© 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.

2024 apparition of Neptune

02 Jul 2024 – Neptune enters retrograde motion
20 Sep 2024 – Neptune at opposition
07 Dec 2024 – Neptune ends retrograde motion

Observing Neptune

Neptune leaves retrograde motion as its 2024 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 23h50m10s 2°28'S Pisces 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 17:38 (EDT), 44° above your southern horizon, as dusk fades to darkness. It will then reach its highest point in the sky at 18:34, 46° above your southern horizon. It will continue to be observable until around 22:25, when it sinks below 22° 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 07 December 2024
Sunrise
07:06
Sunset
16:26
Twilight ends
18:06
Twilight begins
05:26

6-day old moon
Waxing Crescent

48%

6 days old

Planets
Rise Culm. Set
Mercury 06:43 11:31 16:19
Venus 10:18 14:57 19:36
Moon 11:56 17:23 23:02
Mars 20:01 03:25 10:50
Jupiter 16:18 23:44 07:10
Saturn 12:12 17:45 23:17
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

20 Sep 2024  –  Neptune at opposition
23 Sep 2025  –  Neptune at opposition
25 Sep 2026  –  Neptune at opposition
28 Sep 2027  –  Neptune at opposition

Image credit

© NASA/Voyager 2

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