Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

In the southern hemisphere Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.0.

From Ashburn however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 8° above the horizon at dawn.

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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:

14 Jan 2016 08:59 EST – Mercury at inferior solar conjunction
01 Feb 2016 07:12 EST – Mercury at dichotomy
06 Feb 2016 22:30 EST – Mercury at greatest elongation west

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
25 Jan 201607:2105:5912°west
01 Feb 201607:1605:4613°west
08 Feb 201607:0905:4712°west
15 Feb 201607:0105:5310°west
22 Feb 201606:5206:00west
29 Feb 201606:4206:05west
07 Mar 201606:3206:09west
14 Mar 201607:2107:11west
21 Mar 201607:1007:13-1°west
28 Mar 201606:5907:15-3°west
04 Apr 201606:4807:17-5°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 19h13m20s -20°40' Sagittarius 7.5"
Sun 20h57m -17°14' Capricornus 32'27"

The coordinates above are given in J2000.0.

The sky on 01 February 2016
Twilight ends
Twilight begins

22-day old moon
Waning Crescent


22 days old

Rise Culm. Set
Mercury 05:46 10:38 15:30
Venus 05:25 10:10 14:56
Moon 00:59 06:21 11:44
Mars 01:04 06:17 11:30
Jupiter 20:37 02:59 09:16
Saturn 03:24 08:16 13:07
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

29 Dec 2015, 00:59 EST  –  Mercury at greatest elongation east
06 Feb 2016, 22:30 EST  –  Mercury at greatest elongation west
18 Apr 2016, 07:34 EDT  –  Mercury at greatest elongation east
09 May 2016, 10:57 EDT  –  Transit of Mercury

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