Chapter 3

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 earth's equator

Circumpolar stars: always above horizon.

Which stars depends on location

Annual Motions

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


Perihelion: Earth closest to sun (January)

Aphelion: Earth farthest from sun (July)

The Moon

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