136199 Eris will reach opposition, when it lies opposite to the Sun in the sky. Lying in the constellation Cetus, 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 20:54 and 04:38. It will become accessible at around 20:54, when it rises to an altitude of 21° above your south-eastern horizon. It will reach its highest point in the sky at 00:46, 46° above your southern horizon. It will become inaccessible at around 04:38 when it sinks below 21° above your south-western horizon.
A close approach to the Earth
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.
This happens because when 136199 Eris lies opposite to the Sun in the sky, the Earth passes between 136199 Eris and the Sun. The solar system is lined up with 136199 Eris 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, 136199 Eris orbits much further out in the solar system than the Earth – at an average distance from the Sun of 68.11 times that of the Earth, and so its brightness does not vary much as it cycles between opposition and solar conjunction.
Observing 136199 Eris
At opposition, 136199 Eris 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, 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, even through a telescope.
A chart of the path of 136199 Eris across the sky in 2021 can be found here, and a chart of its rising and setting times here.
At the moment of opposition, 136199 Eris will lie at a distance of 94.91 AU, and reach a peak brightness of magnitude 18.6. Its celestial coordinates at the moment it passes opposition will be:
| Object | Right Ascension | Declination | Constellation | Magnitude | Angular Size |
| 136199 Eris | 01h45m20s | 1°25'S | Cetus | 18.6 | 0.0" |
The coordinates above are given in J2000.0.
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.
The sky on 17 Oct 2021
| The sky on 17 October 2021 | ||||||||||||||||||||||||||||||||||
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92% 11 days old |
All times shown in EDT.
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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.
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Image credit
© NASA/Hubble Space Telescope