Asteroid 7 Iris at opposition

2015 Mar 6

Dominic Ford – originally published on

Asteroid 7 Iris will be well placed for observation, lying in the constellation Leo, well above the horizon for much of the night.

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

From London (click to change), it will be visible between 21:04 and 02:50. It will become accessible at around 21:04, when it rises 24° above your south-eastern horizon, and then reach its highest point in the sky at 23:55, 35° above your southern horizon. It will become inaccessible at around 02:50 when it sinks to 24° above your south-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 Iris 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 Iris lies opposite to the Sun in the night sky, the solar system is lined up so that Iris, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as Iris.

On this occasion, Iris will pass within 1.577 AU of us, reaching a peak brightness of magnitude 8.5. Nonetheless, even at its brightest, Iris 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 Iris at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 7 Iris 10h52m20s -02°30′ Leo 8.5

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of