The Motion of Planets

demonstrate an understanding that the planets appear to move within a band called the Zodiac
demonstrate an understanding of the direct and retrograde motion of planets on a star chart
demonstrate an understanding of the terms: perihelion, aphelion, greatest elongation, conjunction, opposition, transit and occultation

Here is an animation of the first 4 planets in orbit around the Sun. The distances from the Sun and orbit times are to scale.

The Earth's orbit around the Sun, like other planet's orbits, is not a perfect circle. It is an ellipse. This means that the Earth is closer to and further away from the Sun at different times in the year.

Perihelion - when the Earth is closest to the Sun (usually happens in January)

Aphelion - when the Earth is furthest from the Sun (usually happens in June)

The diagram below is definitely not to scale. There is only about a 3% difference between perihelion and aphelion for the Earth's orbit.

All the planets, except Pluto, orbit the Sun in more or less the same plane called the ecliptic plane, so viewed from Earth they appear to move in a band, close to the ecliptic, called the zodiac. There are 12 constellations in the zodiac. Some people believe that the position of the Sun, Moon and planets in the zodiac has some affect on our characters and destinies. I don't and neither should you.

Viewed from Earth Mars appears here to stop (stationary point), move backwards (retrograde motion), stop again (another stationary point) and then continue normally. Of course Mars doesn't move backwards it just appears to when the Earth and Mars are in certain positions.

There are a number of other terms which we need to know. (in the pictures below the Sun is the big yellow one and the Earth is blue)

Greatest Elongation	Conjunction	Opposition	Transit	Occultation

The greatest angular separation of an inferior planet (Venus 45⁰ or Mercury 28⁰) from the Sun.	When 2 or more planets appear close to each other in the sky. Here the two inferior planets are in conjunction.	When a superior planet (Mars, Jupiter, Saturn etc) lies directly on the other side of the Earth to the Sun.	When a smaller body passes in front of a larger one. Mercury and Venus may, at times, pass in front of the Sun	When a smaller body passes behind a larger body and so is hidden, e.g. when a planet passes behind the Sun.