© 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 Jan–Feb 1990 morning apparition. It will be shining brightly at mag -0.1.

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 sunrise on 26 Jan 1990.

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

Jan–Feb 1990 morning apparition of Mercury

08 Jan 1990 – Mercury at inferior solar conjunction
24 Jan 1990 – Mercury at highest altitude in morning sky
26 Jan 1990 – Mercury at dichotomy
31 Jan 1990 – Mercury at greatest elongation west

A graph of the phase of Mercury is available here.

Apparitions of Mercury

28 Aug 1989 – Evening apparition
10 Oct 1989 – Morning apparition
23 Dec 1989 – Evening apparition
31 Jan 1990 – Morning apparition
13 Apr 1990 – Evening apparition
31 May 1990 – Morning apparition
11 Aug 1990 – 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 18h51m50s 21°01'S Sagittarius 7.5"
Sun 20h34m -18°41' Capricornus 32'29"

The coordinates above are given in J2000.0.

The sky on 23 January 2022
Sunrise
07:21
Sunset
17:19
Twilight ends
18:53
Twilight begins
05:48

21-day old moon
Waning Gibbous

59%

21 days old

Planets
Rise Culm. Set
Mercury 07:06 12:14 17:23
Venus 05:36 10:45 15:54
Moon 22:05 04:27 10:38
Mars 05:14 09:54 14:33
Jupiter 08:58 14:26 19:54
Saturn 08:00 13:05 18:09
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

25 Jan 1990  –  Mercury at highest altitude in morning sky
31 Jan 1990  –  Mercury at greatest elongation west
08 Apr 1990  –  Mercury at highest altitude in evening sky
13 Apr 1990  –  Mercury at greatest elongation east

Image credit

© NASA/JPL/MESSENGER

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