© 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 2021–Jan 2022 evening apparition. It will be shining brightly at mag -0.5.

From Ashburn , this apparition will not be one of the most prominent and tricky to observe, reaching a peak altitude of 14° above the horizon at sunset on 9 Jan 2022.

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Dec 2021–Jan 2022 evening apparition of Mercury

28 Nov 2021 – Mercury at superior solar conjunction
07 Jan 2022 – Mercury at greatest elongation east
09 Jan 2022 – Mercury at dichotomy
11 Jan 2022 – Mercury at highest altitude in evening sky
23 Jan 2022 – 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 Ashburn local time.

Date Sun
sets at
Mercury
sets at
Altitude
at sunset
Direction
at sunset
Mag Phase
19 Dec 202116:5117:26south-west-0.795%
22 Dec 202116:5417:37south-west-0.792%
25 Dec 202116:5317:48south-west-0.789%
28 Dec 202116:5517:59south-west-0.785%
31 Dec 202116:5818:0911°south-west-0.780%
03 Jan 202217:0018:2012°south-west-0.773%
06 Jan 202217:0318:2713°south-west-0.763%
09 Jan 202217:0518:3614°south-west-0.552%
12 Jan 202217:1118:3413°south-west-0.238%
15 Jan 202217:1318:2712°south-west0.623%
18 Jan 202217:1418:09south-west2.011%

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

05 Jul 2021 – Morning apparition
13 Sep 2021 – Evening apparition
24 Oct 2021 – Morning apparition
07 Jan 2022 – Evening apparition
16 Feb 2022 – Morning apparition
29 Apr 2022 – Evening apparition
16 Jun 2022 – 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 20h41m50s 18°42'S Capricornus 7.2"
Sun 19h22m -22°05' Sagittarius 32'31"

The coordinates above are given in J2000.0.

The sky on 09 January 2022
Sunrise
07:29
Sunset
17:05
Twilight ends
18:41
Twilight begins
05:53

7-day old moon
Waxing Gibbous

52%

7 days old

Planets
Rise Culm. Set
Mercury 08:36 13:36 18:36
Venus 07:05 12:10 17:15
Moon 11:55 18:17 00:54
Mars 05:26 10:08 14:50
Jupiter 09:46 15:10 20:35
Saturn 08:51 13:54 18:56
All times shown in EST.

Warning

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.

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

07 Jan 2022  –  Mercury at greatest elongation east
11 Jan 2022  –  Mercury at highest altitude in evening sky
08 Feb 2022  –  Mercury at highest altitude in morning sky
16 Feb 2022  –  Mercury at greatest elongation west

Image credit

© NASA/JPL/MESSENGER

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