Neptune 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.
This motion was known to ancient observers, and it troubled them as they could not reconcile it with models in which the planets moved in uniform circular orbits around the Earth, as they believed.
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 a planet in the outer solar system.
Not drawn to scale.
2020 apparition of Neptune
| 22 Jun 2020 | – | Neptune enters retrograde motion |
| 11 Sep 2020 | – | Neptune at opposition |
| 28 Nov 2020 | – | Neptune ends retrograde motion |
Observing Neptune
Neptune leaves retrograde motion as its 2020 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 |
| Neptune | 23h17m00s | 5°48'S | Aquarius | 7.9 | 2.3" |
The coordinates above are given in J2000.0.
From Cambridge , it will be visible in the evening sky, becoming accessible around 17:25 (EST), 39° above your southern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 18:28, 41° above your southern horizon. It will continue to be observable until around 22:00, when it sinks below 21° above your south-western horizon.
Over the following weeks, Neptune will reach its highest point in the sky four minutes earlier each night, gradually disappearing into evening twilight.
The sky on 28 Nov 2020
| The sky on 28 November 2020 | ||||||||||||||||||||||||||||||||||
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99% 13 days old |
All times shown in EST.
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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
| 28 Nov 2020 | – Neptune ends retrograde motion |
| 25 Jun 2021 | – Neptune enters retrograde motion |
| 14 Sep 2021 | – Neptune at opposition |
| 01 Dec 2021 | – Neptune ends retrograde motion |
Image credit
© NASA/Voyager 2