Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Jan–Feb 2023 morning apparition. It will be shining brightly at mag -0.1.

From San Diego , this apparition will not be one of the most prominent and tricky to observe, reaching a peak altitude of 16° above the horizon at sunrise on 25 Jan 2023.

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Jan–Feb 2023 morning apparition of Mercury

07 Jan 2023 – Mercury at inferior solar conjunction
24 Jan 2023 – Mercury at highest altitude in morning sky
24 Jan 2023 – Mercury at dichotomy
30 Jan 2023 – Mercury at greatest elongation west

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

Date Sun
rises at
rises at
at sunrise
at sunrise
Mag Phase
10 Jan 202306:4906:13south-east3.64%
13 Jan 202306:4905:4811°south-east1.912%
16 Jan 202306:4805:3014°south-east0.823%
19 Jan 202306:4705:1815°south-east0.334%
22 Jan 202306:4605:1216°south-east0.044%
25 Jan 202306:4505:0916°south-east-0.152%
28 Jan 202306:4305:0816°south-east-0.159%
31 Jan 202306:4205:1016°south-east-0.264%
03 Feb 202306:4005:1315°south-east-0.269%
06 Feb 202306:3705:1714°south-east-0.173%
09 Feb 202306:3505:2113°south-east-0.177%
12 Feb 202306:3205:2512°south-east-0.280%
15 Feb 202306:2905:2911°south-east-0.283%
18 Feb 202306:2605:33south-east-0.285%
21 Feb 202306:2305:37south-east-0.387%
24 Feb 202306:2005:40south-east-0.489%
27 Feb 202306:1605:44east-0.591%

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

27 Aug 2022 – Evening apparition
08 Oct 2022 – Morning apparition
21 Dec 2022 – Evening apparition
30 Jan 2023 – Morning apparition
11 Apr 2023 – Evening apparition
29 May 2023 – Morning apparition
09 Aug 2023 – 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 18h42m50s 21°06'S Sagittarius 7.5"
Sun 20h25m 19°14'S Capricornus 32'29"

The coordinates above are given in J2000.0.

The sky on 24 Jan 2023

The sky on 24 January 2023
Twilight ends
Twilight begins

3-day old moon
Waxing Crescent


3 days old

Rise Culm. Set
Mercury 05:11 10:16 15:22
Venus 08:05 13:31 18:57
Moon 09:07 14:53 20:47
Mars 12:46 19:57 03:08
Jupiter 09:46 15:50 21:55
Saturn 07:57 13:22 18:47
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

24 Jan 2023  –  Mercury at highest altitude in morning sky
30 Jan 2023  –  Mercury at greatest elongation west
11 Apr 2023  –  Mercury at highest altitude in evening sky
11 Apr 2023  –  Mercury at greatest elongation east

Image credit



San Diego



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