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Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

Mercury will reach half phase in its 2017 morning apparition. It will be shining brightly at mag 0.1.

From Cambridge , this apparition will not be one of the most prominent and very difficult to observe, reaching a peak altitude of 9° above the horizon at sunrise on 20 May 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 Cambridge local time.

Date Sun
sets at
rises at
at sunrise
at sunrise
02 May 201705:3905:01north-west
05 May 201705:3504:52north-west
08 May 201705:3104:45north-west
11 May 201705:2804:38north-west
14 May 201705:2504:32north-west
17 May 201705:2204:27north-west
20 May 201705:1904:23north-west
23 May 201705:1704:19north-west
26 May 201705:1504:17north-west
29 May 201705:1304:15north-west
01 Jun 201705:1104:15north-west
04 Jun 201705:1004:16north-west
07 Jun 201705:0904:19north-west
10 Jun 201705:0904:24north-west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2017 morning apparition of Mercury
20 Apr 2017 – Mercury at inferior solar conjunction
17 May 2017 – Mercury at greatest elongation west
23 May 2017 – Mercury at dichotomy
25 May 2017 – Mercury reaches highest point in morning sky

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 02h25m20s +11°01' Aries 7.3"
Sun 04h01m +20°38' Taurus 31'35"

The coordinates above are given in J2000.0.

The sky on 23 May 2017
Twilight ends
Twilight begins

27-day old moon
Waning Crescent


27 days old

Rise Culm. Set
Mercury 04:19 11:04 17:48
Venus 03:21 09:44 16:06
Moon 04:11 10:40 17:09
Mars 06:22 14:02 21:41
Jupiter 15:39 21:28 03:21
Saturn 21:41 02:22 06:59
All times shown in EDT.


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

17 May 2017  –  Mercury at greatest elongation west
25 May 2017  –  Mercury reaches highest point in morning sky
16 Jul 2017  –  Mercury reaches highest point in evening sky
29 Jul 2017  –  Mercury at greatest elongation east

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