Astronomy 100

Lectures

Table of Contents

Astro 100

Lecture 3
Apparent Motions of the Sun and Stars

Outline

The View from Earth: The Celestial Sphere
Daily and Annual Motions of the Sun and Stars

Terms to Know

Celestial Sphere:

North and South Celestial Poles
Celestial Equator
Zenith
Nadir
Horizon

Rotation
Revolution
Ecliptic
Zodiac

1. The View from Earth: The Celestial Sphere

From a dark, clear site, you can see 3000-6000 stars with the naked eye. The sky is divided into 88 official constellations (e.g., Ursa Major, Virgo, Perseus) based on ancient Arab and Greek mythology. In addition, there are familiar asterisms (e.g., The Big Dipper, The Summer Triangle) that are common but not "official."

The contellations are only apparent lineups as seen from Earth of stars at many different distances; the stars' positions as seen in the sky of a planet orbiting a star other than the Sun would be totally different !

The Earth spins on its axis; the axis defines the North and South Poles. The Earth's equator lies exactly between the poles like a belt around a basketball.

Now imagine extending the Earth's axis like a needle north and south into the sky. If the sky were a shell surrounding the Earth (rather than an infinite void) -- the Celestial Sphere -- then the axis would intersect at two points: The North and South Celestial Poles . Likewise, if you could fling the Earth's equator out onto the shell of the sky, it would create a Celestial Equator, a Great Circle overhead exactly between the Celestial Poles.

How bright is Polaris, the North Star, in magnitudes? What's so special about Polaris?

Every person on Earth has a slightly different horizon -- the limit of what can be seen from that precise spot -- due to the Earth's curved surface. Directly overhead is the zenith , and directly under you is the nadir.

2. Daily and Annual Motions of the Sun and Stars

The stars rise in the East and set in the West just as the Sun does. Why? Because the Earth rotates (spins) on its axis from West to East (how many times a day?), bringing new parts of sky into view in the East and carrying them below the horizon in the West.

How can you prove this to yourself? On a clear night, go outside, face anywhere but North (why not North?), locate a bright star, and move around until the star is just next to a tree, telephone pole, or other landmark. Now wait a few (5-10) minutes. What happened?

Are there stars in the sky during the day?

The Earth also revolves (orbits) around the Sun (how many times a year?). Therefore, the Sun appears to glide among the stars throughout the year at the rate of about 1 degree per day (why 1 degree?). The path of that glide is the ecliptic (think: eclipse). If you were standing on the surface of the Sun (whose surface is not only 5000 degrees (K) but also not solid), you would see the Earth glide through the same ecliptic path, but 6 months later (or earlier), as it orbited around you.

Day by day, degree by degree, this causes different constellations to appear in the night sky and disappear into the daylight, until finally, after a year, the same constellations reappear.

The 12 constellations along the ecliptic make up the zodiac . When does the Sun appear to lie in the zodiac sign of your birthday?

Lectures Table of Contents Astro 100