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Lunar Observations

Goals of the Lab

• To observe and study the details of the Moon.

• To estimate the angular size of the Moon.

Requirements: the telescope, a calculator, observation templates (either the small version or the large version) and a moon map (available in lab).

Background

In order to accurately describe and discuss the moon's features, you need to be familiar with some technical terms. The drawing below will help put some of these terms defined below into context.

• Earthshine: Earthshine is reflected Earthlight visible on the Moon's night side.
• Highlands: The Highlands are the rougher, more cratered parts of the moon.
• Lowlands: The Lowlands are the smoother parts of the moon including the Maria.
• Limb: The limb is the edge of the visible disk of the Moon.
• Maria: The "seas" of the moon are not water filled areas; they are large smooth areas.
• Near side: Since the same side of the Moon always faces the Earth, we can only observe features on the "near side". The only people to have observed the "far side" of the Moon are the Apollo astronauts who orbited the Moon.
• Terminator: The terminator is 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

Note:All observations should include N/S and E/W, the time, date, and weather conditions.

1. Center the Moon in the 25mm eyepiece. Sketch the Moon using an observation template. Your drawing should include the moon's phase and any large features you can see.

2. Use the Moon Map to identify and label the maria in your sketch.

3. Question: How does the color of the Moon's surface on the maria compare to the color of the Moon's surface at other places?

4. Size of the Moon: Estimate the angular size (in degrees, or in arc minutes) 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, estimate the angular size 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.

5. Question: Calculate the diameter of the Moon. Knowing the distance to an object and the angular size of that object, you can determine its size. Use 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 diameter of the Earth (12,756 km)? To the size of the USA?

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Part II: The Lunar Terminator and the Lunar Limb

(Note: Skip this step when the moon is completely full, as the terminator is not visible. It is most easily observed during a crescent or half moon, but still possible with a gibbous moon. Your TA's will let you know if this section is possible on any given night.)

1. Locate the lunar terminator in the 25mm eyepiece. Study the features near the terminator; compare the contrast you see near the terminator to the contrast observed near the center of the illuminated portion of the Moon.

   Question: Is it easier to see the details of a crater close to the terminator, or in a fully illuminated part of the moon? Give a possible reason for this phenomenon.

2. Sweep across the terminator using the 10mm eyepiece. Choose a portion of the terminator that looks interesting. Sketch this portion of the terminator, including some 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 Part I.

3. Look at the lunar limb with the 10mm eyepiece. Sketch this portion of the limb.

4. Question: Briefly describe the appearance of the lunar limb. Is it smooth, or jagged? Explain why it appears this way.