Mars and Neptune will share the same right ascension, with Mars passing 2'21" to the north of Neptune.
At around the same time, the two objects will also make a close approach, technically called an appulse.
From Cambridge , the pair will be visible in the evening sky, becoming accessible around 17:24 (EST), 40° above your southern horizon, as dusk fades to darkness. They will then reach their highest point in the sky at 17:38, 40° above your southern horizon. They will continue to be observable until around 21:02, when they sink below 21° above your south-western horizon.
Mars will be at mag 0.1, and Neptune at mag 7.9, both in the constellation Aquarius.
The pair will be close enough to fit within the field of view of a telescope, but will also be visible through a pair of binoculars.
A graph of the angular separation between Mars and Neptune around the time of closest approach is available here.
The positions of the two objects at the moment of conjunction will be as follows:
| Object | Right Ascension | Declination | Constellation | Magnitude | Angular Size |
| Mars | 23h00m30s | 7°21'S | Aquarius | 0.1 | 8"8 |
| Neptune | 23h00m30s | 7°23'S | Aquarius | 7.9 | 2"2 |
The coordinates above are given in J2000.0. The pair will be at an angular separation of 88° from the Sun, which is in Ophiuchus at this time of year.
The sky on 7 Dec 2018
| The sky on 7 December 2018 | ||||||||||||||||||||||||||||||||||
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0% 30 days old |
All times shown in EST.
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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.
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| 24 Nov 2018 | – Neptune ends retrograde motion |
| 21 Jun 2019 | – Neptune enters retrograde motion |
| 10 Sep 2019 | – Neptune at opposition |
| 27 Nov 2019 | – Neptune ends retrograde motion |
Image credit
The Moon in conjunction with Venus and Jupiter, with the Very Large Telescope in the foreground. Image © Y. Beletsky, ESO, 2009.