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
D = linear size of object
 d
θ = angular size of object
(in arcsec)
206265
d = distance to the object
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
 d
206265
D = linear size of object
θ = angular size of object
(in arcsec)
d = distance to the object
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