Lunar Craters
Goals of the Lab
To identify and study lunar craters.
To use lunar craters to learn about the moon's history.
Requirements: a calculator, observation templates, and moon map (available in lab)
Reference the Lunar Observations lab for terminology required to describe the moon and its features.
Part I: Examining individual Craters
Note: All observations should include N/S and E/W, the time, date, and weather conditions.
Craters with Mountains: Use the Moon Map to identify one of the following craters in the 25mm eyepiece: Agrippa, Delambre, Eratosthenes, Langrenus, Theophilus, or Tycho. Examine this crater and the mountain in the center of the crater with the 10mm eyepiece. Sketch the crater in as much detail as possible on an observation template.
Craters with Rays: Identify one of the following craters with the 25mm eyepiece: Aristillus, Copernicus, Langrenus, Kepler, Pickering or Tycho. Choose a different crater than the one you used in step 1. Examine the crater with the 10mm eyepiece. Notice the bright "rays" radiating from the craters; these rays are brighter near the full Moon. Sketch the crater and its rays.
Crater Size: Pick one of the two craters you already sketched, 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 those near the limb. Describe any differences you notice between craters in the two locations.
Draw the outline 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 way is it oriented.) 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:
(a) Use the small angle formula (described in How to Measure Angular Distances), to estimate the physical size of the crater you measured in Step 3. The distance to the Moon is about 380,000 km. How does this compare to the size of the USA? To the size of Tennessee (710 km)?
(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
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 as a result of lava flows throughout history filling in craters and smoothing out the parts of the Moon's surface. Consequently, visible craters on these parts must be a result of impacts occurring after the lava flow. This means that regions with fewer craters represent younger regions of the Moon's surface.
Use the 25mm eyepiece to pick a spot on the Moon somewhere within one of the Maria.
Look at this area with the 10mm eyepiece. Count and record the number of craters in this maria.
Use the 25mm eyepiece to pick a spot on the Moon somewhere within the lunar highlands.
Look around the spot you've chosen with the 10mm eyepiece. Look at an area that is the same size as the area of the maria in which you counted craters in questions #2. Count and record the number of craters in this highland region.
Questions:
(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.