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annular eclipse  A solar eclipse in which the Moon is too far from the Earth to block the entire Sun from view and a thin ring of sunlight appears around the Moon.
ascending node  The point in the Moon’s orbit where it crosses the ecliptic from south to north.
Baily’s beads  Points of light around the limb of the Moon just before and just after a total eclipse of the Sun. Baily’s beads are caused by sunlight shining through valleys on the Moon’s limb.
binary accretion theory  A theory of the origin of the Moon that holds that the Moon and the Earth formed at about the same time out of the same swarm or cloud of material.
capture theory  The theory of the origin of the Moon that holds that the Moon formed elsewhere in the solar system and then was captured into orbit about the Earth.
corona  (A) The outermost layer of the Sun’s atmosphere. Gases in the corona are tenuous and hot. (B) A circular feature on the surface of Venus. Coronae appear to be collapsed volcanic domes and can be as much as several hundred kilometers across. (C) A type of surface feature of Uranus’s satellite Miranda. Coronae consist of parallel ridges and troughs producing a striped appearance. Coronaehave sharp boundaries.
crater density  The number of craters of a given size per unit area of the surface of a solar system body.
crater saturation  The maximum crater density a solar system body can have. Once saturation is reached, new craters can only be produced by eradicating old ones.
descending node  The point in the Moon’s orbit where it crosses the ecliptic from north to south.
diamond ring  The last of Baily’s beads, which seems to shine with special brilliance just before a solar eclipse becomes total.
eclipse season  The times, separated by about 51⁄2 months, when eclipses of the Sun and Moon are possible.
eclipse track  The path of the Moon’s shadow across the Earth duringa solar eclipse.
eclipse year  The interval of time (346.6 days) from one passage of the Sun through a node of the Moon’s orbit to the next passage through the same node.
fission theory  A theory for the origin of the Moon in which the Moon consists of matter that was flung from the primitive Earth because of the Earth’s rapid rotation.
giant impact theory  The theory of the origin of the Moon that holds that the Moon formed from debris blasted into orbit when the Earth was struck by a Mars-sized body.
line of nodes  The line connecting the two nodes of the Moon’s orbit around the Earth.
maria  A dark, smooth region on the Moon formed by flows of basaltic lava.
mascon  A concentration of mass below the surface of the Moon that slightly alters the orbit of a spacecraft orbiting the Moon.
penumbra  The outer part of the shadow of a body where sunlight is partially blocked by the body. perihelion The point in the orbit of a body when it is closest to theSun.
rays  Long, narrow light streaks on the Moon and other bodies that radiate from relatively young craters. Rays consist of material ejected from a crater at the time it was formed by an impact.
regolith  The surface layer of dust and fragmented rock, caused by meteoritic impacts, on a planet, satellite, or asteroid.
rille  A lunar valley, probably the result of volcanic activity.
saros  The length of time between one member of a series of similar eclipses and the next (65851⁄3 days).
sinuous rille  A winding lunar valley possibly caused by the collapse of a lava tube.
synchronous rotation  Rotation for which the period of rotation is equal to the period of revolution. An example of synchronous rotation is the Moon, for which the period of rotation and the period of revolution about the Earth are both 1 month.
terrae  The light-colored, ancient, heavily cratered portions of the surface of the Moon.
umbra  (A) The inner portion of the shadow of a body, within which sunlight is completely blocked. (B) The dark central portion of a sunspot.

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