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 EarthWhat are the motions of spaceship Eearth?

• Earth SPINS on its axis towards the EASTonce every 24 hours

• Earth ORBITS around the Sun once every 365.25 days. Speed ~ 1x105 km/hr.

• Earth, the Sun & the Solar System all orbit around the center of the Milky Way galaxy once every 2.3x108 years. Speed ~ 8x105 km/hr.

• The Milky Way is moving toward the Andromeda galaxy, speed ~ 3x105 km/hr

• The Milky Way (along with the local group) partakes in the expansion of the universe.

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

Earth ORBITS around the Sunonce a year

Whatkeeps theEarthin orbitand stops it from flying off?

The Solar System ORBITS the center of the Milky Way galaxy

Period = 2.3x108 years

What keeps itin orbit so itdoesn’t fly off?

Our motion through the universeThe 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 MotionUsually planets appear to move toward the East re the stars.

Occasionally, they appear to move backward - toward the West, called Retrograde Motion

Retrograde MotionView 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 dueWednesday Jan 28(If you miss a Wednesday class, homeworkis linked to the course web pagehttp://www.pa.msu.edu/courses/isp205/sec-3)

• Pre-class assignment for Monday Jan 26, is linked to the course web page and is to be done on Angel.