© NASA/Hubble Space Telescope

Mars enters retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Mars
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Mars will enter retrograde motion, halting its usual eastward movement through the constellations, and turning to move westwards instead. This reversal of direction is a phenomenon that all the solar system's outer planets periodically undergo, a few months before they reach opposition.

The retrograde motion is caused by the Earth's own motion around the Sun. As the Earth circles the Sun, our perspective changes, and this causes the apparent positions of objects to move from side-to-side in the sky with a one-year period. This nodding motion is super-imposed on the planet's long-term eastward motion through the constellations.

The diagram below illustrates this. The grey dashed arrow shows the Earth's sight-line to the planet, and the diagram on the right shows the planet's apparently movement across the sky as seen from the Earth:

The retrograde motion of a planet in the outer solar system. Not drawn to scale.

2012 apparition of Mars

23 Jan 2012 – Mars enters retrograde motion
03 Mar 2012 – Mars at opposition
05 Mar 2012 – Mars at perigee
13 Apr 2012 – Mars ends retrograde motion

Observing Mars

Mars enters retrograde motion as its 2012 apparition gets underway, although it has already been visible for some weeks in the pre-dawn sky.

Its celestial coordinates as it enters retrograde motion will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
Mars 11h39m40s 6°07'N Virgo -0.3 11.0"

The coordinates above are given in J2000.0.

From Fairfield , it will be visible in the morning sky, becoming accessible around 21:54, when it reaches an altitude of 9° above your eastern horizon. It will then reach its highest point in the sky at 03:25, 54° above your southern horizon. It will be lost to dawn twilight around 06:47, 32° above your south-western horizon.

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Over the following weeks, Mars will reach its highest point in the sky four minutes earlier each night, gradually becoming visible in the evening sky, as well as the pre-dawn sky, as it approaches opposition.

The panels below show the month-by-month change in Mars' apparent size in coming weeks:

28 Nov 2011
26 Dec 2011
23 Jan 2012
20 Feb 2012
19 Mar 2012

The table below lists Mars' angular size at brightness at two-week intervals throughout its apparition:

Date Angular size Mag
14 Nov 20116.4”1.0
28 Nov 20117.0”0.8
12 Dec 20117.7”0.6
26 Dec 20118.6”0.3
09 Jan 20129.7”0.0
23 Jan 201211.0”-0.3
06 Feb 201212.4”-0.7
20 Feb 201213.5”-1.0
05 Mar 201213.9”-1.2
19 Mar 201213.5”-1.0
02 Apr 201212.4”-0.7
The sky on 22 October 2021
Twilight ends
Twilight begins

16-day old moon
Waning Gibbous


16 days old

Rise Culm. Set
Mercury 05:41 11:34 17:27
Venus 11:22 15:44 20:07
Moon 18:46 01:50 09:04
Mars 06:48 12:20 17:51
Jupiter 15:20 20:28 01:36
Saturn 14:32 19:24 00:16
All times shown in EDT.


The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

29 Jan 2010  –  Mars at opposition
03 Mar 2012  –  Mars at opposition
05 Mar 2012  –  Mars at perigee
08 Apr 2014  –  Mars at opposition

Image credit

© NASA/Hubble Space Telescope






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