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 evening sky, shining brightly at mag -0.5.

From Seattle, it will be difficult to observe as it will appear no higher than 12° above the horizon. It will become visible at around 19:34 (PST) as the dusk sky fades, 12° above your western horizon. It will then sink towards the horizon, setting 1 hour and 44 minutes after the Sun at 20:51.

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:

17 Feb 2018 04:12 PST – Mercury at superior solar conjunction
14 Mar 2018 01:59 PDT – Mercury at dichotomy
15 Mar 2018 03:18 PDT – Mercury at greatest elongation east
01 Apr 2018 10:47 PDT – Mercury at inferior 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 Mercury will remain up after sunset each night; all times are given in Seattle local time.

Date Sun
sets at
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
07 Mar 201817:5719:2413°west
14 Mar 201819:0720:5417°west
21 Mar 201819:1720:5315°west
28 Mar 201819:2720:16west
04 Apr 201819:3719:15-3°west
11 Apr 201819:4718:21-14°west
18 Apr 201819:5717:49-21°north-west
25 Apr 201820:0717:39-24°north-west
02 May 201820:1717:46-24°north-west
09 May 201820:2618:06-22°north-west
16 May 201820:3518:37-18°north-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 00h40m00s +06°20' Pisces 7.1"
Sun 23h36m -02°35' Pisces 32'10"

The coordinates above are given in J2000.0.

The sky on 14 March 2018
Twilight ends
Twilight begins

27-day old moon
Waning Crescent


27 days old

Rise Culm. Set
Mercury 07:48 14:21 20:54
Venus 08:00 14:16 20:32
Moon 06:21 11:16 16:11
Mars 03:20 07:31 11:42
Jupiter 00:20 05:04 09:48
Saturn 03:57 08:14 12:32
All times shown in PDT.


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

01 Jan 2018, 16:40 PST  –  Mercury at greatest elongation west
15 Mar 2018, 03:18 PDT  –  Mercury at greatest elongation east
29 Apr 2018, 08:06 PDT  –  Mercury at greatest elongation west
11 Jul 2018, 21:00 PDT  –  Mercury at greatest elongation east

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