Download The Motion of Planets

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
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.
The Zodiac
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.
Retrograde Motion
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
The greatest
angular
separation of an
inferior planet
(Venus 450 or
Mercury 280) from
the Sun.
Conjunction
When 2 or more planets
appear close to each other
in the sky. Here the two
inferior planets are in
conjunction.
Opposition
Transit
Occultation
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.