The Moon's orbital plane is inclined by about 5.1° with respect to the ecliptic plane, whereas the Moon's equatorial plane is tilted by only 1.5°.


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The picture shown below holds the key to the answer. The maximum elevation angle the Sun can make is the sum of the Moon's own equatorial tilt (1.5°) and the angle formed by the ecliptic plane and the Moon as viewed from the Sun. As shown in the diagram, the Moon makes a 5.1° angle with the ecliptic when viewed from the Earth, for the Moon is directly orbiting the Earth. Because the Sun is so much farther away, the ecliptic angle viewed from the Sun is correspondingly smaller, only about 0.01°, and so the Sun's maximum elevation angle at the lunar poles is essentially the 1.5° tilt angle (the 0.01° figure is not a significant digits given the rounded values of the angles).

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This low angle has some physical significance. Because many of the crater walls on the Moon slope with much larger angles than that 1.5°, the craters near the poles contain shadows within where the Sun is too low for its light (and heat) to get past the rim. Where these shadows reach the bottom, the bottom is permanently dark and cold, and scientists believe that water ice may be embedded in such shadowed places (crater bottoms near the poles). The planet Mercury presents a similar situation and despite its being close to the Sun, it could harbor water ice in crater bottoms near the poles too.


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