© NASA/Voyager 2

Neptune enters retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Neptune
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Neptune 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.

This motion was known to ancient observers, and it troubled them as they could not reconcile it with models in which the planets moved in uniform circular orbits around the Earth, as they believed.

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.

2016 apparition of Neptune

13 Jun 2016 – Neptune enters retrograde motion
02 Sep 2016 – Neptune at opposition
19 Nov 2016 – Neptune ends retrograde motion

Observing Neptune

Neptune enters retrograde motion as its 2016 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
Neptune 22h54m10s 7°54'S Aquarius 7.9 2.3"

The coordinates above are given in J2000.0.

From Ashburn , it will be visible in the dawn sky, rising at 00:59 (EST) and reaching an altitude of 33° above the south-eastern horizon before fading from view as dawn breaks around 04:21.

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Over the following weeks, Neptune 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 13 June 2016
Sunrise
05:41
Sunset
20:37
Twilight ends
22:37
Twilight begins
03:45

8-day old moon
Waxing Gibbous

69%

8 days old

Planets
Rise Culm. Set
Mercury 04:33 11:37 18:40
Venus 05:50 13:17 20:43
Moon 14:20 20:27 02:25
Mars 18:15 23:05 03:56
Jupiter 12:21 18:47 01:14
Saturn 19:34 00:26 05:19
All times shown in EDT.

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

31 Aug 2015  –  Neptune at opposition
02 Sep 2016  –  Neptune at opposition
05 Sep 2017  –  Neptune at opposition
07 Sep 2018  –  Neptune at opposition

Image credit

© NASA/Voyager 2

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39.04°N
77.49°W
EST

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