Jupiter 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 a planet in the outer solar system.
Not drawn to scale.
2021 apparition of Jupiter
| 20 Jun 2021 | – | Jupiter enters retrograde motion |
| 19 Aug 2021 | – | Jupiter at opposition |
| 20 Aug 2021 | – | Jupiter at perigee |
| 18 Oct 2021 | – | Jupiter ends retrograde motion |
Observing Jupiter
Jupiter enters retrograde motion as its 2021 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 |
| Jupiter | 22h16m50s | 11°42'S | Aquarius | -2.6 | 42.9" |
The coordinates above are given in J2000.0.
From Cambridge , it will be visible in the dawn sky, rising at 23:46 (EST) and reaching an altitude of 35° above the southern horizon before fading from view as dawn breaks at around 04:43.
Over the following weeks, Jupiter 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 20 Jun 2021
| The sky on 20 June 2021 | ||||||||||||||||||||||||||||||||||
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81% 10 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 Jun 2021 | – Jupiter enters retrograde motion |
| 19 Aug 2021 | – Jupiter at opposition |
| 18 Oct 2021 | – Jupiter ends retrograde motion |
| 28 Jul 2022 | – Jupiter enters retrograde motion |
Image credit
© NASA/Cassini