Asteroid 15 Eunomia at opposition

2015 Oct 3

Dominic Ford – originally published on

Asteroid 15 Eunomia will be well placed for observation, lying in the constellation Pegasus, well above the horizon for much of the night.

Regardless of your location on the Earth, Eunomia will reach its highest point in the sky at around midnight local time.

From London (click to change), it will be visible in the evening sky. It will become visible at around 19:52 (GMT) as the dusk sky fades, 33° above your eastern horizon. It will then reach its highest point in the sky at 00:16, 61° above your southern horizon. It will continue to be observable until around 05:30, when it sinks to 24° above your western horizon.

The geometry of the alignment

This optimal positioning occurs when it makes its closest approach to the point in the sky directly opposite to the Sun – an event termed opposition. 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 Eunomia passes opposition, it also makes its closest approach to the Earth – termed its perigee – making it appear at its brightest in the night sky. This happens because when Eunomia lies opposite to the Sun in the night sky, the solar system is lined up so that Eunomia, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as Eunomia.

On this occasion, Eunomia will pass within 1.2 AU of us, reaching a peak brightness of magnitude 7.4. Nonetheless, even at its brightest, Eunomia is a faint object beyond the reach of the naked eye or binoculars; a telescope of moderate aperture and a good star chart are needed.

The exact position of Eunomia at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 15 Eunomia 00h01m00s +23°04′ Pegasus 7.4

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of