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 evening apparition. It will be shining brightly at mag 0.2.

From Fairfield , this apparition will not be one of the most prominent and tricky to observe, reaching a peak altitude of 14° above the horizon at sunset on 17 Jul 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 Fairfield local time.

Date Sun
sets at
sets at
at sunset
at sunset
29 Jun 201720:2821:09north-west
02 Jul 201720:2821:21north-west
05 Jul 201720:2721:2910°west
08 Jul 201720:2621:3412°west
11 Jul 201720:2521:3813°west
14 Jul 201720:2321:3913°west
17 Jul 201720:2221:3814°west
20 Jul 201720:2021:3613°west
23 Jul 201720:1721:3213°west
26 Jul 201720:1521:2713°west
29 Jul 201720:1221:2013°west
01 Aug 201720:0921:1311°west
04 Aug 201720:0521:0410°west
07 Aug 201720:0220:53west
10 Aug 201719:5820:42west
13 Aug 201719:5420:28west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2017 evening apparition of Mercury
21 Jun 2017 – Mercury at superior solar conjunction
15 Jul 2017 – Mercury reaches highest point in evening sky
27 Jul 2017 – Mercury at dichotomy
29 Jul 2017 – Mercury at greatest elongation east

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 10h14m20s +09°59' Leo 7.4"
Sun 08h28m +19°02' Cancer 31'30"

The coordinates above are given in J2000.0.

The sky on 27 July 2017
Twilight ends
Twilight begins

4-day old moon
Waxing Crescent


4 days old

Rise Culm. Set
Mercury 08:05 14:44 21:23
Venus 02:46 10:10 17:34
Moon 10:32 16:44 22:56
Mars 05:40 12:58 20:17
Jupiter 11:47 17:31 23:16
Saturn 17:11 21:52 02:38
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

16 Jul 2017  –  Mercury reaches highest point in evening sky
29 Jul 2017  –  Mercury at greatest elongation east
12 Sep 2017  –  Mercury at greatest elongation west
14 Sep 2017  –  Mercury reaches highest point in morning sky

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