Mercury at dichotomy

Thu, 30 Mar 2017 at01:32 EDT(356 days ago)
05:32 UTC

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 Ashburn (click to change), it will be difficult to observe as it will appear no higher than 13° above the horizon. It will become visible at around 19:49 (EDT) as the dusk sky fades, 13° above your western horizon. It will then sink towards the horizon, setting 1 hour and 37 minutes after the Sun at 21:03.

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:

06 Mar 2017 19:15 EST – Mercury at superior solar conjunction
30 Mar 2017 01:32 EDT – Mercury at dichotomy
01 Apr 2017 01:52 EDT – Mercury at greatest elongation east
20 Apr 2017 01:48 EDT – 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 Ashburn local time.

Date Sun
sets at
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
23 Mar 201719:2020:4014°west
30 Mar 201719:2621:0617°west
06 Apr 201719:3321:0717°west
13 Apr 201719:4020:3910°west
20 Apr 201719:4719:48west
27 Apr 201719:5318:55-10°west
04 May 201720:0018:18-19°north-west
11 May 201720:0718:01-23°north-west
18 May 201720:1318:00-24°north-west
25 May 201720:1918:14-22°north-west
01 Jun 201720:2418:39-18°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 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 07:33 14:19 21:06
Venus 06:02 12:30 18:58
Moon 08:57 15:39 22:21
Mars 08:25 15:24 22:23
Jupiter 20:03 01:51 07:34
Saturn 01:40 06:27 11:13
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, 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
29 Jul 2017, 20:24 EDT  –  Mercury at greatest elongation east

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