© NASA/Cassini

Saturn enters retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Saturn
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Saturn 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.

2013 apparition of Saturn

18 Feb 2013 – Saturn enters retrograde motion
28 Apr 2013 – Saturn at opposition
08 Jul 2013 – Saturn ends retrograde motion

Observing Saturn

Saturn enters retrograde motion as its 2013 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
Saturn 14h38m50s 12°49'S Libra 0.3 17.6"

The coordinates above are given in J2000.0.

From Fairfield , it will be visible in the morning sky, becoming accessible around 00:26, when it reaches an altitude of 10° above your south-eastern horizon. It will then reach its highest point in the sky at 04:39, 36° above your southern horizon. It will be lost to dawn twilight around 06:10, 31° 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 becoming visible in the evening sky, as well as the pre-dawn sky, as it approaches opposition.

The sky on 18 February 2013
Sunrise
06:42
Sunset
17:30
Twilight ends
19:06
Twilight begins
05:10

8-day old moon
Waxing Gibbous

62%

8 days old

Planets
Rise Culm. Set
Mercury 07:20 13:12 19:03
Venus 06:26 11:32 16:39
Moon 11:12 18:44 02:14
Mars 07:22 12:59 18:35
Jupiter 10:56 18:18 01:39
Saturn 23:22 04:39 09:57
All times shown in EST.

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

15 Apr 2012  –  Saturn at opposition
28 Apr 2013  –  Saturn at opposition
10 May 2014  –  Saturn at opposition
22 May 2015  –  Saturn at opposition

Image credit

© NASA/Cassini

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