Lunar Craters
The Moon is only up during the lab meeting for a few days during the semester. The TA's will tell you on a given night if you should be working on the Moon Craters lab.
Note that during this semester, many of the Moon's best showings are during October and Thanksgiving breaks! Therefore, you really need to take advantage of it when it is available, and should consider coming in on Thursday to make sure that you get good Moon observations.
Goals of the Lab
To carefully observe and draw some lunar craters, practicing careful observation through drawing.
To find similiarities and differences between various different craters on the Moon, and to think about what this may tell us about the shape of the Moon.
To estimate the size of a crater.
To perform some simple observations of lunar craters that tell you something about the history of the Moon.
Requirements: the telescope, your logbook, moon map (available from the TAs)
Terminology: for names for parts of the moon, see the Lunar Observations lab.
Part I: Individual Craters
Craters with Mountains: use the Moon Map to identify one of the following craters: Agrippa, Delambre, Eratosthenes, Langrenus, Theophilus, or Tycho. Observe this crater and the mountain in the center of the crater with the 10mm eyepiece. Make a sketch of it in your logbook in as much detail as you are able. Take your time to do a good job!
Craters with Rays: Find one of the following craters: Aristillus, Copernicus, Langrenus, Kepler, Pickering or Tycho. Do not use the same crater as the one you used in step 1. Notice the bright "rays" radiating from the craters; these rays are brighter near the full Moon. Sketch this crater and its rays.
Crater Size: Pick one of the two craters you sketched in the previous two steps, and measure its angular size using the Method of Transit Times.
Crater Shapes: Compare the shapes of craters near the center of the Moon's disk to craters near the limb. Find craters near the center; what shapes are they generally? Now scan along the illuminated limb of the Moon. What are the shapes of the craters there?
Draw just the outlines of 3-4 craters near the center of the disk. (You don't need to do careful sketches as you did in steps 1 and 2; just indicate the overall shape of the crater, i.e. is it a circle or an oval, which was is it oriented.) Next pick three regions along the limb of the Moon: one towards the center of the illuminated limb, one towards the north, and one towards the south. Sketch the outlines of 3-4 craters in each region. (Again, these need not be detailed drawings, just sketches of the shapes of the craters.)
Questions to answer outside of lab:
(a) Use the small angle formula (described in How to Measure Angular Distances), knowing the distance to the Moon is about 380,000km, to estimate the physical size of the crater you measured in Step 3. How does this compare to the size of the USA? To the size of Tennessee?
(b) What, if anything, do the shapes you observed in Step 4 tell you about the overall shape of the Moon? In particular, what would you expect to see if the Moon were a big, flat disk facing the Earth? What would you expect to see if the Moon were a big ball? Which of these two descriptions better fits the crater shapes you've observed?
Part II: Crater Counts
Crater Counts: the craters on the Moon are the result of impacts from solar system debris. The crater density is different on different parts of the Moons surface because sometimes throughout the history of the Moon, lava flows would wipe out previously existing craters and smooth out the surface on some part of the Moon. Any craters on these parts must result from impacts after the lava flow. As such, regions with fewer craters represent younger regions of the Moon's surface.
Procedures
Use the 25mm eyepiece to pick a spot on the Moon somewhere within one of the Maria.
Put in the 10mm eyepiece and get the telescope back in focus. Count and record the number of creaters within the field of view at this spot in one of the maria.
Use the 25mm eyepiece to pick a spot on the Moon somewhere within the lunar highlands.
Put in the 10mm eyepiece and get the telescope back in focus. Count and record the number of creaters within the field of view at this spot in one of the highlands.
Questions to answer outside of lab:
(a) Which area is younger, the maria or the highlands?
(b) If you assume that the older area is as old as the Moon itself (about 4.5 billion years), and that the cratering rate has been constant throughout the history of the Solar System, how old would you estimate the younger area to be?
(c) In fact, the cratering rate has been decreasing over the age of the Solar System; over time, there is less and less debris to make craters. Given this, is the age you estimated in question (b) too high or too low? Explain.