Unit 1 Vocab Flashcards
(53 cards)
astronomical unit
The average distance between the Earth and the Sun (=1.5 x 108 km).
light-year
The distance light travels in one year.(=9.46 x 1012 km)
precession
The slow wobble of the Earth on its rotation axis
synodic day
The rotation period of the Earth measured relative to the Sun.
sidereal day
The rotation period of the Earth measured relative to the stars
celestial pole
The place in the sky that the Earth’s axis points toward (can be either north or south).
celestial sphere
The imaginary sphere centered on the Earth that holds the stars
celestial equator
The projection of the Earth’s equator onto the celestial sphere
summer solstice
When the Sun is farthest north of the celestial equator (about June 22)
autumnal equinox
When the Sun moves from north to south across the celestial equator (about September 23).
winter solstice
When the Sun is farthest south of the celestial equator (about
December 22).
vernal equinox
When the Sun moves from south to north across the celestial equator (about March 21).
ecliptic
The apparent path of the Sun through the stars on the celestial sphere
zenith
The point directly overhead
meridian
The north-south line passing directly overhead through the zenith.
total eclipse
When the Moon entirely blocks the Sun
annular eclipse
An eclipse of the Sun that occurs when the angular diameter of the Moon is less than the angular diameter of the Sun.
umbra
The shadow area behind the Earth or Moon where the Sun is completely obscured
penumbra
The shadow area behind the Earth or Moon where the Sun is partially obscured.
geocentric
Centered on the Earth
direct motion
The normal eastward movement of a planet against the background of distant stars
retrograde motion
The apparent backward motion of a planet against the
background of stars
epicycle
In Ptolemy’s geocentric solar system, the small circle on which a planet moved
deferent
In Ptolemy’s geocentric solar system, the large circle on which a planet’s epicycle moved around the Earth
