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

In the southern hemisphere Mercury will be well placed for observation in the evening sky, shining brightly at mag -0.3.

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 6° above the horizon at dusk.

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:

23 Nov 2017 21:22 EST – Mercury at greatest elongation east
28 Nov 2017 04:03 EST – Mercury at dichotomy
12 Dec 2017 20:43 EST – Mercury at inferior solar conjunction
27 Dec 2017 18:23 EST – Mercury at dichotomy

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 Mercury will remain up after sunset each night; all times are given in Ashburn local time.

Date Sun
sets at
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
21 Nov 201716:4717:58south-west
28 Nov 201716:4418:0010°south-west
05 Dec 201716:4317:43south-west
12 Dec 201716:4316:55south-west
19 Dec 201716:4516:03-8°west
26 Dec 201716:4815:34-13°west
02 Jan 201816:5415:25-16°west
09 Jan 201817:0015:27-17°west
16 Jan 201817:0715:38-16°west
23 Jan 201817:1515:56-15°west
30 Jan 201817:2316:20-11°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 17h47m50s -25°31' Sagittarius 7.3"
Sun 16h16m -21°19' Scorpius 32'25"

The coordinates above are given in J2000.0.

The sky on 28 November 2017
Twilight ends
Twilight begins

10-day old moon
Waxing Gibbous


10 days old

Rise Culm. Set
Mercury 08:55 13:27 18:00
Venus 06:15 11:15 16:16
Moon 14:02 19:59 00:54
Mars 03:28 09:05 14:42
Jupiter 04:55 10:12 15:28
Saturn 08:41 13:26 18:11
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

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
29 Apr 2018, 11:06 EDT  –  Mercury at greatest elongation west

Image credit





Color scheme