© 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 1984–Jan 1985 morning apparition. It will be shining brightly at mag -0.2.

From Ashburn , this apparition will be well placed but tricky to observe, reaching a peak altitude of 16° above the horizon at sunrise on 30 Dec 1984.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

Dec 1984–Jan 1985 morning apparition of Mercury

14 Dec 1984 – Mercury at inferior solar conjunction
29 Dec 1984 – Mercury at dichotomy
01 Jan 1985 – Mercury at highest altitude in morning sky
03 Jan 1985 – Mercury at greatest elongation west

A graph of the phase of Mercury is available here.

Apparitions of Mercury

31 Jul 1984 – Evening apparition
13 Sep 1984 – Morning apparition
25 Nov 1984 – Evening apparition
03 Jan 1985 – Morning apparition
16 Mar 1985 – Evening apparition
01 May 1985 – Morning apparition
13 Jul 1985 – 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 17h02m00s 20°15'S Ophiuchus 7.4"
Sun 18h36m -23°10' Sagittarius 32'31"

The coordinates above are given in J2000.0.

The sky on 25 January 2022
Sunrise
07:20
Sunset
17:22
Twilight ends
18:55
Twilight begins
05:47

23-day old moon
Waning Crescent

43%

23 days old

Planets
Rise Culm. Set
Mercury 06:49 11:56 17:03
Venus 05:27 10:35 15:44
Moon 00:18 05:59 11:31
Mars 05:13 09:52 14:32
Jupiter 08:51 14:20 19:48
Saturn 07:53 12:58 18:03
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

03 Dec 1984  –  Mercury at highest altitude in evening sky
01 Jan 1985  –  Mercury at highest altitude in morning sky
03 Jan 1985  –  Mercury at greatest elongation west
16 Mar 1985  –  Mercury at highest altitude in evening sky

Image credit

© NASA/JPL/MESSENGER

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