Mercury at dichotomy

Thu, 27 Jul 2017 at19:18 EDT(233 days ago)
23:18 UTC

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 evening sky, shining brightly at mag 0.2.

From Ashburn (click to change) however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 7° above the horizon at dusk.

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:

27 Jul 2017 19:18 EDT – Mercury at dichotomy
29 Jul 2017 20:24 EDT – Mercury at greatest elongation east
26 Aug 2017 16:36 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
20 Jul 201720:2721:4814°west
27 Jul 201720:2221:3713°west
03 Aug 201720:1521:1912°west
10 Aug 201720:0720:53west
17 Aug 201719:5820:19west
24 Aug 201719:4919:39-1°west
31 Aug 201719:3819:04-7°west
07 Sep 201719:2818:43-8°west
14 Sep 201719:1618:38-7°west
21 Sep 201719:0518:41-5°west
28 Sep 201718:5418:44-2°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 10h14m20s +09°59' Leo 7.4"
Sun 08h28m +19°02' Cancer 31'30"

The coordinates above are given in J2000.0.

The sky on 27 July 2017
Sunrise 06:05
Sunset 20:25
Twilight ends
Twilight begins

4-day old moon
Age of Moon
4 days

All times shown in EDT.
Rise Culm. Set
Mercury 08:24 15:01 21:37
Venus 03:09 10:27 17:45
Moon 10:52 17:01 23:11
Mars 06:02 13:15 20:28
Jupiter 12:02 17:48 23:34
Saturn 17:22 22:09 03:01


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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