THE EARTH'S ORBIT AND SEASONS

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.

PRECESSION OF THE EQUINOXES

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.

RETROGRADE MOTION OF THE PLANETS

The Observation:

as seen from earth, the planets occasionally seem to reverse their motion, moving backwards for a while before continuing on in the original direction.

Ptolemy's explanation:

Each planet moves on a small circle called an epicycle, which in turn moves around the earth on a big circle. In more advanced versions of the theory, the earth is not at the center of the circle. By appropriately arranging the speeds of the motions and the sizes of the circles, retrograde motion can be simulated.

Modern explanation (since Copernicus)

The earth and all the other planets move in orbits around the sun. Planets closest to the sun complete their orbits in the least time. We observe the other planets from the moving earth, and retrograde motion appears as a natural consequence.

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.