Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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Mercury will reach half phase in its 2017 evening apparition. It will be shining brightly at mag -0.3.

From Washington , this apparition will not be one of the most prominent and very difficult to observe, reaching a peak altitude of 11° above the horizon at sunset on 1 Dec 2017.

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

The table below lists how high Mercury will appear at sunset over the course of the apparition. All times are given in Washington local time.

Date Sun
sets at
sets at
at sunset
at sunset
29 Oct 201718:3919:10south-west
01 Nov 201718:3519:11south-west
04 Nov 201718:3219:13south-west
07 Nov 201717:2918:14south-west
10 Nov 201717:2718:17south-west
13 Nov 201717:2418:19south-west
16 Nov 201717:2218:2210°south-west
19 Nov 201717:2018:2610°south-west
22 Nov 201717:1818:2811°south-west
25 Nov 201717:1718:3011°south-west
28 Nov 201717:1618:3011°south-west
01 Dec 201717:1518:2711°south-west
04 Dec 201717:1518:2010°south-west
07 Dec 201717:1418:07south-west
10 Dec 201717:1517:47south-west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2017 evening apparition of Mercury
23 Nov 2017 – Mercury at greatest elongation east
26 Nov 2017 – Mercury reaches highest point in evening sky
28 Nov 2017 – Mercury at dichotomy
12 Dec 2017 – Mercury at inferior solar conjunction

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 days 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 hours, 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 17h47m50s -25°31' Sagittarius 7.3"
Sun 16h16m -21°19' Scorpius 32'25"

The coordinates above are given in J2000.0.

The sky on 28 November 2017
Twilight ends
Twilight begins

10-day old moon
Waxing Gibbous


10 days old

Rise Culm. Set
Mercury 09:12 13:51 18:30
Venus 06:35 11:40 16:44
Moon 14:29 20:28 01:24
Mars 03:51 09:29 15:08
Jupiter 05:16 10:35 15:55
Saturn 09:00 13:50 18:40
All times shown in MST.


The circumstances of this event were computed using the DE405 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

27 Nov 2017  –  Mercury reaches highest point in evening sky
29 Dec 2017  –  Mercury reaches highest point in morning sky
01 Jan 2018  –  Mercury at greatest elongation west
15 Mar 2018  –  Mercury at greatest elongation east

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