The Moving Sky
The Celestial Sphere: a convenient model
Diurnal motion - apparent daily motion of the
sun, moon, stars.
Due to earth's rotation west to east on its axis
North and South Celestial Poles
Polaris - currently closest visible star to
North Celestial Pole.
Celestial equator - circle on the sky over
Circumpolar stars: always above horizon.
Which stars depends on location
See stars opposite sun on celestial sphere
Earth moves around sun in one year
Different stars at different times of year
Apparent motion of sun against stars due to earth's motion (30 deg per month)
Ecliptic - sun's apparent annual path on sky
Earth's axis tilted 23.4 deg from perpendicular to plane of orbit
Celestial equator tipped 23.4 deg to plane of orbit
Sun appears north and south of equator on sky - crosses at vernal and autumnal equinoxes.
Summer solstice - farthest north of equator
Winter solstice - farthest south of equator
Zodiac - 18 deg band around sky, centered on ecliptic - moon and naked-eye planets
Precession: change in direction of earth's axis of rotation - 26,000 years
Location of equinoxes moves westward on sky - ~ 30 deg in 2000 years
Co-ordinate System on the Sky
Longitude and latitude on earth
Declination - angle between celestial equator and star, north (+) or south (-)
Hour circle - circle on sky passing through both celestial poles and star
Right ascension - angle measured eastward between hour circles of star and vernal equinox
Change slightly due to precession
Altitude of celestial pole equals observer's latitude
Caused by inclination of earth's axis from perpendicular to plane of orbit
More direct sunlight in summer than winter
NOT DUE TO DISTANCE OF EARTH FROM SUN
Perihelion: Earth closest to sun (January)
Aphelion: Earth farthest from sun (July)
Sidereal period - motion with respect to stars: 27.3 days.
Sidereal period equals rotation period, always see same side of moon from earth
Moon visible by reflected sunlight
Half of moon always illuminated
Phases depend on relative location of earth, moon, sun
Waxing - increasing
Waning - decreasing
Synodic period - phases repeat - 29.5 days
Local meridian - circle on sky passing through celestial poles and zenith
Sidereal day - interval of time between any star ( or vernal equinox) crossing local meridian twice successively. Equals earth's rotation period.
Apparent solar day - time interval between sun crossing local meridian twice successively. Varies slightly.
Mean solar day - length of solar day averaged over the year
Solar and sidereal days not equal. Sidereal day 4 minutes shorter
Varying lengths of year as measured by sun, stars,
lunar months has caused problems with the calendar
Prof. Donna Weistrop
University of Nevada, Las Vegas