© NASA/Cassini

Jupiter ends retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Jupiter
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Jupiter 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 a planet in the outer solar system. Not drawn to scale.

2020 apparition of Jupiter

14 May 2020 – Jupiter enters retrograde motion
14 Jul 2020 – Jupiter at opposition
15 Jul 2020 – Jupiter at perigee
12 Sep 2020 – Jupiter ends retrograde motion

Observing Jupiter

Jupiter 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
Jupiter 19h14m20s 22°45'S Sagittarius -2.5 41.9"

The coordinates above are given in J2000.0.

From Washington , it will be visible in the evening sky, becoming accessible around 20:01 (MDT), 27° above your southern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 21:18, 30° above your southern horizon. It will continue to be observable until around 01:16, when it sinks below 7° above your south-western horizon.

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Over the following weeks, Jupiter will reach its highest point in the sky four minutes earlier each night, gradually disappearing into evening twilight.

The sky on 12 Sep 2020

The sky on 12 September 2020
Twilight ends
Twilight begins

24-day old moon
Waning Crescent


24 days old

Rise Culm. Set
Mercury 08:53 14:42 20:31
Venus 03:42 10:41 17:39
Moon 01:38 09:16 16:53
Mars 21:32 03:55 10:19
Jupiter 16:28 21:18 02:07
Saturn 16:58 21:52 02:47
All times shown in MDT.


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

12 Sep 2020  –  Jupiter ends retrograde motion
20 Jun 2021  –  Jupiter enters retrograde motion
19 Aug 2021  –  Jupiter at opposition
17 Oct 2021  –  Jupiter ends retrograde motion

Image credit

© NASA/Cassini





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