Explain the phrases of a strip of sky.
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Answer:
The Moon is the most conspicuous of the denizens of the night sky, and for several nights each month completely dominates the sky, so it always was of major interest to ancient astronomers. Its motion from day-to-day against the star background is also faster (about 13 degrees per day) than those of the Sun or planets and can be followed easily without the need for complicated instruments.
Because it orbits the Earth rather than the Sun and is very nearby by planetary standards, the Moon is associated with a number of unique phenomena, some of which are quite dramatic.
The lunar phases are the most obvious of the important cyclic phenomena associated with the Moon. Others are eclipses (two types), changing positions of north/south maximum distances from the celestial equator, and polar precession, all of which we discuss here, and the tides, which are covered in Study Guide 13.
All ancient cultures must have been aware of the phases and eclipses. (Precession is too subtle to detect without careful observations, and tides are only obvious along coastlines.) Although the Greeks were the first to arrive at the explanation of them (around 500 BC), others left traces of their fascination with them.
A. Lunar Phases
Unlike the planets and stars, the Moon is perceptibly extended on the sky to the unaided eye. Throughout a given month, the Moon exhibits drastic changes in apparent shape, from crescent to round and back. The shapes are called phases of the Moon. Overall lunar brightness changes in proportion to the bright area. .
A montage of photographs of the lunar phases during a month is shown at the right. Click for an enlargement.
Other than the apparent daily and annual motions of the Sun, the lunar phases are the most dramatic of the cycles visible in the sky. The Moon repeats its phases after a period of 29.5 days.