AS101 LESSON 3
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29 Terms
Celestial sphere
Imaginary sphere around Earth where sky objects appear located
Celestial equator
Earth’s equator projected onto celestial sphere
Declination
Sky latitude measured north or south of celestial equator
Right ascension
Sky longitude measured eastward in hours
0h right ascension
Vernal (spring) equinox
Right ascension direction
Eastward
Declination of celestial equator
0°
Declination of north celestial pole
+90°
Solar day
Average time between successive Sun meridian crossings
Sidereal day
Time between successive star meridian crossings
Sidereal day length
23 h 56 m
Why sidereal day is shorter
Earth moves in orbit while rotating
Synodic month
Moon phase cycle relative to Sun
Synodic month length
29.5 days
Sidereal month
Moon cycle relative to stars
Sidereal month length
27.3 days
Sidereal year
Earth orbit relative to stars
Tropical year
Time between spring equinoxes
Sidereal year compared to tropical year
About 20 minutes longer
Precession
Slow wobble of Earth’s axis
Local meridian
Imaginary line through north point, zenith, south point
Noon
Sun crosses local meridian
a.m.
Before noon
p.m.
After noon
Time zones
Regions sharing same standard time
Who proposed time zones
Sandford Fleming
Egyptian calendar flaw
365-day year caused seasons to drift
Julian calendar
Added leap day every 4 years
Gregorian calendar rule
Century years leap only if divisible by 4