Don’t Panic We are both learning how to use Astronomy Place. –Remember to “Join the Class” –There have been problems submitting answers In the future I will set up Angel to accept answers in the cases where you can’t submit them to Astronomy Place If problems persist come to my office so we can work together For this time deadline is extended until Monday

Spaceship Earth What are the motions of spaceship Eearth? Earth SPINS on its axis towards the EAST once every 24 hours Earth ORBITS around the Sun once every days. Speed ~ 1x10 5 km/hr. Earth, the Sun & the Solar System all orbit around the center of the Milky Way galaxy once every 2.3x10 8 years. Speed ~ 8x10 5 km/hr. The Milky Way is moving toward the Andromeda galaxy, speed ~ 3x10 5 km/hr The Milky Way (along with the local group) partakes in the expansion of the universe.

Earth Spins (Rotates) on its axis toward the East, once every 24 hours

Earth ORBITS around the Sun once a year What keeps the Earth in orbit and stops it from flying off?

The Solar System ORBITS the center of the Milky Way galaxy Period = 2.3x10 8 years What keeps it in orbit so it doesn’t fly off?

Our motion through the universe The Milky Way is moving through the universe ~ 600 km/s

Expanding Universe Model - Balloon

The Seasons What could produce our seasons - the difference between summer and winter? Changing energy output by the Sun Changing amount of energy we receive from the Sun Distance from the Sun Orientation of the ground re. Sun’s light (Tilt of Earth’s axis of rotation)

Sun’s heating & Distance

Sun’s heating & Orientation

Sun’s heating & Rays

The Cause of the Seasons

Why doesn’t Distance Matter? Small variation for Earth — about 3% Distance does matter for some other planets, notably Mars and Pluto. Surprisingly, seasons are more extreme in N. hemisphere, even thought Earth is closer to Sun in S. hemisphere summer (and farther in S. hemisphere winter) because of land/ocean distribution

Moon’s Phases

Phases of the Moon

A Solar Eclipse

Eclipses The Earth & Moon cast shadows. When either passes through the other’s shadow, we have an eclipse. Why don’t we have an eclipse every full & new Moon?

Planetary Motion Usually planets appear to move toward the East re the stars. Occasionally, they appear to move backward - toward the West, called Retrograde Motion

Retrograde Motion View from Earth: explained by big circles + epicycles Epicycle, rolls on big circle. Explains retrograde loop. big circle=‘deferent’. Explains most of planets motion

Retrograde Motion Like the Sun, planets usually appear to move eastward relative to the stars. But as we pass them by in our orbit, they move west relative to the stars for a few weeks or months. Noticeable over many nights; on a single night, a planet rises in east and sets in west…

Next Week I will be away Monday and Wednesday. Classes will occur in BPS 1410 as usual both days. Dr. Horace Smith will take over for me.

Assignment for Next Week Make sure you pick up homework due Wednesday Jan 28 (If you miss a Wednesday class, homework is linked to the course web page Pre-class assignment for Monday Jan 26, is linked to the course web page and is to be done on Angel.