© 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.

2033 apparition of Neptune

23 Jul 2033 – Neptune enters retrograde motion
11 Oct 2033 – Neptune at opposition
28 Dec 2033 – Neptune ends retrograde motion

Observing Neptune

Neptune enters retrograde motion as its 2033 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 01h15m00s 6°10'N Pisces 7.9 2.3"

The coordinates above are given in J2000.0.

From Ashburn , it will be visible in the dawn sky, rising at 23:58 (EST) and reaching an altitude of 51° above the south-eastern horizon before fading from view as dawn breaks around 04:50.

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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 07 December 2021
Sunrise
07:13
Sunset
16:49
Twilight ends
18:25
Twilight begins
05:37

3-day old moon
Waxing Crescent

18%

3 days old

Planets
Rise Culm. Set
Mercury 07:45 12:21 16:58
Venus 10:06 14:46 19:27
Moon 10:54 15:38 20:30
Mars 05:41 10:39 15:37
Jupiter 11:39 16:56 22:14
Saturn 10:51 15:50 20:50
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

09 Oct 2032  –  Neptune at opposition
11 Oct 2033  –  Neptune at opposition
14 Oct 2034  –  Neptune at opposition
16 Oct 2035  –  Neptune at opposition

Image credit

© NASA/Voyager 2

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Ashburn

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Longitude:
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39.04°N
77.49°W
EST

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