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 evening sky, shining brightly at mag 0.2.

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 5° above the horizon at dusk.

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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:

16 Aug 2016 11:19 EDT – Mercury at greatest elongation east
17 Aug 2016 06:44 EDT – Mercury at dichotomy
12 Sep 2016 19:34 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
10 Aug 201620:0721:1412°west
17 Aug 201619:5820:5811°west
24 Aug 201619:4820:37west
31 Aug 201619:3820:09west
07 Sep 201619:2719:33west
14 Sep 201619:1618:55-4°west
21 Sep 201619:0518:28-7°west
28 Sep 201618:5318:17-7°west
05 Oct 201618:4218:15-5°west
12 Oct 201618:3218:15-3°west
19 Oct 201618:2218:15-2°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 11h27m40s +01°16' Leo 7.5"
Sun 09h48m +13°16' Leo 31'35"

The coordinates above are given in J2000.0.

The sky on 17 August 2016
Twilight ends
Twilight begins

15-day old moon
Waxing Gibbous


15 days old

Rise Culm. Set
Mercury 08:45 14:52 20:58
Venus 08:02 14:29 20:56
Moon 19:39 00:07 05:19
Mars 15:00 19:38 00:19
Jupiter 08:54 15:07 21:19
Saturn 15:02 19:55 00:52
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

16 Aug 2016, 11:19 EDT  –  Mercury at greatest elongation east
28 Sep 2016, 13:07 EDT  –  Mercury at greatest elongation west
11 Dec 2016, 02:45 EST  –  Mercury at greatest elongation east
19 Jan 2017, 08:45 EST  –  Mercury at greatest elongation west

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