The dwarf planet 136199 Eris will reach opposition (i.e. be directly opposite the Sun seen from Earth) on Wednesday 17 October 2023 at 4.18 pm GMT. This means that it will both be at its closest to the Earth this year, about 97.74 AU (97.74 times the average distance between the Earth and the Sun, or about 14, 173 000 000 km), and completely illuminated by the Sun. While it is not visible to the naked eye observer, the planets have phases just like those of the Moon; being further from the Sun than the Earth, 136199 Eris is 'full' when directly opposite the Sun. As this falls only four days after the Full Moon, the prospects for viewing for those equipped with suitable telescopes is not as good as it might be. The planet will be in the constellation of Cetus and at its highest point in the sky at about midnight local time from anywhere on Earth (this is because the rising and setting of objects in the sky is caused by the Earth's rotation, not the movement of the object). (Even at it's very brightest 136199 Eris will only have a Magnitude of 18.6, making it almost impossible to see with any but the largest of Earth-based telescopes, and where resolvable it will only be possible to see it as a point of light indistinguishable from a faint star.
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136199 Eris orbits the Sun on an eccentric orbit tilted at an angle of 44.1° to the plane of the Solar System, which takes it from 35.9 AU from the Sun (35.9 times the average distance at which the Earth orbits the Sun) to 97.5 AU from the Sun (97.5 times the average distance at which the Earth orbits the Sun. With an average distance of 67.74 AU, 136199 Eris completes one orbit around the Sun every 558 years. This means that the planet is almost stationary compared to the faster moving Earth, so that it reaches Opposition only four days later each year than the year before, and reaches Solar Conjunction (when it is directly on the opposite side of the Sun to the Earth), roughly six months later.
136199 Eris was discovered on 5 January 2005 by a team led by Mike Brown of the Palomar Observatory in California. With a diameter of 2326 km it is considered to be the second largest dwarf planet in the Solar System (after 134340 Pluto) as well as the sixteenth largest body in the Solar System, excluding the Sun (though several moons, including our own, are larger). It is also the largest body in the Solar System never to have been visited by a spacecraft (again, with the exception of the Sun), and (currently) the second most distant known object in the Solar System, after trans-Neptunian Object 2018 VG18. 136199 Eris has a single moon, Dysnomia, which has a diameter of about 700 km and obits at a distance of about 37 350 km.
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