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

Asteroid 30 Urania at opposition

Dominic Ford, Editor
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The sky at

Asteroid 30 Urania will be well placed for observation, lying in the constellation Pisces, well above the horizon for much of the night.

Regardless of your location on the Earth, 30 Urania will reach its highest point in the sky at around midnight local time.

From Ashburn, it will be visible between 20:48 and 05:08. It will become accessible at around 20:48, when it rises 21° above your eastern horizon, and then reach its highest point in the sky at 01:00, 51° above your southern horizon. It will become inaccessible at around 05:08 when it sinks to 22° above your western 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 30 Urania 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 30 Urania lies opposite to the Sun in the night sky, the solar system is lined up so that 30 Urania, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as 30 Urania.

On this occasion, 30 Urania will pass within 1.114 AU of us, reaching a peak brightness of magnitude 9.6. Nonetheless, even at its brightest, 30 Urania 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 30 Urania

The star charts below mark the path of 30 Urania 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 30 Urania at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 30 Urania 23h38m50s +00°58' Pisces 9.6

The coordinates above are given in J2000.0.

The sky on 18 September 2018
Twilight ends
Twilight begins

9-day old moon
Waxing Gibbous


9 days old

Rise Culm. Set
Mercury 06:41 12:58 19:14
Venus 10:25 15:25 20:25
Moon 15:45 20:36 00:39
Mars 17:00 21:38 02:18
Jupiter 11:22 16:28 21:35
Saturn 14:45 19:29 00:16
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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