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Lunar Observations

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 lab.

Goals of the Lab

• To observe the Moon – an object so familiar as to be unremarkable – and to see how much interesting detail is available to a small telescope.

• To practice drawing complex features through the telescope (lunar terminator).

• To estimate the size of the Moon.

Requirements: the telescope, your logbook, moon map (available from the TAs)

Terminology: This lab will refer to a few parts of the Moon as it appears to us. The drawing below explains some of these terms.

• Earthshine: The portion of the Moon not illuminated by the sun is not always completely dark. Look at a half or crescent Moon with your naked eye; can you see the dark half? This is the result of reflected light from the earth lighting up the "dark" portion of the Moon.
• Highlands: The rougher, more cratered parts of the moon.
• Lowlands: The smoother parts of the moon including the Maria.
• Limb: the edge of the visible disk of the Moon.
• Maria: The "seas" of the moon are not really water, but they are the large smooth areas. They are indicated and named on the Moon Map which may be borrowed from the TAs during lab time.
• Near side: one side of the Moon always faces the Earth. That is the "near side". The only people to have seen the far side of the Moon are the Apollo astronauts who orbited the Moon.
• Terminator: the line on the Moon that delineates the portion illuminated by the Sun from the dark portion.

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Part I: The Moon at Large

Procedures

1. Center the Moon in the field of view of the 25mm eyepiece. Sketch the Moon as it appears in your field of view, using a generic observation template (either the small version or the large version). Make sure your drawing includes the phase, and any large features you can see.

2. On your drawing from step 1, identify all of the maria you can see. Use the Moon Map to figure out which is which. How does the color of the Moon's surface on the maria compare to the color of the Moon's surface at other places?

3. Size of the Moon: Estimate the angular size (in degrees, or in arcminutes) of the Moon. If the Moon is near full, or if the earthshine is bright enough that you can see the full disk of the Moon during another phase, do this using the Method of Transit Times with the 25mm eyepiece. If you cannot see the full disk of the Moon, then estimate it by measuring the size of the Moon and the field of view in your drawing of step 1. If your drawing is good, the fraction that the Moon occupies on your drawing should be the Moon's angular size divided by the field of view of the telescope (as measured during the Telescope Basics lab). Use the method of transit times if possible, as this will give you a more reliable measurement.

4. Calculation outside of lab: what is the physical size of the Moon? Knowing the distance to an object and the angular size of that object, you can figure out its size, using the small angle formula (see Measuring Angular Distances. The distance from the surface of the Earth to the Moon is about 380,000km. Use this and your measured angular size of the Moon to determine the physical diameter of the Moon. How does this compare to the size of the Earth? To the size of the USA?

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Part II: Lunar Features

1. The Lunar Terminator: (Note: this step will not be possible when the moon is completely full. It is easiest with a crescent or half moon, but still possible with a gibbous moon.) The terminator is often the most interesting place on the Moon to look at, as long as the Moon is not near full. Look at the lunar terminator, and compare the contrast you see in features there to the contrast you see near the center of the illuminated portion of the Moon.

   Sweep across the terminator using the 10mm eyepiece. Pick a portion of the terminator which looks interesting to you. Make a sketch of this portion of the terminator, including some of the high-contrast features (craters or mountains) on the "illuminated" side of the terminator.

   Indicate the location of the region you are drawing on your sketch from step 1 of Part I.

2. The Lunar Limb: Look at the lunar limb with the 10mm eyepiece. Sketch this portion of the limb. In particular, notice whether the limb is perfectly smooth, or jagged.

3. Questions to answer outside of lab:

   • (a) Explain why you see the contrast difference between the lunar terminator and the center of the illuminated part of the moon in Step 1.

   • (b) In Step 2, was the edge of a perfect circle, or can you detect some jaggedness? If you do detect jaggedness, can you explain why?

   • (c) (OPTIONAL) If you are very ambitious you may try to estimate the size of the features that make up any jaggedness you saw in Step 2 using the methods you've learned for estimating the size of things seen through your telescope.