The diagram shows a simplified view of the moon's orbit around the earth, seen from above the earth's North Pole, with the sun far off screen to the left. The numbered images at right show the appearance of the moon when it is at the corresponding positions on its orbit.
This diagram is very useful for understanding and memorizing the relationship between the moon's orbital geometry and its phases. But keep in mind that there is no particular reason why we need to draw the sun's light coming in from the left — this is an arbitrary choice, and in fact some diagrams do it differently. The only important thing is the angle between the sun, earth, and moon. When the moon is between the earth and sun it will always be new, regardless of the diagram's orientation. Turn the monitor upside down and the diagram is still true.
The geometry of the earth-moon system also allows us to make various conclusions based on the moon's phase. There is a concrete connection between (A) the moon's phase, (B) the time, and (C) the moon's location in the sky. In fact, given any two of those details, it is always possible to determine the third. Consider the following situation:
Suppose Sherlock Holmes is investigating a crime that took place at 3 AM, away from street lamps and any other artificial light sources. A witness claims he recognized the perpetrator in the light cast by a first quarter moon. Should the detective believe him?
Before we can answer this question we need to look into the details of telling time and describing the locations of objects in the sky.
The diagram shows a simplified view of the moon's orbit around the earth, seen from above the earth's North Pole, with the sun far off screen to the left. The numbered images at right show the appearance of the moon when it is at the corresponding positions on its orbit.This diagram is very useful for understanding and memorizing the relationship between the moon's orbital geometry and its phases. But keep in mind that there is no particular reason why we need to draw the sun's light coming in from the left — this is an arbitrary choice, and in fact some diagrams do it differently. The only important thing is the angle between the sun, earth, and moon. When the moon is between the earth and sun it will always be new, regardless of the diagram's orientation. Turn the monitor upside down and the diagram is still true.The geometry of the earth-moon system also allows us to make various conclusions based on the moon's phase. There is a concrete connection between (A) the moon's phase, (B) the time, and (C) the moon's location in the sky. In fact, given any two of those details, it is always possible to determine the third. Consider the following situation:Suppose Sherlock Holmes is investigating a crime that took place at 3 AM, away from street lamps and any other artificial light sources. A witness claims he recognized the perpetrator in the light cast by a first quarter moon. Should the detective believe him?Before we can answer this question we need to look into the details of telling time and describing the locations of objects in the sky.
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