Astronomy 2, Spring 2007
Homework set 1, due April 17.
1. The scale of the Universe. This problem is to give you some sense of astronomical
distances, which are so great that light takes significant amounts of time to
cross them even at its high speed of 3x105 km/sec. Astronomers use as a measure of
distance the light-year, which is the
distance light travels in one year.
This problem also serves to give a review of powers-of-ten or scientific notation*, which will be used extensively during the
course.
(a) How long does it take light to reach us from the moon,
which is 3.84x105 km away?
(b) The astronomical unit is
the average distance of the earth from the sun, about 1.50x108
km. How many times further away is
the sun than the moon? How many
minutes will it take light to travel from the sun to the earth?
(c) The nearest star other than the sun is Proxima Centauri,
about 4.2 light-years away. How
long does it take light to travel from that star to us?
2. Around 240
BC Erathosthenes found that the altitude of the noonday sun on the same day-
the first day of summer- differed by about 7o when viewed from two
cities whose north-south distance was about 5000 stadia. An alien on a planet going around
another star (extra-solar planet) made the same set of measurements for two
cities also separated by 5000 stadia north-south. In this case the altitude of the sun differed by 30o
on the same day at noon. Answer
the following, explaining your answers.
(a) Is the extra-solar planet bigger or smaller than the
earth?
(b)Assuming this planet is spherical, calculate its
circumference and radius in stadia.
Show your calculations.
3. Suppose you
lived on the moon near the center of the side we see from earth.
(a) At the phase of full moon, what phase would you see for
earth? Would it be daylight or
dark where you live?
(b)When the moon is a waning crescent as seen from the
earth, what phase would the earth have as seen by you on the moon.
(c) If you lived on the moon, would you see the earth rise
and set from everywhere on the moon or not? Explain.
4. Eclipses can occur at only two particular phases of the
moon. Explain each answer carefully.
(a) What are these phases?
(b) These two phases occur every lunar month. Do eclipses occur every lunar month?
(c) Can you see annular solar eclipses from the moon
(d) Over 100 years would you expect to see more total and annular eclipses of the sun or eclipses of the moon?
*Scientific notation:
