Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Aug–Sep 2025 morning apparition. It will be shining brightly at mag -0.4.

From Cambridge , this apparition will be well placed but tricky to observe, reaching a peak altitude of 16° above the horizon at sunrise on 22 Aug 2025.

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Aug–Sep 2025 morning apparition of Mercury

31 Jul 2025 – Mercury at inferior solar conjunction
19 Aug 2025 – Mercury at greatest elongation west
21 Aug 2025 – Mercury at dichotomy
21 Aug 2025 – Mercury at highest altitude in morning sky
13 Sep 2025 – 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 Cambridge local time.

Date Sun
rises at
rises at
at sunrise
at sunrise
Mag Phase
07 Aug 202505:4105:05east3.36%
10 Aug 202505:4704:4810°east2.212%
13 Aug 202505:5004:3213°east1.220%
16 Aug 202505:5204:2515°east0.530%
19 Aug 202505:5504:2015°east-0.141%
22 Aug 202505:5704:2416°east-0.553%
25 Aug 202506:0304:3315°east-0.865%
28 Aug 202506:0504:4514°east-1.076%
31 Aug 202506:0704:5811°east-1.285%
03 Sep 202506:1305:1710°east-1.392%
06 Sep 202506:1405:33east-1.596%

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

07 Mar 2025 – Evening apparition
21 Apr 2025 – Morning apparition
04 Jul 2025 – Evening apparition
19 Aug 2025 – Morning apparition
29 Oct 2025 – Evening apparition
07 Dec 2025 – Morning apparition
19 Feb 2026 – 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 08h48m50s 17°26'N Cancer 7.0"
Sun 10h01m +12°04' Leo 31'37"

The coordinates above are given in J2000.0.

The sky on 21 August 2025
Twilight ends
Twilight begins

28-day old moon
Waning Crescent


28 days old

Rise Culm. Set
Mercury 04:27 11:37 18:47
Venus 03:10 10:34 17:58
Moon 03:46 11:35 19:06
Mars 09:27 15:19 21:10
Jupiter 02:19 09:52 17:24
Saturn 20:55 02:51 08:46
All times shown in EDT.


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

19 Aug 2025  –  Mercury at greatest elongation west
21 Aug 2025  –  Mercury at highest altitude in morning sky
29 Oct 2025  –  Mercury at greatest elongation east
01 Nov 2025  –  Mercury at highest altitude in evening sky

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