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BAA Observing calendar

Mercury at inferior solar conjunction

Wednesday, 2017, December 13 - 01:43

Dominic Ford – originally published on

From our vantage point on the Earth, Mercury will appear very close to the Sun in the sky as it passes between the Sun and Earth.

At closest approach, Mercury and the Sun will appear at a separation of only 1°43', making Mercury totally unobservable for several weeks while it is lost in the Sun's glare.

Mercury will also pass perigee – the time when it is closest to the Earth – within a few days of the same time, since it will lie on exactly the same side of the Sun as the Earth in the Solar System. It will move to within a distance of 0.68 AU from the Earth, making it appear with its largest angular size. If it could be observed, it would measure 9.9 arcsec in diameter, whilst appearing completely unilluminated.

Mercury's reaching inferior conjunction marks the end of its apparition in the evening sky and its transition to become a morning object over the next few weeks.

The exact position of Mercury at the moment it passes solar conjunction will be:

Object Right Ascension Declination Constellation Angular Size
Mercury 17h21m10s -21°24' Ophiuchus 9.9"
Sun 17h20m -23°07' Ophiuchus 32'29"

The coordinates above are given in J2000.0.

The details of this observing event were provided courtesy of