Neptune will reach opposition, when it lies opposite to the Sun in the sky. Lying in the constellation Aquarius, it will be visible for much of the night, reaching its highest point in the sky around midnight local time.

From Cambridge, it will be visible between 21:18 and 04:11. It will become accessible at around 21:18, when it rises to an altitude of 21° above your south-eastern horizon. It will reach its highest point in the sky at 00:45, 40° above your southern horizon. It will become inaccessible at around 04:11 when it sinks below 21° above your south-western horizon.

2018 apparition of Neptune

A close approach to the Earth

At around the same time that Neptune passes opposition, it also makes its closest approach to the Earth – termed its perigee – making it appear at its brightest.

This happens because when Neptune lies opposite to the Sun in the sky, the Earth passes between Neptune and the Sun. The solar system is lined up with Neptune and the Earth on the same side of the Sun, as shown by the configuration labelled perigee in the diagram below:

When a planet is at opposition, the solar system is aligned such that the planet lies on the same side of the Sun as the Earth. At this time, the planet makes its perigee, or closest approach to the Earth. Not drawn to scale.

In practice, however, Neptune 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, and so its angular size does not vary much as it cycles between opposition and solar conjunction.

Observing Neptune

At opposition, Neptune is visible for much of the night. When it lies opposite to the Sun in the sky, this means that it rises at around the time the Sun sets, and it sets at around the time the Sun rises. It reaches its highest point in the sky at around midnight local time.

But even when it is at its closest point to the Earth, 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 2018 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. Its celestial coordinates at the moment it passes opposition will be:

The coordinates above are given in J2000.0.

Over the weeks following its opposition, Neptune will reach its highest point in the sky 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 7 Sep 2018

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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Image credit

© NASA/Voyager 2

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