Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.5.

From Ashburn, it will be difficult to observe as it will appear no higher than 11° above the horizon. It will rise at 05:21 (EST) – 1 hour and 30 minutes before the Sun – and reach an altitude of 11° above the eastern horizon before fading from view as dawn breaks at around 06:28.

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Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is very difficult to observe most of the time.

It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.

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 in coming weeks

The key moments in this apparition of Mercury are as follows:

26 Aug 2017 16:36 EDT – Mercury at inferior solar conjunction
12 Sep 2017 05:11 EDT – Mercury at greatest elongation west
12 Sep 2017 23:34 EDT – Mercury at dichotomy
08 Oct 2017 16:38 EDT – Mercury at superior solar conjunction

Over coming weeks, the distance between Mercury and the Sun will decrease each night as it sinks back into the Sun's glare. The table below lists how long before sunrise Mercury will rise each night; all times are given in Ashburn local time.

Date Sun
sets at
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
06 Sep 201706:4005:3111°west
13 Sep 201706:4605:1816°west
20 Sep 201706:5305:3613°west
27 Sep 201706:5906:10west
04 Oct 201707:0606:46west
11 Oct 201707:1207:20-1°west
18 Oct 201707:1907:52-6°west
25 Oct 201707:2708:22-11°south-west
01 Nov 201707:3408:50-14°south-west
08 Nov 201706:4208:17-17°south-west
15 Nov 201706:5008:39-19°south-west

A graph of the phase of Mercury is available here.

Mercury's position

The coordinates of Mercury when it reaches dichotomy will be:

Object Right Ascension Declination Constellation Angular Size
Mercury 10h18m20s +11°07' Leo 7.0"
Sun 11h24m +03°50' Leo 31'47"

The coordinates above are given in J2000.0.

The sky on 12 September 2017
Twilight ends
Twilight begins

22-day old moon
Waning Gibbous


22 days old

Rise Culm. Set
Mercury 05:18 11:58 18:38
Venus 04:22 11:16 18:10
Moon 23:14 06:13 13:12
Mars 05:29 12:09 18:48
Jupiter 09:36 15:12 20:48
Saturn 14:16 19:03 23:50
All times shown in EDT.


Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.


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

12 Sep 2017, 05:11 EDT  –  Mercury at greatest elongation west
23 Nov 2017, 21:22 EST  –  Mercury at greatest elongation east
01 Jan 2018, 19:40 EST  –  Mercury at greatest elongation west
15 Mar 2018, 06:18 EDT  –  Mercury at greatest elongation east

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