Dominic Ford, Editor
From the Asteroids feed
Asteroid 7 Iris will be well placed for observation, lying in the constellation Aries, well above the horizon for much of the night.
Regardless of your location on the Earth, 7 Iris will reach its highest point in the sky at around midnight local time.
From Ashburn (click to change), it will be visible between 19:29 and 05:44. It will become accessible at around 19:29, when it rises 22° above your eastern horizon, and then reach its highest point in the sky at 00:39, 72° above your southern horizon. It will become inaccessible at around 05:44 when it sinks to 23° 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 7 Iris 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 7 Iris lies opposite to the Sun in the night sky, the solar system is lined up so that 7 Iris, the Earth and the Sun lie in a straight line with the Earth in the middle, on the same side of the Sun as 7 Iris.
On this occasion, 7 Iris will pass within 0.844 AU of us, reaching a peak brightness of magnitude 6.8. Nonetheless, even at its brightest, 7 Iris 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 7 Iris
The star charts below mark the path of 7 Iris across the sky around the time of its opposition.
This star chart is also available to download:
|Light-on-dark||PNG image||PDF document|
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The exact position of 7 Iris at the moment of opposition will be as follows:
|Asteroid 7 Iris||02h03m50s||+21°21'||Aries||6.8|
The coordinates above are given in J2000.0.
|The sky on 30 October 2017|
11 days old
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.
© NASA/Galileo 1993. Pictured asteroid is 243 Ida.