Asteroid 25 Phocaea at opposition

2017 Aug 17

Dominic Ford – originally published on

Asteroid 25 Phocaea will be well placed for observation, lying in the constellation Vulpecula, well above the horizon for much of the night.

Regardless of your location on the Earth, 25 Phocaea will reach its highest point in the sky at around midnight local time.

From London (click to change), it will be visible all night. It will become visible at around 21:48 (BST) as the dusk sky fades, 56° above your south-eastern horizon. It will be lost to dawn twilight at around 04:14, 33° 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 25 Phocaea 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 25 Phocaea lies opposite to the Sun in the night sky, the solar system is lined up so that 25 Phocaea, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as 25 Phocaea.

On this occasion, 25 Phocaea will pass within 0.93 AU of us, reaching a peak brightness of magnitude 9.0. Nonetheless, even at its brightest, 25 Phocaea 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 25 Phocaea

The star charts below mark the path of 25 Phocaea 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 25 Phocaea at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 25 Phocaea 20h26m50s +25°29' Vulpecula 9.0

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of