Comets 彗星 are loose collections of ice, dust, and small rocky particles in the Solar System that orbit the Sun and, when close enough to the Sun, exhibits a visible coma (or atmosphere) and/or a tail — both primarily from the effects of solar radiation upon the comet's nucleus. The nucleus itself measures a few kilometres or tens of kilometres across, and is composed mostly of rock, dust and ice. 月蚀 A lunar eclipse occurs whenever the Moon passes through some portion of the Earth’s shadow (the moon gets behind the earth or the sun, earth, and then the moon). This can occur only when the Sun, Earth, and Moon are aligned exactly, or very closely so, with the Earth in the middle. Hence, the Moon is always full near a lunar eclipse. The type and length of an eclipse depend upon the Moon's location relative to its orbital nodes
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