Astronomy World

Planetary Events

by Hal

The position of a planet in its orbit relative to the Earth can significantly enhance it’s view from the Earth.


For planets inside the Earth’s orbit (Mercury and Venus), Elongations are important. Elongations occurs when the planet position in its orbital path is at tangent to the view from Earth. Because these inner planets are inside the Earth’s orbits their positions as viewed from the  Earth are never very far from the position of the Sun.  When a planet is at Elongation is it furthest from the Sun as viewed from Earth so it’s view is best at that point.

Elongation Example
On November 3rd, 2005, both Mercury and Venus were coincidentally at their Greatest Eastern Elongation.  The line-of-sight from Earth is tangent to the orbits or Mercury and Venus placing the planets position in the sky at the furthest position from the Sun.

There are two kinds of Elongations: The Eastern Elongation occurs when the planet is in the evening sky and the Western Elongation Occurs when a planet is in the morning sky.

Also calculate below is the angular distance from the Sun.  Since the sky moves at 15 degrees per hour, the angular distance can be used to calculate how much time after sunset or before sunrise the planet has before it set or rises.

For example, On February 23, 2006 Mercury is at it’s Greatest Western Elongation at 18° 8’ from the Sun – This means that Mercury will set about 1 hour and 12 minutes after the Sun.

Elongations for this year are:

Elongation Type
Angular Separation


For planets outside the Earth’s orbit (Mars, Jupiter, Saturn, Uranus, Neptune and Pluto), the months around Oppositions are the best time to view these. An Opposition occurs when the planet is opposite from the Sun relative to the Earth.  At Opposition the planet will rise as the Sun sets and will set as the Sun rises providing an entire night of observation. Also at Opposition the planet comes physically closest to the Earth in it’s orbit so it appears as large as possible.

Opposition Example

Oppositions for this year are:

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