© NASA/Hubble Space Telescope

Mars ends retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Mars
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Mars will reach the end of its retrograde motion, ending its westward movement through the constellations and returning to more usual eastward motion instead. This reversal of direction is a phenomenon that all the solar system's outer planets periodically undergo, a few months after they pass 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 leaves retrograde motion as its 2012 apparition comes to an end, although it will remain visible for some weeks in the dusk sky.

Its celestial coordinates as it leaves retrograde motion will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
Mars 10h25m40s 12°46'N Leo -0.4 11.4"

The coordinates above are given in J2000.0.

From Fairfield , it will be visible in the evening sky, becoming accessible around 19:55 (EDT), 52° above your south-eastern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 21:49, 61° above your southern horizon. It will continue to be observable until around 03:45, when it sinks below 8° above your western horizon.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

Over the following weeks, Mars will reach its highest point in the sky four minutes earlier each night, gradually disappearing into evening twilight.

The panels below show the month-by-month change in Mars' apparent size in coming weeks, as it recedes from the Earth:

17 Feb 2012
16 Mar 2012
13 Apr 2012
11 May 2012
08 Jun 2012

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

Date Angular size Mag
03 Feb 201212.1”-0.6
17 Feb 201213.3”-1.0
02 Mar 201213.9”-1.2
16 Mar 201213.6”-1.1
30 Mar 201212.6”-0.7
13 Apr 201211.4”-0.4
27 Apr 201210.2”-0.1
11 May 20129.1”0.2
25 May 20128.2”0.4
08 Jun 20127.5”0.6
22 Jun 20126.9”0.8
The sky on 13 April 2012
Twilight ends
Twilight begins

22-day old moon
Waning Crescent


22 days old

Rise Culm. Set
Mercury 05:25 11:18 17:12
Venus 08:06 15:50 23:34
Moon 02:02 07:06 12:16
Mars 15:00 21:49 04:38
Jupiter 07:19 14:20 21:20
Saturn 19:31 01:08 06:44
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

05 Mar 2012  –  Mars at perigee
08 Apr 2014  –  Mars at opposition
14 Apr 2014  –  Mars at perigee
22 May 2016  –  Mars at opposition

Image credit

© NASA/Hubble Space Telescope






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