© NASA/Galileo 1993. Pictured asteroid is 243 Ida.

Asteroid 10 Hygiea at opposition

Dominic Ford, Editor
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The sky at

Asteroid 10 Hygiea will be well placed for observation, lying in the constellation Sagittarius, well above the horizon for much of the night.

Regardless of your location on the Earth, 10 Hygiea will reach its highest point in the sky at around midnight local time.

From Cambridge, it will be visible between 23:36 and 01:57. It will become accessible at around 23:36, when it rises 21° above your southern horizon, and then reach its highest point in the sky at 00:48, 23° above your southern horizon. It will become inaccessible at around 01:57 when it sinks to 22° above your southern horizon.

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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 10 Hygiea 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 10 Hygiea lies opposite to the Sun in the night sky, the solar system is lined up so that 10 Hygiea, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as 10 Hygiea.

On this occasion, 10 Hygiea will pass within 1.851 AU of us, reaching a peak brightness of magnitude 9.1. Nonetheless, even at its brightest, 10 Hygiea 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 10 Hygiea

The star charts below mark the path of 10 Hygiea across the sky around the time of its opposition.

This star chart is also available to download:

Light-on-dark PNG image PDF document
Dark-on-light PNG image PDF document

The exact position of 10 Hygiea at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 10 Hygiea 18h34m20s -23°44' Sagittarius 9.1

The coordinates above are given in J2000.0.

The sky on 29 June 2017
Twilight ends
Twilight begins

5-day old moon
Waxing Crescent


5 days old

Rise Culm. Set
Mercury 05:49 13:29 21:09
Venus 02:37 09:41 16:46
Moon 11:36 17:56 00:16
Mars 05:46 13:23 21:00
Jupiter 13:15 19:03 00:55
Saturn 19:04 23:41 04:23
All times shown in EDT.


The circumstances of this event were computed from orbital elements made available by Ted Bowell of the Lowell Observatory. The conversion to geocentric coordinates was performed using the position of the Earth recorded in the DE405 ephemeris published by the Jet Propulsion Laboratory (JPL).

The star chart above shows the positions and magnitudes of stars as they appear in the Tycho catalogue. The data was reduced by the author and plotted using PyXPlot. A gnomonic projection of the sky has been used; celestial coordinates are indicated in the J2000.0 coordinate system.

Image credit

© NASA/Galileo 1993. Pictured asteroid is 243 Ida.




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