Four Kinds of Motion that
Cause Changes in the Sky
1) Earth's Rotation (spin)
- counterclockwise as seen from above the Earth's North Pole
- 24 hour period
- causes: the "celestial sphere" (an imaginary sphere around the Earth to which the stars appear to be attached) to rotate around us from East to West with a period of 24 hours, thus causing things like the Sun and Moon to "rise" and "set"
- celestial poles: points directly over Earth.s North and South poles
- celestial equator: a great circle (one that divides a sphere into two equal hemispheres) on the celestial sphere directly above Earth's equator.
2) Earth's Revolution (orbit) Around the Sun
- counterclockwise as seen from above the Sun's (or Earth's) North Pole
- one year period (that's how a year is defined!)
- causes: the Sun to appear to move from West to East against the background of the stars during the course of a year
- causes: what stars are seen at a given time of night (midnight, for example) to change gradually as the year goes on
- the Sun's path through the constellations is called the ecliptic. It takes a full year for the Sun to go all the way around.
3) The Revolution (orbit) of the Moon Around the Earth
- counterclockwise as seen from the North
- takes about a month to go all the way around
- causes: the Moon to move in the sky from West to East against the background of the stars by about 12 degrees per day
- since the Moon shines only by reflected sunlight, this causes the Moon to change its "phase" (which we'll cover in Chapter 3)
4) The Other Planets' Orbital Motion Around the Sun
- all go counterclockwise as seen from the North
- takes longer for planets farther from the Sun (see Kepler's third law)
- causes: (along with Earth's motion -- we're on a moving platform!) complicated motion of the planets through the constellations as seen from Earth: usually from West to East along the celestial sphere, but sometimes from East to West, or "retrograde".
- each planet will appear (from Earth) to "go backward" (retrograde) for a few weeks whenever we catch up with it in orbit or it catches up with us in orbit
Note the following two definitions of rotation and revolution as they are used in Astronomy:
Rotation means spin on an axis. For example, the Earth's rotation period is 24 hours.
Revolution means transverse (orbital) motion around another body. For example, the Earth's revolution period around the Sun is one year.