Neptune will reach opposition – the optimal time to observe it, when it will be visible for much of the night in the constellation Aquarius.
From South El Monte, it will be visible between 20:57 and 04:42. It will become accessible at around 20:57, when it rises to an altitude of 21° above your south-eastern horizon. It will reach its highest point in the sky at 00:49, 49° above your southern horizon. It will become inaccessible at around 04:42 when it sinks below 21° above your south-western horizon.
A close approach to the Earth
When a planet is at opposition, the solar system is aligned with that planet on the same side of the Sun as the Earth.
The term opposition refers to the moment when a planet passes opposite to the Sun in the sky. For those planets which orbit the Sun at a greater distance than the Earth – like Neptune – this geometry occurs as the two planets pass each other in their orbits and they make closest approach – termed its perigee.
At opposition / perigee, planets are visible for much of the night, reaching their highest point in the sky around midnight local time, just as the Sun, 180° away, dips to its lowest point below the horizon.
Because it passes closest to the Earth at this time, the planet also appears at its brightest around opposition.
In practice, the variation for Neptune is quite modest since it orbits much further out in the solar system than the Earth – at an average distance from the Sun of 30.07 times that of the Earth. Consequently, its distance and angular size does not vary much as it cycles between opposition and solar conjunction. The variation is much greater for Mars, since it lies much closer to the Earth.
Observing Neptune
At opposition, Neptune is visible for much of the night. Even when it is at its closest point to the Earth, however, it is not possible to distinguish it as more than a star-like point of light without the aid of a telescope.
A chart of the path of Neptune across the sky in 2019 can be found here, and a chart of its rising and setting times here.
At the moment of opposition, Neptune will lie at a distance of 28.93 AU, and its disk will measure 2.4 arcsec in diameter, shining at magnitude 7.8. At opposition, its celestial coordinates will be:
| Object | Right Ascension | Declination | Constellation | Magnitude | Angular Size |
| Neptune | 23h13m50s | 6°05'S | Aquarius | 7.8 | 2.4" |
The coordinates above are given in J2000.0.
Over the weeks following its opposition, Neptune will reach its highest point in the sky around four minutes earlier each night, gradually receding from the pre-dawn morning sky while remaining visible in the evening sky for a few months.
The sky on 10 Sep 2019
| The sky on 10 September 2019 | ||||||||||||||||||||||||||||||||||
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89% 11 days old |
All times shown in PDT.
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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.
Related news
| 10 Sep 2019 | – Neptune at opposition |
| 27 Nov 2019 | – Neptune ends retrograde motion |
| 22 Jun 2020 | – Neptune enters retrograde motion |
| 11 Sep 2020 | – Neptune at opposition |
Image credit
© NASA/Voyager 2