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Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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Mercury will reach half phase in its 2017–2018 morning apparition. It will be shining brightly at mag -0.2.

From Fairfield , this apparition will be well placed but tricky to observe, reaching a peak altitude of 16° above the horizon at sunrise on 28 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 sunrise over the course of the apparition. All times are given in Fairfield local time.

Date Sun
sets at
rises at
at sunrise
at sunrise
16 Dec 201707:1306:34south-west
19 Dec 201707:1506:0610°west
22 Dec 201707:1705:4713°west
25 Dec 201707:1805:3615°west
28 Dec 201707:1905:3116°west
31 Dec 201707:2005:3115°west
03 Jan 201807:2005:3415°west
06 Jan 201807:2005:3914°west
09 Jan 201807:1905:4613°west
12 Jan 201807:1905:5311°west
15 Jan 201807:1806:0110°west
18 Jan 201807:1606:08west
21 Jan 201807:1506:15west
24 Jan 201807:1306:22west
27 Jan 201807:1006:28west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2017–2018 morning apparition of Mercury
27 Dec 2017 – Mercury at dichotomy
29 Dec 2017 – Mercury reaches highest point in morning sky
01 Jan 2018 – Mercury at greatest elongation west

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 16h53m40s -20°01' Ophiuchus 7.4"
Sun 18h26m -23°17' Sagittarius 32'31"

The coordinates above are given in J2000.0.

The sky on 27 December 2017
Twilight ends
Twilight begins

9-day old moon
Waxing Gibbous


9 days old

Rise Culm. Set
Mercury 05:31 10:20 15:10
Venus 07:06 11:40 16:13
Moon 12:48 19:10 00:26
Mars 02:52 08:04 13:16
Jupiter 03:16 08:23 13:30
Saturn 06:51 11:30 16:08
All times shown in EST.


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

28 Nov 2017  –  Mercury reaches highest point in evening sky
30 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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