Greatest Elongation

by Dominic Ford, Editor
Last updated: 16 Jul 2020
Greatest Elongation

Venus. © NASA/Ricardo Nunes.

The inferior planets of the Solar System – Mercury and Venus – are those that orbit closer to the Sun than the Earth, as distinct from the superior planets, which orbit further out in the Solar System.

Because we look in on the orbits of the inferior planets from the outside, they never appear to venture very far from the Sun in the sky, whereas the superior planets can pass almost directly opposite to the Sun in the sky when at opposition.

As the inferior planets orbit the Sun, they pass alternately in front of and behind it – events which are termed inferior and superior conjunctions respectively. On such occasions they are almost completely unobservable because they are so close to the Sun in the sky that they are completely lost in its glare.

At other times, they set and rise either a few hours before, or a few hours after, the Sun, becoming visible for a few hours in either the morning or evening sky respectively.

They are visible for longest at those times when they are furthest away from the Sun in one or other direction; at such times, they are said to be at either greatest western elongation or greatest eastern elongation, depending whether they appear to the west or to the east of the Sun.

List of greatest elongations

The table below lists greatest elongations in 2021, computed from NASA's DE430 planetary ephemeris. To show events in other years, use the control below.

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Greatest elongations in 2021

Date Event Separation
from Sun
Date Event Separation
from Sun
23 Jan 2021 18:10 PSTMercury at greatest elongation east18°33'More information »
06 Mar 2021 01:58 PSTMercury at greatest elongation west27°16'More information »
17 May 2021 01:14 PDTMercury at greatest elongation east22°01'More information »
04 Jul 2021 21:11 PDTMercury at greatest elongation west21°32'More information »
13 Sep 2021 14:55 PDTMercury at greatest elongation east26°45'More information »
24 Oct 2021 20:45 PDTMercury at greatest elongation west18°23'More information »
29 Oct 2021 07:57 PDTVenus at greatest elongation east47°02'More information »






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