite Stars: Locate these

bright stars in the sky and

learn about their

characteristics. Refer to the

star charts in Appendix B

Star Brightness

• Magnitude scale• The astronomical brightness scale; the larger the number the fainter the star.

• Brightest stars: ~1st magnitude

• Faintest stars (unaided eye): 6th magnitude

• More quantitative:• First magnitude stars appear 100x brighter than sixth magnitude stars

• First magnitude difference gives a factor of 2.512 in apparent brightness • Larger magnitude → fainter object

• Apparent Visual Magnitude• The brightness of a star as seen by human eyes on earth.

The Scale of Apparent Visual Magnitudes

• The magnitude scale system can be extended

Magnitude and Flux

• Brightness is subjective• Flux = measure of light energy from a star that hits a collecting area of one

square meter in one second

2-2 The Sky and Celestial Motions

• The sky above seems like a great blue dome in the daytime and a sparkling ceiling at night

• The first astronomers kept this in mind long ago as they tried to understand the Universe

The Celestial Sphere

• Celestial Sphere• The imaginary sphere of very large radius surrounding Earth to which the planets, stars, Sun,

and Moon seem to be attached.

• Zenith• Point on the celestial sphere (CS) directly overhead

• Nadir • Point on the CS directly underneath

• Celestial equator• Projection of Earth’s equator onto CS

• North Celestial Pole• Projection of Earth’s north pole onto CS

• South Celestial Pole• Projection of Earth’s south pole onto CS

The Celestial Sphere (cont’d.)

The eastward rotation of Earth causes the Sun,

Moon, planets, and stars to move westward in the

sky as if the celestial sphere were rotating

westward around Earth. From any location on

Earth you see only half of the celestial sphere, the

half above the horizon. The zenith marks the

point of the celestial sphere directly above your

head, and the nadir marks the point of the

celestial sphere directly under your feet. The

drawing at right shows the view for an observer in

North America. An observer in South America

would have a completelydifferent horizon, zenith, and nadir.

The Celestial Sphere (cont’d.)

• The distance between two stars on the celestial sphere can only be given as the difference between the directions in which we see the stars.

• Therefore, distances on the celestial sphere are measured as angles in:• Degrees (º): full circle = 360º

• Arc minutes (‘): 1º= 60’

• Arc seconds (“): 1’=60”

The Celestial South Pole Is Not Visible from the Northern Hemisphere

North

Celestial

North Pole

40.7º

New York City: l ≈ 40.7º

South

49.30

Celestial

Equator

Horizon

Measuring Distance Across the Sky as Angles

Apparent Motion of the Celestial Sphere

• Looking north, you will see stars apparently circling counterclockwise around the Celestial North Pole

• Some constellations around the Celestial North Pole never set

• Circumpolar constellation: Any of the constellations so close to the celestial pole that they never set (or never rise) as seen from a given latitude.

The “Circumpolar Circle”

Apparent Motion of the Celestial Sphere (cont’d.)

Looking east, you see stars rising and moving to the upper right (south)

Looking south, you see stars moving to the right (west)

How Do We Know? 2-1

• Scientific models (e.g., celestial sphere)• A carefully devised conception of how something

works• Idealized models: some complex aspects of nature

can be simplified or omitted• e.g., ball-and-stick model of a molecule doesn’t

show the relative strength of the chemical bonds• It is important to remember the limitations of a

model

Common Misconceptions

• Misconception: The stars are not in the sky during the daytime

• Truth: The stars are actually there day and night; they are just invisible during the day because the sky is lit by the Sun

• Misconception: Polaris is the brightest star in the sky • It is actually the 50th visually brightest star, and important

because of its position, not because of its brightness

Precession

• The resulting “wobbling” of Earth’s axis of rotation around the vertical with respect to the Ecliptic (takes about 26,000 years)

Precession

• The slow change in the direction of Earth’s axis of rotation; one cycle take 26K years.

• (a) The rotation axis of a spinning top precesses in a conical motion around the perpendicular to the floor because its weight tends to make it fall over.

• (b) Earth’s axis precesses around the perpendicular to its orbit because the gravity of the Sun and Moon acting on Earth’s equatorial bulge tend to twist it “upright.”

• (c) Precession causes the north celestial pole to move slowly among the stars, completing a circle in about 26,000 years .

The Celestial North Pole

2-3 Sun and Planets

• Earth’s rotation on its axis causes the cycle of day and night, but its motion around the Sun in its orbit defines the year

• Rotation: the turning of a body on its axis

• Revolution: the motion of a body around a point outside the body

Annual Motion of the Sun

• Earth’s rotation is causing the day/night cycle

• This view of Earth as if looking down from above the North Pole shows how the time of day or night depends on your location.

The Sun’s Apparent Path on the Sky – The Ecliptic• Earth’s orbit is a nearly perfect

circle, but it is shown in an inclined view in this diagram and consequently looks oval. Earth’s motion around the Sun makes the Sun appear to move against the background of the stars. Earth’s orbit is thus projected on the sky as the path of the Sun, the ecliptic. If you could see the stars in the daytime, you would notice the Sun crossing in front of the distant constellations as Earth moves along its orbit.

Seasons• Earth’s axis of rotation is

inclined relative to the normal to its orbital plane by 23.5º, which causes the seasons

Common Misconception

• Misconception: Seasons are caused by Earth moving closer to, or farther from, the Sun• Truth: The seasons arise because Earth’s axis is not

perpendicular to its orbit• Note: In January, the Earth is actually 1.7% closer to

the Sun than its average orbital distance

The Angle of Incidence of the Sun’s Rays

The Angle of Incidence of the Sun’s Rays

We receive more energy from the sun when it is shining onto the Earth’s surface under a steeper angle of incidence.

Seasons and Earth's Inclination

Seasons are not related to Earth’s distance from the sun. In fact, Earth is slightly

closer to the sun in (northern-hemisphere) winter than in summer.

Additional Important Terms• Vernal Equinox: The place on the celestial sphere where the Sun crosses the

celestial equator moving northward; also, the time of year when the Sun crosses this point, about March 21, and spring begins in Northern Hemisphere.

• Autumnal Equinox: The place on the celestial sphere where the Sun crosses the celestial equator moving southward; also, the time of year when the Sun crosses this point, about September 22, and autumn or fall begins in Northern Hemisphere.

• Summer Solstice: The point on the celestial sphere where the sun is at its most northerly point (highest point); also, the time when the Sun passes this point, about June 22, and summer begins in the Northern Hemisphere.

• Winter Solstice: The point on the celestial sphere when the sun is farthest south (lowest point); also, the time of year when the sun passes this point, December 22, and winter begins in the Northern Hemisphere.

Additional Important Terms (cont’d.)• Perihelion: the orbital point of closest approach to the sun.

• Aphelion: the orbital point of greatest distance from the sun.

• Evening Star: any planet visible in the sky just after sunset.

• Morning Star: any planet visible in the sky just before sunrise.

• Zodiac: the band around the sky centered on the ecliptic within which the planets move.

How Do We Know? 2-2

• What is the difference between a science and a pseudoscience?

• Pseudoscience = a set of beliefs that appears to include scientific ideas, but does not follow basic rules of science

• Scientific claims can be tested and verified

• Astrology: a pseudoscience• There is no connection between the positions of the Sun,

Moon, and planets with people’s personalities or events in their lives

Motions of the Planets

• The planets are orbiting the sun almost exactly in the plane of the ecliptic

Venus

Mercury

Mercury and Venus

• Mercury and Venus follow orbits that keep them near the Sun, and they are visible only soon after sunset or before sunrise when the brilliant Sun is hidden below the horizon. Venus takes 584 days to move from the morning sky to the evening sky and back again, but Mercury zips around in only 116 days.

• Mercury: appears at most ~28° from the Sun

• Venus: at most ~46° from the Sun

2-4 Astronomical Influences on Earth's Climate

• Factors affecting Earth’s climate• Eccentricity of Earth’s orbit around the Sun (varies over

period of ~ 100,000 years)• Precession (Period of ~ 26,000 years)• Inclination of Earth’s axis versus orbital plane

• Milankovitch hypothesis • Changes in all three of these aspects are responsible for

long-term global climate changes (ice ages)

Predicting Earth's Temperatures

(a) Milankovitch Hypothesis: the hypothesis that small changes in Earth’s orbital and rotational motions cause the ice ages.

(b) Calculations based on the Milankovitch hypothesis can be used to predict temperatures on Earth over time. The warming illustrated by the Earth globes shown here took place from 25,000 to 10,000 years ago and ended the last glacial advance. Relatively cool temperatures are represented by violet and blue, warm temperatures by yellow and red.

(c) Over the last 400,000 years , changes in ocean temperatures measured from fossils found in sediment layers from the seabed approximately match calculated changes in solar heating.

The globes in panel (a) illustrate events in only a short segment near the recent (right) end of the timelines.

How Do We Know? 2-3

• Why is evidence so important in science?• All scientific knowledge is based on evidence from

observations and experiments• Evidence = reality

• Scientists constantly check their ideas against reality

• Every theory or conclusion should have supporting evidence • If you can find and understand the evidence, the science

will make sense

How Do We Know? 2-4

• Scientific arguments• Argument = a summary of evidence and principles leading

to a conclusion

• A good scientific argument • Hypothesis (explanation)• Description of observations and/or experiments• Other evidence and alternate explanations

Discussion Questions

• All cultures on Earth named constellations. Why do you suppose this was such a common practice?

• Hint: Is it human nature to see patterns and images in ordinary objects like the sky?

• Does another planet in the Solar System or an extrasolar planet have an ecliptic? Could it have seasons? If so, name two possible sources for the seasons.