Earth, Moon, and Sky

 

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