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

Asteroid 21 Lutetia at opposition

Dominic Ford, Editor
From the Asteroids feed

Objects: 21 Lutetia
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The sky at

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

Regardless of your location on the Earth, 21 Lutetia will reach its highest point in the sky around midnight local time.

From Seattle, it will be visible between 21:43 and 04:35. It will become accessible around 21:43, when it rises to an altitude of 21° above your south-eastern horizon. It will reach its highest point in the sky at 01:09, 38° above your southern horizon. It will become inaccessible around 04:35 when it sinks below 22° above your south-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 21 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 21 Lutetia lies opposite to the Sun in the night sky, the solar system is lined up so that 21 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 21 Lutetia.

On this occasion, 21 Lutetia will pass within 1.089 AU of us, reaching a peak brightness of magnitude 9.6. Nonetheless, even at its brightest, 21 Lutetia is a faint object beyond the reach of the naked eye; binoculars or a telescope of moderate aperture are needed.

Finding 21 Lutetia

The chart below indicates the path of 21 Lutetia across the sky around the time of opposition.

It was produced using StarCharter and is available for download, either on dark background, in PNG, PDF or SVG formats, or on a light background, in PNG, PDF or SVG formats.

The position of 21 Lutetia at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 21 Lutetia 00h25m30s 3°26'S Pisces 9.6

The coordinates above are given in J2000.0.

The sky on 28 September 2019
Twilight ends
Twilight begins

29-day old moon
Waxing Crescent


29 days old

Rise Culm. Set
Mercury 08:46 14:07 19:27
Venus 08:08 13:45 19:23
Moon 06:40 13:11 19:27
Mars 06:16 12:30 18:44
Jupiter 13:31 17:47 22:02
Saturn 15:25 19:41 23:57
All times shown in PDT.


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 DE430 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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