Monday January 18th 1999
Announcements:

Lecture notes:
Class 4.
 

Earth has a 13 mile equatorial bulge due to its rotation

Last time we learned about rotation axis of the Earth and how it
is tipped by 23.5 degrees from the plane of Earth's orbit around
the sun.

The north pole points towards a star in the sky called Polaris

However there is a subtle point that we missed last time.  It turns out that the rotation of the Earth actually wobbles around very slowly.

Like a gyroscope -- if you have played with a gyroscope you know the motion of the gyroscope keeps it relatively stable but does wobble some due to the Earth's gravity trying to pull the gyroscope over.

A similar effect occurs for Earth - gravity of sun and moon tugs
on Earth's equatorial bulge and this causes slow precession.

Noticed over 2000 years ago  Hipparchus who compared star positions over 200 years

Very slow process - 1 cycle takes 26000 years

• 4800 years ago Thuban was the north star according to Egyptian records
• Now it is Polaris
• 12000 years from now it will be Vega

The Moon always keeps one side facing us
Locking of Moon's rotation and orbit -- the moon's rotation period equals the period of revolution about the earth.  27.3 days

Earth also makes tidal bulges on the Moon.  This caused the moon's rotation to slow down and  causes tidal locking.

Lunar Eclipses.  Occurs when Earth blocks the Sun's light from the Moon
They are relatively rare circumstances when Earth's shadow falls on the Moon.

Sometimes also the moon can block out the sun called a solar eclipse.
This is only possible because the Sun and Moon have nearly equal angular diameters about .5 degrees

During solar eclipse you can see stars in daytime can also see solar chromosphere and the corona
There are nice pictures of solar eclipse on page 33-34 of the text.

Umbra of Moon's shadow is small on Earth always smaller than 168 miles in diameter
Takes about an hour for the moon to cover sun's disk but totality only lasts for a few minutes.
Moon's motion and Earth's rotation make umbra move at about 1000 miles/hour

total eclipse = perfect alignment
partial eclipse = imperfect alignment
annular eclipse = moon happens to be further from earth due to its eccentric orbit so doesn't cover whole sun.
 

Planetary Motions
ecliptic - plane of Earth's orbit around the Sun
Astronomical unit - average distance from Earth to Sun about 93 million miles or 150 million km

All planets go around the sun in roughly the same plane roughly the plane of the ecliptic