Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Nov 2005–Jan 2006 morning apparition. It will be shining brightly at mag -0.4.

From Ashburn , this apparition will be well placed but tricky to observe, reaching a peak altitude of 17° above the horizon at sunrise on 10 Dec 2005.

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Nov 2005–Jan 2006 morning apparition of Mercury

24 Nov 2005 – Mercury at inferior solar conjunction
08 Dec 2005 – Mercury at dichotomy
09 Dec 2005 – Mercury at highest altitude in morning sky
12 Dec 2005 – Mercury at greatest elongation west

A graph of the phase of Mercury is available here.

Apparitions of Mercury

08 Jul 2005 – Evening apparition
23 Aug 2005 – Morning apparition
03 Nov 2005 – Evening apparition
12 Dec 2005 – Morning apparition
23 Feb 2006 – Evening apparition
08 Apr 2006 – Morning apparition
20 Jun 2006 – Evening apparition

Observing Mercury

Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is lost in the Sun's glare much of the time.

It is observable for only a few weeks each time it reaches greatest separation from the Sun – moments referred to as greatest elongation. These apparitions repeat roughly once every 3–4 months.

Mercury's phase

Mercury's phase varies depending on its position relative to the Earth. When it passes between the Earth and Sun, for example, the side that is turned towards the Earth is entirely unilluminated, like a new moon.

Conversely, when it lies opposite to the Earth in its orbit, passing almost behind the Sun, it appears fully illuminated, like a full moon. However, at this time it is also at its most distant from the Earth, so it is actually fainter than at other times.

Mercury shows an intermediate half phase – called dichotomy – at roughly the same moment that it appears furthest from the Sun, at greatest elongation. The exact times of the two events may differ by a few days, only because Mercury's orbit is not quite perfectly aligned with the ecliptic.

Mercury's position

The coordinates of Mercury when it reaches dichotomy will be:

Object Right Ascension Declination Constellation Angular Size
Mercury 15h38m30s 16°48'S Libra 7.3"
Sun 17h01m -22°46' Ophiuchus 32'28"

The coordinates above are given in J2000.0.

The sky on 23 January 2022
Twilight ends
Twilight begins

21-day old moon
Waning Gibbous


21 days old

Rise Culm. Set
Mercury 07:06 12:14 17:23
Venus 05:36 10:45 15:54
Moon 22:05 04:27 10:38
Mars 05:14 09:54 14:33
Jupiter 08:58 14:26 19:54
Saturn 08:00 13:05 18:09
All times shown in EST.


The circumstances of this event were computed using the DE430 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

05 Nov 2005  –  Mercury at highest altitude in evening sky
09 Dec 2005  –  Mercury at highest altitude in morning sky
12 Dec 2005  –  Mercury at greatest elongation west
23 Feb 2006  –  Mercury at greatest elongation east

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