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.
2099 apparition of Mars
| 13 Sep 2099 | – | Mars enters retrograde motion |
| 10 Oct 2099 | – | Mars at perigee |
| 18 Oct 2099 | – | Mars at opposition |
| 18 Nov 2099 | – | Mars ends retrograde motion |
Observing Mars
Mars leaves retrograde motion as its 2099 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 | 01h07m00s | 6°29'N | Pisces | -1.7 | 17.4" |
The coordinates above are given in J2000.0.
From Cambridge , it will be visible in the evening sky, becoming accessible around 16:34 (EDT), 22° above your eastern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 20:57, 54° above your southern horizon. It will continue to be observable until around 02:38, when it sinks below 7° above your western horizon.
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:
The table below lists Mars' angular size at brightness at two-week intervals throughout its apparition:
| Date | Angular size | Mag |
| 09 Sep 2099 | 19.3” | -1.9 |
| 23 Sep 2099 | 21.2” | -2.2 |
| 07 Oct 2099 | 22.3” | -2.5 |
| 21 Oct 2099 | 21.8” | -2.5 |
| 04 Nov 2099 | 20.0” | -2.1 |
| 18 Nov 2099 | 17.4” | -1.7 |
| 02 Dec 2099 | 14.9” | -1.2 |
| 16 Dec 2099 | 12.7” | -0.8 |
| 30 Dec 2099 | 10.9” | -0.4 |
| 13 Jan 2100 | 9.5” | -0.0 |
| 27 Jan 2100 | 8.4” | 0.3 |
The sky on 10 May 2024
| The sky on 10 May 2024 | ||||||||||||||||||||||||||||||||||
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10% 2 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
| 18 Nov 2099 | – Mars ends retrograde motion |
| 02 Nov 2101 | – Mars enters retrograde motion |
| 04 Dec 2101 | – Mars at perigee |
| 11 Dec 2101 | – Mars at opposition |
Image credit
© NASA/Hubble Space Telescope