MOTIONS OF THE SUN, MOON, STARS AND PLANETS

MOTIONS OF EARTH AND SUN

1. daily rising and setting, due to the rotation of the Earth on its axis. Period of rotation = 1 day.

stars appear to move in circles around the celestial pole, as if attached to a celestial sphere. Circumpolar stars near the celestial pole never rise or set.
Position of objects in the sky is specified by altitude (angle above the horizon) and azimuth (angle from North).
Position of objects on celestial sphere is specified by declination (analogous to latitude) and right ascension (analogous to longitude)

2. slow drift of the Sun, Moon and planets with respect to the stars, caused by the revolution of the Earth about the Sun. period of revolution = 1 year or approximately 365 ¹/₄ days

solar day vs. sidereal day
different constellations visible at night at different times of year.

Seasons

The seasons are caused by the angle between tilt of Earth's axis and the Earth's orbit. (NOT because the Earth gets closer or further from the Sun!)

path of the Sun across the celestial sphere is called the ecliptic (where the plane of the Earth's orbit intersects the celestial sphere)
when it is summer in the northern hemisphere, it is winter in the southern hemisphere, and v.v.
longest day of summer = summer solstice, shortest day of winter = winter solstice, and day and night have equal length at vernal or autumnal equinox.

Precession of the equinoxes

Direction of Earth's north pole varies slowly with time, making a complete circle in 26,000 years.

At present Sun is in Pisces, not Aries, at the vernal equinox; soon vernal equinox will be in Aquarius

MOON

The Moon revolves around the Earth. period of revolution = 1 month or approximately 29 ¹/₂ days

Moon shines not by its own light but by reflected light of Sun; has different phases, depending on how much of the lit half faces Earth

Eclipses

A solar eclipse occurs when the Moon comes between the Earth and the Sun and casts its shadow on the Earth. Solar eclipses can be:

TYPE OF ECLIPSE	RELATIVE POSITION OF MOON AND SUN	PART OF SHADOW
total	Moon completely covers Sun	umbra (region of total shadow)
partial	Moon only partly covers Sun	penumbra (region of partial shadow)
annular	a ring of Sun appears around Moon	umbra does not touch Earth

Total solar eclipses are only visible from small area on Earth because apparent angular size of Moon is very close to angular size of Sun. Sun, Moon and observer must be in very nearly perfect straight line for Moon to cover Sun completely.
Annular eclipse occurs when Moon is little further from the Earth, and looks smaller.
there aren't eclipses every month because Moon's orbit is not in same plane as Earth's.

A lunar eclipse occurs when the Earth casts its shadow on the Moon.

Lunar eclipses are more common than solar because Earth is much bigger than the Moon, so alignment does not have to be so perfect for shadow to fall on Moon
Even during total eclipse, the Moon appears reddish because of light bent by the Earth's atmosphere
Aristotle observes Earth is round from shape of its shadow on Moon.

THE OTHER PLANETS.

Revolve around the Sun; plane of their orbits not same as the plane of Earth's orbit.

3. retrograde motion occurs when the Earth catches up with a slower moving planet and passes it by: it appears temporarily to be moving backwards

4. small N-S motion with respect to plane of the ecliptic

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