Download Motions of the Planets

Alignments and Motions of the Planets
The positions and motions of the five planets known in antiquity fascinated and
mystified many ancient societies. Models of the “universe” were constructed to
explain these phenomena:

All of the planets (and the Moon) orbit within the ecliptic plane, therefore
they appear to move along similar “tracks” on the sky. The Sun, Moon, and
planets move amongst the constellations of the zodiac.

Mercury & Venus, known as the inferior planets, orbit inside the Earth’s
orbit. They are always visible near the Sun on the sky.

Mercury’s maximum separation from the Sun on the sky, known as greatest
elongation is 27o; for Venus it is 46o.

Mercury and Venus are colloquially referred to as morning or evening “stars”.
Both planets appear at dusk or dawn, when they can be easily seen against
the dim sky. During the day, Venus is visible, if you know exactly where to
look.

Mars, Jupiter & Saturn, termed the superior planets, orbit outside of Earth’s
orbit (further from the Sun).

Superior planets can appear with any separation from the Sun up to 180 o.

A planet is said to be “in conjunction with the Sun” when it appears in the
same direction as the Sun on the sky.

A planet is said to be “in opposition with the Sun” when it appears directly
opposite the Sun (180 o or 12 hrs of right ascension) on the sky.

Only the superior planets can ever appear opposite the Sun on the sky.

All of these alignments result from Earth’s position in the Solar System as the
THIRD planet from the Sun.

The remaining outer planets, Uranus, Neptune, and Pluto, were unknown
prior to the invention of the telescope.