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 -0.4.

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 8° 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:

11 Dec 2016 02:45 EST – Mercury at greatest elongation east
14 Dec 2016 11:40 EST – Mercury at dichotomy
28 Dec 2016 13:41 EST – 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
07 Dec 201616:4218:0111°south-west
14 Dec 201616:4318:0912°south-west
21 Dec 201616:4617:5410°south-west
28 Dec 201616:5017:02south-west
04 Jan 201716:5516:04-9°west
11 Jan 201717:0215:33-16°west
18 Jan 201717:1015:23-20°west
25 Jan 201717:1715:27-20°west
01 Feb 201717:2615:40-19°west
08 Feb 201717:3416:01-17°west
15 Feb 201717:4216:27-14°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 18h57m30s -24°16' Sagittarius 7.3"
Sun 17h29m -23°14' Ophiuchus 32'30"

The coordinates above are given in J2000.0.

The sky on 14 December 2016
Twilight ends
Twilight begins

15-day old moon
Waning Gibbous


15 days old

Rise Culm. Set
Mercury 08:54 13:32 18:09
Venus 10:25 15:18 20:11
Moon 18:11 00:17 07:23
Mars 11:13 16:30 21:47
Jupiter 02:03 07:46 13:29
Saturn 06:59 11:47 16:35
All times shown in EST.


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

11 Dec 2016, 02:45 EST  –  Mercury at greatest elongation east
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

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