© 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 Nov–Dec 2023 evening apparition. It will be shining brightly at mag -0.3.

From Cambridge , this apparition will not be one of the most prominent and very difficult to observe, reaching a peak altitude of 10° above the horizon at sunset on 9 Dec 2023.

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Nov–Dec 2023 evening apparition of Mercury

04 Dec 2023 – Mercury at greatest elongation east
08 Dec 2023 – Mercury at dichotomy
08 Dec 2023 – Mercury at highest altitude in evening sky
22 Dec 2023 – Mercury at inferior solar conjunction

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

Date Sun
sets at
Mercury
sets at
Altitude
at sunset
Direction
at sunset
Mag Phase
21 Nov 202316:1817:06south-west-0.486%
24 Nov 202316:1917:09south-west-0.483%
27 Nov 202316:1517:15south-west-0.479%
30 Nov 202316:1217:17south-west-0.573%
03 Dec 202316:1317:21south-west-0.567%
06 Dec 202316:1417:26south-west-0.458%
09 Dec 202316:1117:2810°south-west-0.347%
12 Dec 202316:1317:22south-west0.135%
15 Dec 202316:1417:15south-west0.921%
18 Dec 202316:1516:58south-west2.39%

Mercury will fade rapidly towards the end of the apparition as it heads towards inferior conjunction, when it will pass between the Earth and Sun. At inferior conjunction, the planet turns its unilluminated side towards the Earth, and so appears as a thin, barely illuminated crescent.

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 before it reaches its highest point in the sky than in the days after.

Altitude of Mercury at sunset

A graph of the phase of Mercury is available here.

Apparitions of Mercury

29 May 2023 – Morning apparition
09 Aug 2023 – Evening apparition
22 Sep 2023 – Morning apparition
04 Dec 2023 – Evening apparition
12 Jan 2024 – Morning apparition
24 Mar 2024 – Evening apparition
09 May 2024 – Morning 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 18h28m00s 25°02'S Sagittarius 7.3"
Sun 16h58m -22°40' Ophiuchus 32'28"

The coordinates above are given in J2000.0.

The sky on 08 December 2023
Sunrise
06:59
Sunset
16:11
Twilight ends
17:55
Twilight begins
05:19

25-day old moon
Waning Crescent

16%

25 days old

Planets
Rise Culm. Set
Mercury 08:43 13:06 17:29
Venus 03:28 08:51 14:14
Moon 02:17 08:01 13:34
Mars 06:34 11:10 15:46
Jupiter 14:05 20:54 03:43
Saturn 11:35 16:51 22:07
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

04 Dec 2023  –  Mercury at greatest elongation east
08 Dec 2023  –  Mercury at highest altitude in evening sky
08 Jan 2024  –  Mercury at highest altitude in morning sky
12 Jan 2024  –  Mercury at greatest elongation west

Image credit

© NASA/JPL/MESSENGER

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