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 2017 evening apparition. It will be shining brightly at mag -0.5.

From Cambridge , this apparition will be exceptionally well placed but tricky to observe, reaching a peak altitude of 18° above the horizon at sunset on 1 Apr 2017.

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 Cambridge local time.

Date Sun
sets at
sets at
at sunset
at sunset
14 Mar 201718:4819:23west
17 Mar 201718:5219:4210°west
20 Mar 201718:5520:0112°west
23 Mar 201718:5820:1815°west
26 Mar 201719:0220:3216°west
29 Mar 201719:0520:4318°west
01 Apr 201719:0920:5018°west
04 Apr 201719:1220:5118°west
07 Apr 201719:1520:4716°west
10 Apr 201719:1920:3613°west
13 Apr 201719:2220:2010°west
16 Apr 201719:2620:00west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2017 evening apparition of Mercury
06 Mar 2017 – Mercury at superior solar conjunction
30 Mar 2017 – Mercury at dichotomy
01 Apr 2017 – Mercury at greatest elongation east
03 Apr 2017 – 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 01h41m00s +12°52' Pisces 7.1"
Sun 00h34m +03°45' Pisces 32'01"

The coordinates above are given in J2000.0.

The sky on 30 March 2017
Twilight ends
Twilight begins

2-day old moon
Waxing Crescent


2 days old

Rise Culm. Set
Mercury 07:02 13:54 20:46
Venus 05:33 12:04 18:36
Moon 08:25 15:12 22:00
Mars 07:52 14:59 22:05
Jupiter 19:40 01:26 07:07
Saturn 01:25 06:01 10:38
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

19 Jan 2017  –  Mercury at greatest elongation west
01 Apr 2017  –  Mercury at greatest elongation east
03 Apr 2017  –  Mercury reaches highest point in evening sky
17 May 2017  –  Mercury at greatest elongation west

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