Asteroid 21 Lutetia at opposition

2015 Aug 15

Dominic Ford – originally published on

Asteroid 21 Lutetia will be well placed for observation, lying in the constellation Capricornus, well above the horizon for much of the night.

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

From London (click to change) however, it will not be readily observable since it will lie so far south that it will never rise more than 19° above the 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 Lutetia 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 Lutetia lies opposite to the Sun in the night sky, the solar system is lined up so that Lutetia, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as Lutetia.

On this occasion, Lutetia will pass within 1.029 AU of us, reaching a peak brightness of magnitude 9.0. Nonetheless, even at its brightest, Lutetia 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 Lutetia at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 21 Lutetia 21h44m30s -19°12′ Capricornus 9.0

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of