© 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.

2023 apparition of Neptune

30 Jun 2023 – Neptune enters retrograde motion
19 Sep 2023 – Neptune at opposition
06 Dec 2023 – Neptune ends retrograde motion

Observing Neptune

Neptune enters retrograde motion as its 2023 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 23h52m10s 2°10'S 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 00:36 (EDT) and reaching an altitude of 40° above the south-eastern horizon before fading from view as dawn breaks around 04:28.

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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 30 June 2023
Sunrise
05:48
Sunset
20:40
Twilight ends
22:40
Twilight begins
03:48

12-day old moon
Waxing Gibbous

93%

12 days old

Planets
Rise Culm. Set
Mercury 05:40 13:10 20:40
Venus 09:16 16:10 23:04
Moon 18:03 22:51 03:33
Mars 09:31 16:23 23:16
Jupiter 02:19 09:06 15:54
Saturn 23:48 05:16 10:44
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

16 Sep 2022  –  Neptune at opposition
19 Sep 2023  –  Neptune at opposition
20 Sep 2024  –  Neptune at opposition
23 Sep 2025  –  Neptune at opposition

Image credit

© NASA/Voyager 2

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Ashburn

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39.04°N
77.49°W
EDT

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