© 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 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 leaves retrograde motion as its 2018 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 20h07m00s 26°15'S Sagittarius -2.2 21.6"

The coordinates above are given in J2000.0.

From Washington , it will be visible in the evening sky, becoming accessible around 20:26 (MDT), 15° above your south-eastern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 23:15, 26° above your southern horizon. It will continue to be observable until around 02:57, when it sinks below 7° above your south-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:

02 Jul 2018
30 Jul 2018
27 Aug 2018
24 Sep 2018
22 Oct 2018

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

Date Angular size Mag
18 Jun 201818.5”-1.8
02 Jul 201821.2”-2.2
16 Jul 201823.4”-2.6
30 Jul 201824.3”-2.8
13 Aug 201823.6”-2.6
27 Aug 201821.6”-2.2
10 Sep 201819.2”-1.8
24 Sep 201816.8”-1.5
08 Oct 201814.7”-1.1
22 Oct 201812.9”-0.8
05 Nov 201811.4”-0.5

The sky on 27 Aug 2018

The sky on 27 August 2018
Twilight ends
Twilight begins

16-day old moon
Waning Gibbous


16 days old

Rise Culm. Set
Mercury 05:30 12:23 19:15
Venus 10:46 16:17 21:49
Moon 20:36 02:14 07:58
Mars 18:39 23:15 03:51
Jupiter 12:52 18:05 23:19
Saturn 16:29 21:19 02:08
All times shown in MDT.


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

27 Aug 2018  –  Mars ends retrograde motion
23 Aug 2020  –  Mars 2020: a great chance to see the red planet
09 Sep 2020  –  Mars enters retrograde motion
06 Oct 2020  –  Mars at perigee

Image credit

© NASA/Hubble Space Telescope





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