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Moon Mapping: What Changes, If Any, Occur in the Moon's Apparent Shape From Day to Day?

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Author: Janice VanCleave

Problem

What changes, if any, occur in the Moon's apparent shape from day to day?

Materials

• calendar with times of moonrise and moonset (if available)
• drawing compass
• ruler
• 5 sheets of typing paper
• pencil
• clipboard

Procedure

Prior to starting your Moon observations, study a calendar with the times of moonrise (when the Moon rises above the eastern horizon) and moonset (when the Moon sets or sinks below the western horizon) for the 29 dates of your observation. (The horizon is an imaginary line where the sky seems to meet Earth.) If a calendar is not available, use the table below to determine approximately when the Moon will rise and set each day. The times on the table vary during different seasons of the year. Newspapers and television weather reports are also a source of moonrise and moonset times.

• Use the compass and ruler to draw six circles with 3-inch (7.5-cm) diameters on 4 of the sheets of paper and five circles on the last sheet. Each circle will represent the Moon.
• Date the circles, starting with the date of day 1 and ending with the date of day 29.
• On day 1, observe the Moon and use the pencil to shade the portion of the first circle on the paper to represent the portion of the Moon that is not lighted by the Sun and therefore not visible. NOTE: Make no observation for at least 3 days before and after new moon—when the side of the Moon facing Earth is dark. The new moon is close to the Sun and you could damage your eyes if you look at it.
• Repeat steps 3 and 4 each day for as many days as possible. If the weather does not permit, or you are unable to make an observation for another reason, leave the circles for those dates empty.
• On day 29, after all the drawings are completed, study them to determine if the Moon's apparent shape changed from day to day.

Results

The apparent shape of the Moon changed from day to day. The changes are called phases of the Moon or Moon phases.

Why?

Generally half of the Moon is lighted by the Sun, but to observers on Earth, all of the lighted side is not always visible.

The relative position of the Sun and the Moon changes daily. The farther the Moon is from the Sun in the Moon's orbit around Earth, the more of the lighted side we see. When the Moon is opposite the Sun with Earth in between, the lighted side faces the Earth. This is called full moon. When the Moon is between Earth and the Sun, the lighted side faces away from the Earth. This is called new moon.

The rotation of Earth on its axis causes the Sun and the Moon generally to rise in the east and set in the west. But while the time of sunrise varies by only about 1 minute each day, moon-rise is about 50 minutes later each day. This delay in moonrise is due to the eastward rotation of the Moon around the Earth. The new moon rises with the Sun. But the next day, the Moon rises about 50 minutes after the Sun rises. The day after that, it rises 50 minutes later, and so on. About 1 week after new moon, the first quarter moon rises at noon. About 2 weeks after new moon, the full moon rises at sunset, and about 3 weeks after new moon, the third quarter moon rises at midnight. Finally, about 29 days after new moon, the new moon again rises at sunrise.

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