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Astronomy

Key Terms

AstronomyKey Terms
apparent solar time
time as measured by the position of the Sun in the sky (the time that would be indicated by a sundial)
declination
the angular distance north or south of the celestial equator
great circle
a circle on the surface of a sphere that is the curve of intersection of the sphere with a plane passing through its center
International Date Line
an arbitrary line on the surface of Earth near longitude 180° across which the date changes by one day
lunar eclipse
an eclipse of the Moon, in which the Moon moves into the shadow of Earth; lunar eclipses can occur only at the time of full moon
mean solar time
time based on the rotation of Earth; mean solar time passes at a constant rate, unlike apparent solar time
meridian
a great circle on the terrestrial or celestial sphere that passes through the poles
phases of the Moon
the different appearance of light and dark on the Moon as seen from Earth during its monthly cycle, from new moon to full moon and back to new moon
right ascension
the coordinate for measuring the east-west positions of celestial bodies; the angle measured eastward along the celestial equator from the vernal equinox to the hour circle passing through a body
sidereal day
Earth’s rotation period as defined by the positions of the stars in the sky; the time between successive passages of the same star through the meridian
sidereal month
the period of the Moon’s revolution about Earth measured with respect to the stars
solar day
Earth’s rotation period as defined by the position of the Sun in the sky; the time between successive passages of the Sun through the meridian
solar eclipse
an eclipse of the Sun by the Moon, caused by the passage of the Moon in front of the Sun; solar eclipses can occur only at the time of the new moon
solar month
the time interval in which the phases repeat—say, from full to full phase
synchronous rotation
when a body (for example, the Moon) rotates at the same rate that it revolves around another body
tides
alternate rising and falling of sea level caused by the difference in the strength of the Moon’s gravitational pull on different parts of Earth
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