Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Jul–Aug 1981 morning apparition. It will be shining brightly at mag -0.3.

From Seattle , this apparition will not be one of the most prominent and very difficult to observe, reaching a peak altitude of 13° above the horizon at sunrise on 20 Jul 1981.

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Jul–Aug 1981 morning apparition of Mercury

21 Jun 1981 – Mercury at inferior solar conjunction
14 Jul 1981 – Mercury at greatest elongation west
18 Jul 1981 – Mercury at dichotomy
19 Jul 1981 – Mercury at highest altitude in morning sky
09 Aug 1981 – Mercury at superior solar conjunction

A graph of the phase of Mercury is available here.

Apparitions of Mercury

01 Feb 1981 – Evening apparition
15 Mar 1981 – Morning apparition
27 May 1981 – Evening apparition
14 Jul 1981 – Morning apparition
23 Sep 1981 – Evening apparition
02 Nov 1981 – Morning apparition
16 Jan 1982 – 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 06h27m30s 21°34'N Gemini 7.0"
Sun 07h53m +20°53' Gemini 31'28"

The coordinates above are given in J2000.0.

The sky on 20 January 2022
Twilight ends
Twilight begins

18-day old moon
Waning Gibbous


18 days old

Rise Culm. Set
Mercury 07:49 12:40 17:30
Venus 06:10 10:59 15:48
Moon 18:36 02:16 09:42
Mars 05:47 09:55 14:04
Jupiter 09:19 14:34 19:49
Saturn 08:31 13:14 17:57
All times shown in PST.


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

14 Jul 1981  –  Mercury at greatest elongation west
19 Jul 1981  –  Mercury at highest altitude in morning sky
08 Sep 1981  –  Mercury at highest altitude in evening sky
23 Sep 1981  –  Mercury at greatest elongation east

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