© NASA/Cassini

Saturn ends retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Saturn
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Saturn 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.

1990 apparition of Saturn

04 May 1990 – Saturn enters retrograde motion
14 Jul 1990 – Saturn at opposition
23 Sep 1990 – Saturn ends retrograde motion

Observing Saturn

Saturn leaves retrograde motion as its 1990 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
Saturn 19h21m30s 22°08'S Sagittarius 0.2 17.2"

The coordinates above are given in J2000.0.

From Fairfield , it will be visible in the evening sky, becoming accessible around 19:16 (EST), 25° above your southern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 20:01, 26° above your southern horizon. It will continue to be observable until around 23:29, when it sinks below 10° above your south-western horizon.

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

The sky on 04 December 2021
Twilight ends
Twilight begins

30-day old moon
Waxing Crescent


30 days old

Rise Culm. Set
Mercury 07:22 11:55 16:27
Venus 10:06 14:37 19:09
Moon 07:26 12:07 16:44
Mars 05:29 10:24 15:19
Jupiter 11:38 16:51 22:04
Saturn 10:52 15:47 20:41
All times shown in EST.


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

14 Jul 1990  –  Saturn at opposition
26 Jul 1991  –  Saturn at opposition
07 Aug 1992  –  Saturn at opposition
19 Aug 1993  –  Saturn at opposition

Image credit

© NASA/Cassini






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