© NASA/JPL/MESSENGER

Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Dec 2024–Jan 2025 morning apparition. It will be shining brightly at mag -0.3.

From Fairfield , this apparition will be well placed but tricky to observe, reaching a peak altitude of 16° above the horizon at sunrise on 22 Dec 2024.

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Dec 2024–Jan 2025 morning apparition of Mercury

05 Dec 2024 – Mercury at inferior solar conjunction
20 Dec 2024 – Mercury at dichotomy
21 Dec 2024 – Mercury at highest altitude in morning sky
25 Dec 2024 – 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 Fairfield local time.

Date Sun
rises at
Mercury
rises at
Altitude
at sunrise
Direction
at sunrise
Mag Phase
10 Dec 202407:0806:15south-east2.68%
13 Dec 202407:1005:5012°south-east1.019%
16 Dec 202407:1305:3615°south-east0.233%
19 Dec 202407:1205:2815°south-east-0.245%
22 Dec 202407:1405:2616°south-east-0.455%
25 Dec 202407:1705:2816°south-east-0.464%
28 Dec 202407:1505:3415°south-east-0.471%
31 Dec 202407:1805:4114°south-east-0.476%
03 Jan 202507:1605:4912°south-east-0.480%
06 Jan 202507:1905:5912°south-east-0.384%
09 Jan 202507:1706:0610°south-east-0.387%
12 Jan 202507:1506:13south-east-0.489%
15 Jan 202507:1706:20south-east-0.491%
18 Jan 202507:1506:31south-east-0.493%

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

21 Jul 2024 – Evening apparition
04 Sep 2024 – Morning apparition
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

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 16h25m20s 19°02'S Scorpius 7.3"
Sun 17h55m -23°25' Sagittarius 32'30"

The coordinates above are given in J2000.0.

The sky on 20 December 2024
Sunrise
07:12
Sunset
16:28
Twilight ends
18:08
Twilight begins
05:32

19-day old moon
Waning Gibbous

67%

19 days old

Planets
Rise Culm. Set
Mercury 05:28 10:21 15:14
Venus 10:10 15:06 20:02
Moon 21:00 04:11 11:05
Mars 18:59 02:28 09:56
Jupiter 15:20 22:45 06:11
Saturn 11:24 16:58 22:31
All times shown in EST.

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

20 Nov 2024  –  Mercury at highest altitude in evening sky
21 Dec 2024  –  Mercury at highest altitude in morning sky
25 Dec 2024  –  Mercury at greatest elongation west
08 Mar 2025  –  Mercury at highest altitude in evening sky

Image credit

© NASA/JPL/MESSENGER

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