© NASA/Cassini

Saturn ends retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Saturn
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The sky at

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.

2017 apparition of Saturn

06 Apr 2017 – Saturn enters retrograde motion
15 Jun 2017 – Saturn at opposition
25 Aug 2017 – Saturn ends retrograde motion

Observing Saturn

Saturn leaves retrograde motion as its 2017 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 17h20m50s 21°57'S Ophiuchus 0.2 17.1"

The coordinates above are given in J2000.0.

From Fairfield , it will become visible around 20:08 (EDT), 26° above your southern horizon, as dusk fades to darkness. It will then sink towards the horizon, setting at 00:37.

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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 25 Aug 2017

The sky on 25 August 2017
Twilight ends
Twilight begins

4-day old moon
Waxing Crescent


4 days old

Rise Culm. Set
Mercury 06:32 12:56 19:19
Venus 03:23 10:41 17:59
Moon 10:09 16:07 21:58
Mars 05:21 12:19 19:16
Jupiter 10:15 15:53 21:32
Saturn 15:15 19:56 00:37
All times shown in EDT.


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

15 Jun 2017  –  Saturn at opposition
27 Jun 2018  –  Saturn at opposition
09 Jul 2019  –  Saturn at opposition
20 Jul 2020  –  Saturn at opposition

Image credit

© NASA/Cassini






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