© 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 Mars. Not drawn to scale.

2020 apparition of Mars

09 Sep 2020 – Mars enters retrograde motion
06 Oct 2020 – Mars at perigee
13 Oct 2020 – Mars at opposition
13 Nov 2020 – Mars ends retrograde motion

Observing Mars

Mars enters retrograde motion as its 2020 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 01h49m30s 6°45'N Pisces -2.0 20.2"

The coordinates above are given in J2000.0.

From Fairfield , it will be visible in the morning sky, becoming accessible around 21:46, when it reaches an altitude of 7° above your eastern horizon. It will then reach its highest point in the sky at 03:27, 55° above your southern horizon. It will be lost to dawn twilight around 06:08, 40° 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:

15 Jul 2020
12 Aug 2020
09 Sep 2020
07 Oct 2020
04 Nov 2020

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

Date Angular size Mag
01 Jul 202011.5”-0.5
15 Jul 202012.8”-0.8
29 Jul 202014.3”-1.0
12 Aug 202016.1”-1.3
26 Aug 202018.1”-1.7
09 Sep 202020.2”-2.0
23 Sep 202022.0”-2.3
07 Oct 202022.6”-2.6
21 Oct 202021.6”-2.4
04 Nov 202019.4”-2.0
18 Nov 202016.7”-1.5

The sky on 9 Sep 2020

The sky on 9 September 2020
Twilight ends
Twilight begins

21-day old moon
Waning Crescent


21 days old

Rise Culm. Set
Mercury 08:02 13:57 19:51
Venus 02:48 09:58 17:08
Moon 22:31 05:54 13:26
Mars 21:00 03:27 09:54
Jupiter 16:11 20:49 01:27
Saturn 16:40 21:24 02:07
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

09 Sep 2020  –  Mars enters retrograde motion
06 Oct 2020  –  Mars at perigee
13 Oct 2020  –  Mars at opposition
13 Nov 2020  –  Mars ends retrograde motion

Image credit

© NASA/Hubble Space Telescope





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