Announcements

• First exam will be on Friday, September 17

• All lecture notes and a practice exam are available at the class web site at http://phobos.physics.uiowa.edu/~kaaret/sgu_f04

• Astronomy tutorials will be held in 618 VAN on Tuesday 12:30-2:30 pm, Thursday 11:00 am - noon, and Friday 10:30-11:30 am.

• Observing continues on the roof of Van Allen Hall this week, Monday-Thursday 9-11 pm. Go and get the extra credit.

Announcements

• Wednesday will be a review session

• Please hand-in or e-mail questions for the review session.

• If time permits, we will play “Who wants to be a millionaire astronomer” based on the questions on the practice exam. Anyone who can reach the millionaire level gets an automatic 100% score on Friday’s exam.

Today’s topics

• Phases of the Moon

• Angular size and physical size

• Precession of the Earth

• Reading sections 2.3-2.4, 3.1-3.5

Phases of the Moon

Phases of the Moon

• The phases of the Moon are caused by the orbit of the Moon around the Earth, but how does this work?

• Any thoughts?

Moon’s Orbit• Relative to the Sun, the

Moon orbits the Earth once every 29.5 days (relative to the stars the orbit is 27.3 days)

• The orbit of the Moon is tilted by about 5 degrees relative to the ecliptic

• The Moon rotates at the same rate that it orbits, so the same face of the Moon always points towards Earth

Phases of the Moon

Picture taken by Galileo spacecraft from 4 million miles away

Phases of the Moon

Does the Earth have phases? (as seen from the Moon)

Do the planets have phases?

Phases of the Planets

Sizes of Astronomical Objects

• How can we measure the sizes of astronomical objects?

• The same way that we measure distances – using triangles

The Small-Angle Formula D = linear size of object

θ = angular size of object (in arcsec)

d = distance to the object206265

dD

Example: On November 28, 2000, the planet Jupiter was 609 million kilometers from Earth

and had an angular diameter of 48.6″. Using the small-angle formula, determine Jupiter’s actual

diameter.

D = 48.6″ x 609,000,000 km / 206265 = 143,000 km

The Small-Angle Formula D = linear size of object

θ = angular size of object (in arcsec)

d = distance to the object206265

dD

Precession

• If you spin a top, its very hard to get it to spin exactly straight – usually it wobbles around in a circle

• The spinning Earth wobbles in exactly the same way – this is called precession

Precession of the Earth