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 1987 morning apparition. It will be shining brightly at mag -0.3.

From Seattle , this apparition will be well placed but tricky to observe, reaching a peak altitude of 14° above the horizon at sunrise on 28 Jul 1987.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

Jul–Aug 1987 morning apparition of Mercury

03 Jul 1987 – Mercury at inferior solar conjunction
25 Jul 1987 – Mercury at greatest elongation west
27 Jul 1987 – Mercury at highest altitude in morning sky
28 Jul 1987 – Mercury at dichotomy
19 Aug 1987 – Mercury at superior solar conjunction

A graph of the phase of Mercury is available here.

Apparitions of Mercury

11 Feb 1987 – Evening apparition
26 Mar 1987 – Morning apparition
07 Jun 1987 – Evening apparition
25 Jul 1987 – Morning apparition
03 Oct 1987 – Evening apparition
13 Nov 1987 – Morning apparition
26 Jan 1988 – 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 07h09m20s 21°08'N Gemini 7.0"
Sun 08h31m +18°53' Cancer 31'30"

The coordinates above are given in J2000.0.

The sky on 21 January 2022
Twilight ends
Twilight begins

19-day old moon
Waning Gibbous


19 days old

Rise Culm. Set
Mercury 07:41 12:31 17:21
Venus 06:05 10:54 15:43
Moon 19:47 03:02 10:03
Mars 05:46 09:55 14:03
Jupiter 09:15 14:31 19:47
Saturn 08:27 13:11 17:54
All times shown in PST.


Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.


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

27 Jul 1987  –  Mercury at highest altitude in morning sky
24 Sep 1987  –  Mercury at highest altitude in evening sky
03 Oct 1987  –  Mercury at greatest elongation east
12 Nov 1987  –  Mercury at highest altitude in morning sky

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