Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Please wait
Loading 0/4
Click and drag to rotate
Mouse wheel to zoom in/out
Touch with mouse to dismiss
The sky at

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.1.

From Ashburn, it will be difficult to observe as it will appear no higher than 10° above the horizon. It will rise at 05:49 (EST) – 1 hour and 40 minutes before the Sun – and reach an altitude of 10° above the south-eastern horizon before fading from view as dawn breaks at around 06:59.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

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:

28 Dec 2016 13:41 EST – Mercury at inferior solar conjunction
13 Jan 2017 17:02 EST – Mercury at dichotomy
19 Jan 2017 08:45 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
06 Jan 201707:2806:0512°west
13 Jan 201707:2605:4814°west
20 Jan 201707:2305:5014°west
27 Jan 201707:1906:0111°west
03 Feb 201707:1306:12west
10 Feb 201707:0606:23west
17 Feb 201706:5706:31west
24 Feb 201706:4806:37west
03 Mar 201706:3806:41west
10 Mar 201706:2706:43-3°west
17 Mar 201707:1707:43-5°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 18h02m10s -21°11' Sagittarius 7.4"
Sun 19h42m -21°21' Sagittarius 32'31"

The coordinates above are given in J2000.0.

The sky on 13 January 2017
Twilight ends
Twilight begins

15-day old moon
Waning Gibbous


15 days old

Rise Culm. Set
Mercury 05:48 10:38 15:28
Venus 09:48 15:25 21:02
Moon 19:02 00:57 07:58
Mars 10:09 15:56 21:43
Jupiter 00:21 06:00 11:39
Saturn 05:17 10:04 14:51
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

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
17 May 2017, 18:58 EDT  –  Mercury at greatest elongation west

Image credit





Color scheme