Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

Mercury will reach half phase in its 2018 evening apparition. It will be shining brightly at mag -0.5.

From Fairfield , this apparition will be exceptionally well placed but tricky to observe, reaching a peak altitude of 18° above the horizon at sunset on 14 Mar 2018.

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

The table below lists how high Mercury will appear at sunset over the course of the apparition. All times are given in Fairfield local time.

Date Sun
sets at
sets at
at sunset
at sunset
24 Feb 201817:3618:03west
27 Feb 201817:4018:21west
02 Mar 201817:4318:3811°west
05 Mar 201817:4718:5513°west
08 Mar 201817:5019:1115°west
11 Mar 201818:5320:2316°west
14 Mar 201818:5720:3118°west
17 Mar 201819:0020:3417°west
20 Mar 201819:0320:3116°west
23 Mar 201819:0620:2113°west
26 Mar 201819:1020:0510°west
29 Mar 201819:1319:44west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2018 evening apparition of Mercury
17 Feb 2018 – Mercury at superior solar conjunction
14 Mar 2018 – Mercury at dichotomy
15 Mar 2018 – Mercury at greatest elongation east
16 Mar 2018 – Mercury reaches highest point in evening sky

Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is lost in the Sun's glare much of the time.

It is observable for only a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation. These apparitions repeat roughly once every 3–4 months.

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'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 07:39 14:05 20:31
Venus 07:47 14:00 20:12
Moon 05:47 10:53 16:00
Mars 02:40 07:15 11:50
Jupiter 23:44 04:48 09:48
Saturn 03:19 07:59 12:38
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

01 Jan 2018  –  Mercury at greatest elongation west
15 Mar 2018  –  Mercury at greatest elongation east
16 Mar 2018  –  Mercury reaches highest point in evening sky
29 Apr 2018  –  Mercury at greatest elongation west

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