4.9: Key Terms
- Page ID
- 66444
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- 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