136199 Eris at opposition

2017 Oct 16

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

136199 Eris will be well placed for observation, in the constellation Cetus. 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 22:10 and 03:56. It will become accessible at around 22:10, when it rises 24° above your south-eastern horizon, and then reach its highest point in the sky at 01:05, 36° above your southern horizon. It will become inaccessible at around 03:56 when it sinks to 25° above your south-western horizon.

136199 Eris in coming weeks

Over the weeks following its opposition, 136199 Eris 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 136199 Eris across the sky in 2017 can be found here, and a chart of its rising and setting times here.

The position of 136199 Eris at the moment it passes opposition will be:

Object Right Ascension Declination Constellation Magnitude Angular Size
136199 Eris 01h43m20s -02°28' Cetus 18.6 0.0"

The coordinates above are given in J2000.0.

The geometry of the solar system

This optimal positioning occurs when 136199 Eris 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 136199 Eris 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 136199 Eris lies opposite the Sun in the sky, the solar system is lined up so that 136199 Eris, the Earth and the Sun form a straight line with the Earth in the middle, on the same side of the Sun as 136199 Eris.

In practice, however, 136199 Eris orbits much further out in the solar system than the Earth – at an average distance from the Sun of 67.75  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, 136199 Eris will lie at a distance of 95.19 AU, and reach a peak brightness of magnitude 18.6. Even at its closest approach to the Earth, however, 136199 Eris 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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