Neptune at opposition

2016 Sep 2

Dominic Ford – originally published on In-The-Sky.org

Neptune will be well placed for observation, in the constellation Aquarius. It will be visible for much of the night, reaching its highest point in the sky at around midnight local time.

From London (click to change), it will be visible between 23:02 and 02:59. It will become accessible at around 23:02, when it rises 24° above your south-eastern horizon, and then reach its highest point in the sky at 01:03, 30° above your southern horizon. It will become inaccessible at around 02:59 when it sinks to 25° above your south-western horizon.

Neptune in coming weeks

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.

A chart of the path of Neptune across the sky in 2016 can be found here, and a chart of its rising and setting times here.

The position of Neptune at the moment it passes opposition will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
Neptune 22h48m50s -08°29' Aquarius 7.8 2.4"

The coordinates above are given in J2000.0.

The geometry of the solar system

This optimal positioning occurs when Neptune is almost directly opposite the Sun in the sky. Since the Sun reaches its greatest distance below the horizon at midnight, the point opposite to it is highest in the sky at the same time.

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 and largest.

This happens because when Neptune lies opposite the Sun in the sky, the solar system is lined up so that Neptune, the Earth and the Sun form a straight line with the Earth in the middle, on the same side of the Sun as Neptune.

In practice, however, Neptune orbits much further out in the solar system than the Earth – at an average distance from the Sun of 30.27  times that of the Earth, and so its angular size does not vary much as it cycles between opposition and solar conjunction.

On this occasion, Neptune will lie at a distance of 28.95 AU, and its disk will measure 2.4 arcsec in diameter, shining at magnitude 7.8. Even at its closest approach 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.


The details of this observing event were provided courtesy of In-The-Sky.org

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