AST 112

Motion of Objects in the Sky

The Celestial Sphere

• Imagine you’re where Earth is, but there’s no Earth.

• What do you see?

• Keep in mind:

– Nearest star is 4.4 light years away• Our orbit around the Sun is microscopic compared to this

distance

– Stars are points of light in the night sky• Some are brighter than others, but they’re still points

The Celestial Sphere

• Now imagine that you go around Earth’s orbit (still no Earth present):

– Do the positions of stars change perceptibly?

The Celestial Sphere

• What you see is the Celestial Sphere.

• Earth’s orbit around the Sun does not cause the stars to shift.

Celestial Sphere Terminology

• North Celestial Pole is Earth’s North Pole projected onto the Celestial Sphere– Same for South Celestial

Pole

• Celestial Equator is Earth’s equator projected onto the Celestial Sphere

Motion of the Night Sky

So we know what the entire sky looks like from Earth. We just can’t always see all of it.

Motion of the Night Sky

Earth gets in the way!

The Sun in the sky…

If the Sun comes into the sky, the stars in the sky are still there, but they cannot be seen.

How does Earth block the Celestial Sphere?

• What does it look like when:– There’s no Earth?– Earth is the size of a basketball?– Earth is the size of a boulder?– Earth is flat and of infinite extent?

• Earth is large enough to where it acts like a “plane of infinite extent”

How does Earth block the Celestial Sphere?

Earth blocks exactly HALF of the celestial sphere

at a given time.

The half that’s blocked is constantly changing.

Earth is round and large.

• The sky therefore looks like a dome

• When I refer to “The sky”, that means “the half of the Celestial Sphere that Earth isn’t blocking.”

Sky Terminology

• Zenith: the point straight above your head

• Meridian: arc that starts at North, passes through zenith, ends on South

• Horizon: where the sky stops and Earth starts

• Altitude: angle of object above horizon

What’s in the sky?

• In order to visualize what’s in the sky at any given time, you must visualize:

– The Celestial Sphere

– Earth blocking out half of it

So why do the stars rise in the Eastand arc over to the West?

Hourly Motion of the Stars

Earth rotates on its axis.

• Looking down on the Solar system (at Earth’s North Pole):

– Earth rotates counterclockwise about its axis

Day and Night

• At a given time:– Half of Earth is

illuminated (daytime)– Half of Earth is dark

(nighttime)

• Where’s the Sun in this image?

DaytimeNighttime

Day and Night

• Which side is going from day to night?• Which side is going from night to day?

The view depends on latitude.

• The motion of the sky changes as your latitude changes

• Objects in the sky also change

Rotation of the Night Sky

• How does the sky appear to rotate:

– If you’re at the North Pole?

– If you’re at the equator?

– If you’re anywhere in between?

Rotation of the Night Sky

Questions

• At the North Pole, what star is always at your zenith?

• At the equator, what star is always on the horizon?

Day-To-Day Motion of the Stars

So why do the stars shift one degree upward(when looking East) from day to day?

Earth orbits the sun.

• Goes counter-clockwise (looking at North Pole)

• In January:– Which set of stars is seen at

night?– Can the other set of stars be

seen?– Does the other set of stars

come up?

• And July?

Planets are not so far away.

• Because the planets are closer, they move relative to the stars.

• The motion of the planets in the sky is due entirely to the orbital motion of the planets around the Sun.

Sun, planets, moon move along the ecliptic.

• Solar System is more or less planar

• Move along a line in the sky called “The Ecliptic”

• Why is the ecliptic tilted relative to the celestial equator?

Zodiac Constellations

• The constellations along the ecliptic are Zodiac constellations

• The Sun and planets move along these constellations– 12 Zodiac constellations– The Sun spends roughly equal time in each– Corresponds to astrological sign

Zodiac Constellations