We may represent the earth's orbit around the sun as a circle.
Since the earth is "tilted" as it journeys around the sun, the
pole which is tilted toward the sun has summer while the pole
which is tilted away has winter.
SUMMER IN NORTHERN HEMISPHERE (point A)
SUMMER IN SOUTHERN HEMISPHERE (point c)
Days are longer in summer than in winter. At the "equinoxes" day and night have equal length. These occur on approximately March 21 and September 23. (points B and D) The position of the sun at sunset changes too.
The Earth is not spherical, but has a bulge at the equator. The Moon and Sun pull on the bulge. Sun and moon lie approximately in this plane, called the ecliptic. The result is a motion of the earth that is like the motion of a top: it is called precession. The rotation axis moves around a circle. The period of this motion, ie the time to complete one circle, is about 26,000 years. To an observer on the earth, it looks as if the celestial sphere is moving: for example, the pole star changes. Our present pole star, Polaris, will not be at the North pole a few thousand years in the future. Notice that precession causes the celestial equator to move relative to the ecliptic. Precession causes different constellations to appear on the horizon at the equinox --as the constellation changes a new "world age" dawns, e.g.
AGE CONSTELLATION ON THE HORIZON
Golden age Gemini, Sagittarius
Silver age Taurus, Scorpio
Copper age Aries
Iron age Pisces
World ages last about 2000 years.
The diagram below shows retrograde motion of an outer planet (i.e.
one farther from
the sun than the earth). At times labelled 1, 2, etc. the dots
show the positions of both planets in their respective orbits.
The distance between successive dots is greater for the inner planet
because its speed is greater. Sight lines from the inner plant to the
outer planet show the apparent path of the outer planet, viewed from
the inner, against distant stars. The retrograde motion is
visible between times 5 and 7.