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.
1990–1991 apparition of Mars
| 20 Oct 1990 | – | Mars enters retrograde motion |
| 19 Nov 1990 | – | Mars at perigee |
| 27 Nov 1990 | – | Mars at opposition |
| 01 Jan 1991 | – | Mars ends retrograde motion |
Observing Mars
Mars enters retrograde motion as its 1990–1991 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 | 04h53m40s | 22°09'N | Taurus | -1.4 | 15.9" |
The coordinates above are given in J2000.0.
From Cambridge , it will be visible in the morning sky, becoming accessible around 21:05, when it reaches an altitude of 7° above your north-eastern horizon. It will then reach its highest point in the sky at 03:44, 69° above your southern horizon. It will be lost to dawn twilight around 06:45, 47° above your western horizon.
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:
The table below lists Mars' angular size at brightness at two-week intervals throughout its apparition:
| Date | Angular size | Mag |
| 11 Aug 1990 | 9.7” | -0.1 |
| 25 Aug 1990 | 10.6” | -0.3 |
| 08 Sep 1990 | 11.6” | -0.5 |
| 22 Sep 1990 | 12.8” | -0.8 |
| 06 Oct 1990 | 14.3” | -1.1 |
| 20 Oct 1990 | 15.9” | -1.4 |
| 03 Nov 1990 | 17.3” | -1.7 |
| 17 Nov 1990 | 18.1” | -1.9 |
| 01 Dec 1990 | 17.7” | -1.9 |
| 15 Dec 1990 | 16.1” | -1.5 |
| 29 Dec 1990 | 14.1” | -1.1 |
The sky on 2 May 2024
| The sky on 2 May 2024 | ||||||||||||||||||||||||||||||||||
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32% 24 days old |
All times shown in EDT.
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Source
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
| 20 Oct 1990 | – Mars enters retrograde motion |
| 19 Nov 1990 | – Mars at perigee |
| 27 Nov 1990 | – Mars at opposition |
| 01 Jan 1991 | – Mars ends retrograde motion |
Image credit
© NASA/Hubble Space Telescope