Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Jun–Jul 2021 morning apparition. It will be shining brightly at mag -0.2.

From Ashburn , this apparition will not be one of the most prominent and tricky to observe, reaching a peak altitude of 14° above the horizon at sunrise on 10 Jul 2021.

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Jun–Jul 2021 morning apparition of Mercury

10 Jun 2021 – Mercury at inferior solar conjunction
05 Jul 2021 – Mercury at greatest elongation west
09 Jul 2021 – Mercury at highest altitude in morning sky
09 Jul 2021 – Mercury at dichotomy
01 Aug 2021 – Mercury at superior solar conjunction

The table below lists the altitude of Mercury at sunrise over the course of the apparition. All times are given in Ashburn local time.

Date Sun
rises at
rises at
at sunrise
at sunrise
Mag Phase
22 Jun 202105:4205:01east2.610%
25 Jun 202105:4204:49east1.915%
28 Jun 202105:4404:3911°east1.321%
01 Jul 202105:4504:3212°east0.828%
04 Jul 202105:4604:2714°east0.435%
07 Jul 202105:4804:2514°east0.143%
10 Jul 202105:5004:2514°east-0.251%
13 Jul 202105:5204:2914°east-0.560%
16 Jul 202105:5404:3613°east-0.870%
19 Jul 202105:5704:4612°east-1.079%
22 Jul 202105:5905:0010°east-1.387%
25 Jul 202106:0205:17north-east-1.594%

Mercury will brighten rapidly at the start of its morning apparition as it emerges from inferior conjunction. Prior to its apparition, it passed between the Earth and Sun, at which time it had its unilluminated side turned towards the Earth and so appeared as a thin, barely illuminated crescent. As the apparition proceeds, this crescent waxes and becomes gibbous.

Since Mercury can only ever be observed in twilight, it is particularly difficult to find when it is in a thin crescent phase. Thus, it will be significantly easier to see in the days after it reaches its highest point in the sky – when it will show a gibbous phase – than in the days beforehand.

Altitude of Mercury at sunrise

A graph of the phase of Mercury is available here.

Apparitions of Mercury

23 Jan 2021 – Evening apparition
06 Mar 2021 – Morning apparition
17 May 2021 – Evening apparition
05 Jul 2021 – Morning apparition
13 Sep 2021 – Evening apparition
24 Oct 2021 – Morning apparition
07 Jan 2022 – 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 05h46m20s 21°19'N Taurus 7.1"
Sun 07h15m 22°18'N Gemini 31'27"

The coordinates above are given in J2000.0.

The sky on 9 Jul 2021

The sky on 9 July 2021
Twilight ends
Twilight begins

29-day old moon
Waxing Crescent


29 days old

Rise Culm. Set
Mercury 04:27 11:44 19:01
Venus 08:05 15:09 22:13
Moon 05:09 12:57 20:45
Mars 08:17 15:17 22:17
Jupiter 22:51 04:14 09:38
Saturn 21:55 02:57 07:59
All times shown in EDT.


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

09 Jul 2021  –  Mercury at highest altitude in morning sky
02 Sep 2021  –  Mercury at highest altitude in evening sky
13 Sep 2021  –  Mercury at greatest elongation east
25 Oct 2021  –  Mercury at highest altitude in morning sky

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