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

Asteroid 3 Juno at opposition

Sun, 02 Jul 2017 at11:27 EDT(199 days ago)
15:27 UTC

Dominic Ford, Editor
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The sky at

Asteroid 3 Juno will be well placed for observation, lying in the constellation Scutum, well above the horizon for much of the night.

Regardless of your location on the Earth, 3 Juno will reach its highest point in the sky at around midnight local time.

From Ashburn (click to change), it will be visible all night. It will become visible at around 21:56 (EST) as the dusk sky fades, 27° above your south-eastern horizon. It will be lost to dawn twilight at around 04:26, 26° 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 3 Juno 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 3 Juno lies opposite to the Sun in the night sky, the solar system is lined up so that 3 Juno, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as 3 Juno.

On this occasion, 3 Juno will pass within 2.101 AU of us, reaching a peak brightness of magnitude 9.8. Nonetheless, even at its brightest, 3 Juno 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 3 Juno

The star charts below mark the path of 3 Juno 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 3 Juno at the moment of opposition will be as follows:

Object Right Ascension Declination Constellation Magnitude
Asteroid 3 Juno 18h39m50s -05°04' Scutum 9.8

The coordinates above are given in J2000.0.

The sky on 02 July 2017
Sunrise 05:47
Sunset 20:38
Twilight ends
Twilight begins

8-day old moon
Age of Moon
8 days

All times shown in EDT.
Rise Culm. Set
Mercury 06:44 14:09 21:34
Venus 03:08 10:08 17:08
Moon 15:00 20:36 01:44
Mars 06:20 13:45 21:10
Jupiter 13:28 19:17 01:10
Saturn 19:07 23:54 04:45


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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