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Celestial Sphere Model: Moon, Sun, Earth defintions
definitions describing various events/locations within the Celestial Sphere Model
29
Astronomy
Undergraduate 1
09/23/2009

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Term
horizon
Definition
where sea meets sky
Term
zenith
Definition
point directly above observer’s head
Term
meridian
Definition
line from north horizon point through the zenith to south horizon point
Term
altitude
Definition
number of degrees above horizon
Term
azimuth
Definition
degrees east of north around the horizon
Term
circumpolar stars
Definition
around the pole stars, never set
Term
rise and set stars
Definition
spend some time below and above horizon
Term
never-seen stars
Definition
never rise above the horizon
Term
ecliptic
Definition
sun’s path through the stars
Term
ecliptic
Definition
being tilted relative to the celestial equator by 23.5 degrees
Term
obliquity
Definition
tilt of ecliptic
Term
summer solstice
Definition
point on C.S. furthest from the Celestial Equator towards NCP (June 20-21), sun shines directly over Tropic of Cancer
Term
winter solstice
Definition
point on C.S. furthest from Celestial Equator towards SCP (December 20-21), sun shines directly on Tropic of Capricorn
Term
vernal equinox
Definition
point where ecliptic crosses C. Eq, sun going from south to north (March 20th)
Term
autumnal equinox
Definition
point where ecliptic crosses C. Eq., sun going from north to south (September 21)
Term
seasonal visability
Definition
when the sun is in the same part of the sky as star, its brightness overwhelms mere stars
Term
zodiac
Definition
constellations that lie along the ecliptic
Term
causes of seasons
Definition
1. Days in the summer longer, shorter in the winter, hence, more time for the sun to heat the earth during the summer
2. In the summer, the sun is higher in the sky-sun light is more direct in the summer than in the winter
Term
synodic period
Definition
time for moon to pass through one complete set of phases-about 29.5 days
Term
sidereal month
Definition
time for the moon to return to the same place among the stars about 27.3 days
Term
synodic month
Definition
Moon’s rotation period
Term
penumbra
Definition
(lighter)-part of the shadow where some light enters
Term
umbra
Definition
(darker)-part of the shadow where no light enters
Term
lunar eclipse
Definition
-moon is in the shadow of Earth (only occurs at full moon) --everyone on night-side of Earth can see a lunar eclipse
Term
penumbral eclipse
Definition
moon passes only through Earth’s penumbra
Term
total eclipse
Definition
Moon passes through Earth’s umbra
Term
partial eclipse
Definition
moon is never completely within umbra
Term
solar eclipse
Definition
occurs at New Moon, when the moon’s shadow crosses the surface of the Earth (only visible to people within the path of the moon’s shadow)
Term
annular eclipse
Definition
when the moon eclipses the sun at its point farthest away from Earth.
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