Astronomy 120: Fall 2008
Homework 3: Due in September 19/22 recitation
Name ___________________________ Section _______
What is wrong with the following pictures?
picture from a Florida Keys tour company (If you have problems with the website link above, you can find
a copy of it here.)
- This view of the Moon and sun at sunset:
In lecture, we've talked about the phases of the moon - as seen from Earth.
But if you were an astronaut visiting the Moon, and looked back at the Earth
over the course of a month, you would see the Earth go through phases as well
(i.e. "crescent Earth," "full Earth," etc.)
Viewed from the Moon, what phase would the Earth show when Earthlings see
a waxing gibbous moon? Explain in a few sentences, adding a simple diagaram
if it will help.
The planet Mars has two moons - Phobos and Deimos. Mars rotates on its axis
in 24 hours and 37 minutes (1 Mars day = 1.02 Earth days). Phobos orbits Mars
in 0.32 Earth days, and Deimos orbits Mars in 1.26 days.
- What direction (east or west) does Deimos move as viewed from the surface
- What direction (east or west) does Phobos move as viewed from the surface
- If your answers to a) and b) are different, explain (in a few sentences)
- Do these small fast-moving moons show a full set of phases like our own
Moon does? Why or why not?
- BONUS: How long is 1 month on Mars, if you use Deimos to measure
the martian month?
The planet Mars (and Jupiter, and Saturn...) appear to make loops as they
move with respect to the stars. These retrograde loops were explained in the
ancient past (when the Earth-centered solar system was still the best model).
using epicyclic motion (the Ptolemaic model).
- If you had a telescope back during the time of the Greeks, what
observation of Mars could you make to test the Ptolemaic model?
- Retrograde loops only occur around opposition (when the Sun is in the
opposite side of the sky from the planet). Can the Ptolemaic model explain
this? What constraints does this place on the size of the epicycle, and
on the time taken to do the loop.