Pluto at opposition

2015 Jul 6

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

Across much of the world Pluto will be well placed for observation, in the constellation Sagittarius. 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) however, it will not be readily observable since it will lie so far south that it will never rise more than 17° above the horizon.

Pluto in coming weeks

Over the weeks following its opposition, Pluto 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 Pluto across the sky in 2015 can be found here, and a chart of its rising and setting times here.

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

Object Right Ascension Declination Constellation Magnitude Angular Size
Pluto 19h00m00s -20°43' Sagittarius 14.5 0.1"

The coordinates above are given in J2000.0.

The geometry of the solar system

This optimal positioning occurs when Pluto 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 Pluto passes opposition, it also makes its closest approach to the Earth – termed its perigee – making it appear at its brightest.

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

In practice, however, Pluto orbits much further out in the solar system than the Earth – at an average distance from the Sun of 39.68  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, Pluto will lie at a distance of 31.89 AU, and reach a peak brightness of magnitude 14.5. Even at its closest approach to the Earth, however, Pluto is so distant from the Earth that it is not possible to distinguish it as more than a star-like point of light.


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

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