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 05:34 (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 06:41.

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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:

12 Sep 2016 19:34 EDT – Mercury at inferior solar conjunction
28 Sep 2016 13:07 EDT – Mercury at greatest elongation west
28 Sep 2016 13:35 EDT – Mercury at dichotomy
27 Oct 2016 12:00 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
21 Sep 201606:5505:4711°west
28 Sep 201607:0105:3316°west
05 Oct 201607:0805:5014°west
12 Oct 201607:1506:2110°west
19 Oct 201607:2206:54west
26 Oct 201607:2907:27west
02 Nov 201607:3707:58-4°south-west
09 Nov 201606:4507:28-8°south-west
16 Nov 201606:5207:57-11°south-west
23 Nov 201607:0008:23-14°south-west
30 Nov 201607:0708:44-17°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 11h17m20s +05°47' Leo 7.1"
Sun 12h21m -02°17' Virgo 31'55"

The coordinates above are given in J2000.0.

The sky on 28 September 2016
Twilight ends
Twilight begins

27-day old moon
Waning Crescent


27 days old

Rise Culm. Set
Mercury 05:33 11:55 18:17
Venus 09:35 14:52 20:10
Moon 04:50 11:20 17:49
Mars 14:10 18:41 23:11
Jupiter 06:53 12:54 18:55
Saturn 12:25 17:17 22:09
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

28 Sep 2016, 13:07 EDT  –  Mercury at greatest elongation west
11 Dec 2016, 02:45 EST  –  Mercury at greatest elongation east
19 Jan 2017, 08:45 EST  –  Mercury at greatest elongation west
01 Apr 2017, 01:52 EDT  –  Mercury at greatest elongation east

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