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.
Greatest elongations in 2021
|23 Jan 2021 18:10 PST||Mercury at greatest elongation east||18°33'||More information »|
|06 Mar 2021 01:58 PST||Mercury at greatest elongation west||27°16'||More information »|
|17 May 2021 01:14 PDT||Mercury at greatest elongation east||22°01'||More information »|
|04 Jul 2021 21:11 PDT||Mercury at greatest elongation west||21°32'||More information »|
|13 Sep 2021 14:55 PDT||Mercury at greatest elongation east||26°45'||More information »|
|24 Oct 2021 20:45 PDT||Mercury at greatest elongation west||18°23'||More information »|
|29 Oct 2021 07:57 PDT||Venus at greatest elongation east||47°02'||More information »|