Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

In the southern hemisphere Mercury will be well placed for observation in the dawn sky, shining brightly at mag 0.1.

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 3° above the horizon at dawn.

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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 May 2017 18:58 EDT – Mercury at greatest elongation west
23 May 2017 10:35 EDT – Mercury at dichotomy
21 Jun 2017 10:02 EDT – Mercury at superior 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 before sunrise Mercury will rise each night; all times are given in Ashburn local time.

Date Sun
sets at
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
16 May 201705:5504:5810°north-west
23 May 201705:4904:5010°north-west
30 May 201705:4504:4710°north-west
06 Jun 201705:4304:52north-west
13 Jun 201705:4205:07north-west
20 Jun 201705:4205:35north-west
27 Jun 201705:4406:14-5°north-west
04 Jul 201705:4806:55-11°west
11 Jul 201705:5207:32-16°west
18 Jul 201705:5708:01-21°west
25 Jul 201706:0208:21-23°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 02h25m20s +11°01' Aries 7.3"
Sun 04h01m +20°38' Taurus 31'35"

The coordinates above are given in J2000.0.

The sky on 23 May 2017
Twilight ends
Twilight begins

27-day old moon
Waning Crescent


27 days old

Rise Culm. Set
Mercury 04:50 11:29 18:09
Venus 03:49 10:09 16:29
Moon 04:38 11:06 17:35
Mars 06:59 14:27 21:55
Jupiter 16:03 21:54 03:48
Saturn 21:56 02:47 07:34
All times shown in EDT.


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

17 May 2017, 18:58 EDT  –  Mercury at greatest elongation west
29 Jul 2017, 20:24 EDT  –  Mercury at greatest elongation east
12 Sep 2017, 05:11 EDT  –  Mercury at greatest elongation west
23 Nov 2017, 21:22 EST  –  Mercury at greatest elongation east

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