© NASA/JPL/MESSENGER

Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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The sky at

Mercury will reach half phase in its Apr–May 2025 morning apparition. It will be shining brightly at mag 0.2.

From Fairfield , this apparition will not be one of the most prominent and very difficult to observe, reaching a peak altitude of 9° above the horizon at sunrise on 20 Apr 2025.

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Apr–May 2025 morning apparition of Mercury

24 Mar 2025 – Mercury at inferior solar conjunction
16 Apr 2025 – Mercury at highest altitude in morning sky
21 Apr 2025 – Mercury at greatest elongation west
24 Apr 2025 – Mercury at dichotomy
30 May 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 Fairfield local time.

Date Sun
rises at
Mercury
rises at
Altitude
at sunrise
Direction
at sunrise
Mag Phase
02 Apr 202506:3205:57east2.79%
05 Apr 202506:3005:44east1.915%
08 Apr 202506:2405:35east1.321%
11 Apr 202506:1805:27east1.027%
14 Apr 202506:1205:22east0.733%
17 Apr 202506:1005:17east0.538%
20 Apr 202506:0505:14east0.443%
23 Apr 202505:5905:09east0.348%
26 Apr 202505:5805:06east0.252%
29 Apr 202505:5205:05east0.157%
02 May 202505:4705:01east-0.061%
05 May 202505:4604:59east-0.266%
08 May 202505:4204:59east-0.370%
11 May 202505:3704:56east-0.475%
14 May 202505:3304:56east-0.680%
17 May 202505:3204:58east-0.885%

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

16 Nov 2024 – Evening apparition
25 Dec 2024 – Morning apparition
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

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 00h31m00s 0°23'N Cetus 7.5"
Sun 02h08m +12°58' Aries 31'48"

The coordinates above are given in J2000.0.

The sky on 24 April 2025
Sunrise
05:59
Sunset
19:43
Twilight ends
21:27
Twilight begins
04:15

26-day old moon
Waning Crescent

10%

26 days old

Planets
Rise Culm. Set
Mercury 05:11 11:16 17:20
Venus 04:21 10:27 16:32
Moon 04:13 10:05 16:03
Mars 11:38 19:03 02:28
Jupiter 08:27 15:57 23:26
Saturn 04:40 10:33 16:25
All times shown in EDT.

Source

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

21 Apr 2025  –  Mercury at greatest elongation west
26 Jun 2025  –  Mercury at highest altitude in evening sky
04 Jul 2025  –  Mercury at greatest elongation east
19 Aug 2025  –  Mercury at greatest elongation west

Image credit

© NASA/JPL/MESSENGER

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