© 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 2027–Jan 2028 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 18 Jan 2028.

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Dec 2027–Jan 2028 evening apparition of Mercury

11 Dec 2027 – Mercury at superior solar conjunction
17 Jan 2028 – Mercury at highest altitude in evening sky
17 Jan 2028 – Mercury at greatest elongation east
19 Jan 2028 – Mercury at dichotomy
02 Feb 2028 – 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
31 Dec 202716:5617:36south-west-0.894%
03 Jan 202816:5817:48south-west-0.892%
06 Jan 202817:0118:01south-west-0.888%
09 Jan 202817:0318:1311°south-west-0.883%
12 Jan 202817:0618:2512°south-west-0.876%
15 Jan 202817:1018:3414°south-west-0.867%
18 Jan 202817:1318:4114°south-west-0.655%
21 Jan 202817:1618:4314°south-west-0.341%
24 Jan 202817:2018:3713°south-west0.427%
27 Jan 202817:2318:2410°south-west1.613%
30 Jan 202817:2718:02south-west3.54%

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

15 Jul 2027 – Morning apparition
24 Sep 2027 – Evening apparition
04 Nov 2027 – Morning apparition
17 Jan 2028 – Evening apparition
27 Feb 2028 – Morning apparition
09 May 2028 – Evening apparition
26 Jun 2028 – 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 21h17m50s 15°47'S Capricornus 7.2"
Sun 20h01m -20°29' Sagittarius 32'30"

The coordinates above are given in J2000.0.

The sky on 19 January 2028
Sunrise
07:23
Sunset
17:14
Twilight ends
18:48
Twilight begins
05:49

23-day old moon
Waning Crescent

39%

23 days old

Planets
Rise Culm. Set
Mercury 08:23 13:34 18:44
Venus 09:23 14:53 20:22
Moon 01:16 06:26 11:29
Mars 08:13 13:15 18:16
Jupiter 21:57 04:07 10:18
Saturn 11:14 17:36 23:59
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

17 Jan 2028  –  Mercury at greatest elongation east
20 Feb 2028  –  Mercury at highest altitude in morning sky
27 Feb 2028  –  Mercury at greatest elongation west
07 May 2028  –  Mercury at highest altitude in evening sky

Image credit

© NASA/JPL/MESSENGER

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