© NASA/Voyager 2

Neptune ends retrograde motion

Dominic Ford, Editor
From the Outer Planets feed

Objects: Neptune
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Neptune 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.

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.

2015 apparition of Neptune

12 Jun 2015 – Neptune enters retrograde motion
31 Aug 2015 – Neptune at opposition
18 Nov 2015 – Neptune ends retrograde motion

Observing Neptune

Neptune leaves retrograde motion as its 2015 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
Neptune 22h35m20s 9°45'S Aquarius 7.9 2.3"

The coordinates above are given in J2000.0.

From Fairfield , it will be visible in the evening sky, becoming accessible around 17:41 (EDT), 37° above your southern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 18:40, 39° above your southern horizon. It will continue to be observable until around 21:57, when it sinks below 21° above your south-western horizon.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

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

The sky on 18 November 2015
Sunrise
06:45
Sunset
16:33
Twilight ends
18:09
Twilight begins
05:09

7-day old moon
Waxing Crescent

49%

7 days old

Planets
Rise Culm. Set
Mercury 06:51 11:42 16:32
Venus 02:53 08:47 14:40
Moon 12:08 17:36 23:13
Mars 02:15 08:18 14:21
Jupiter 01:05 07:26 13:47
Saturn 07:29 12:20 17:11
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

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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