© 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.

2016 apparition of Mars

17 Apr 2016 – Mars enters retrograde motion
22 May 2016 – Mars at opposition
30 May 2016 – Mars at perigee
29 Jun 2016 – Mars ends retrograde motion

Observing Mars

Mars leaves retrograde motion as its 2016 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 15h18m50s 20°59'S Libra -1.4 16.5"

The coordinates above are given in J2000.0.

From Ashburn , it will be visible in the evening sky, becoming accessible around 20:59 (EST), 28° above your southern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 21:54, 29° above your southern horizon. It will continue to be observable until around 01:53, when it sinks below 7° 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 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:

04 May 2016
01 Jun 2016
29 Jun 2016
27 Jul 2016
24 Aug 2016

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

Date Angular size Mag
20 Apr 201614.6”-1.2
04 May 201616.7”-1.6
18 May 201618.2”-2.0
01 Jun 201618.6”-2.0
15 Jun 201617.9”-1.7
29 Jun 201616.5”-1.4
13 Jul 201614.9”-1.1
27 Jul 201613.4”-0.9
10 Aug 201612.1”-0.6
24 Aug 201611.0”-0.4
07 Sep 201610.0”-0.2
The sky on 29 June 2016
Twilight ends
Twilight begins

24-day old moon
Waning Crescent


24 days old

Rise Culm. Set
Mercury 05:06 12:33 20:00
Venus 06:13 13:39 21:05
Moon 02:00 08:39 15:26
Mars 17:03 21:54 02:45
Jupiter 11:25 17:49 00:12
Saturn 18:21 23:14 04:07
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

30 May 2016  –  Mars at perigee
27 Jul 2018  –  Mars at opposition
31 Jul 2018  –  Mars at perigee
23 Aug 2020  –  Mars 2020: a great chance to see the red planet

Image credit

© NASA/Hubble Space Telescope






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