Asteroid 20 Massalia at opposition

2017 Dec 17

Dominic Ford – originally published on

Asteroid 20 Massalia will be well placed for observation, lying in the constellation Taurus, well above the horizon for much of the night.

Regardless of your location on the Earth, 20 Massalia will reach its highest point in the sky at around midnight local time.

From London (click to change), it will be visible between 18:46 and 05:08. It will become accessible at around 18:46, when it rises 24° above your eastern horizon, and then reach its highest point in the sky at 23:55, 60° above your southern horizon. It will become inaccessible at around 05:08 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 20 Massalia 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 20 Massalia lies opposite to the Sun in the night sky, the solar system is lined up so that 20 Massalia, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as 20 Massalia.

On this occasion, 20 Massalia will pass within 1.086 AU of us, reaching a peak brightness of magnitude 8.3. Nonetheless, even at its brightest, 20 Massalia 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.

Finding 20 Massalia

The star charts below mark the path of 20 Massalia across the sky around the time of its opposition.

This star chart is also available as a PDF download, or in a light-on-dark color scheme (large image, small image or PDF download).

The exact position of 20 Massalia at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 20 Massalia 05h41m50s +22°12' Taurus 8.3

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of