Neptune 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.
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.
1979 apparition of Neptune
| 23 Mar 1979 | – | Neptune enters retrograde motion |
| 10 Jun 1979 | – | Neptune at opposition |
| 30 Aug 1979 | – | Neptune ends retrograde motion |
Observing Neptune
Neptune enters retrograde motion as its 1979 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 |
| Neptune | 17h20m10s | 21°42'S | Ophiuchus | 7.9 | 2.3" |
The coordinates above are given in J2000.0.
From London however, it will not be readily observable since it will lie so far south that it will never rise more than 16° above the horizon.
Over the following weeks, Neptune 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 sky on 29 May 2024
| The sky on 29 May 2024 | ||||||||||||||||||||||||||||||||||
|
59% 21 days old |
All times shown in BST.
|
|||||||||||||||||||||||||||||||||
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
| 23 Mar 1979 | – Neptune enters retrograde motion |
| 10 Jun 1979 | – Neptune at opposition |
| 30 Aug 1979 | – Neptune ends retrograde motion |
| 24 Mar 1980 | – Neptune enters retrograde motion |
Image credit
© NASA/Voyager 2