© 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.

2018 apparition of Mars

26 Jun 2018 – Mars enters retrograde motion
26 Jul 2018 – Mars at opposition
31 Jul 2018 – Mars at perigee
27 Aug 2018 – Mars ends retrograde motion

Observing Mars

Mars enters retrograde motion as its 2018 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 20h50m50s 22°32'S Capricornus -2.0 20.1"

The coordinates above are given in J2000.0.

From San Diego , it will be visible in the morning sky, becoming accessible around 23:07, when it reaches an altitude of 7° above your south-eastern horizon. It will then reach its highest point in the sky at 03:21, 34° above your southern horizon. It will be lost to dawn twilight around 05:22, 27° 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:

01 May 2018
29 May 2018
26 Jun 2018
24 Jul 2018
21 Aug 2018

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

Date Angular size Mag
17 Apr 20189.8”-0.1
01 May 201811.2”-0.4
15 May 201812.9”-0.8
29 May 201814.9”-1.2
12 Jun 201817.4”-1.6
26 Jun 201820.1”-2.0
10 Jul 201822.6”-2.5
24 Jul 201824.1”-2.8
07 Aug 201824.1”-2.7
21 Aug 201822.5”-2.4
04 Sep 201820.2”-2.0

The sky on 26 Jun 2018

The sky on 26 June 2018
Twilight ends
Twilight begins

13-day old moon
Waxing Gibbous


13 days old

Rise Culm. Set
Mercury 07:16 14:21 21:27
Venus 08:47 15:39 22:31
Moon 17:56 --:-- 04:39
Mars 22:20 03:21 08:22
Jupiter 15:49 21:12 02:36
Saturn 19:54 00:55 05:56
All times shown in PDT.


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

26 Jun 2018  –  Mars enters retrograde motion
26 Jul 2018  –  Mars at opposition
31 Jul 2018  –  Mars at perigee
27 Aug 2018  –  Mars ends retrograde motion

Image credit

© NASA/Hubble Space Telescope


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