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 4° 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:

06 Nov 2018 09:59 EST – Mercury at greatest elongation east
11 Nov 2018 11:22 EST – Mercury at dichotomy
27 Nov 2018 04:09 EST – Mercury at inferior solar conjunction
11 Dec 2018 07:31 EST – Mercury at dichotomy

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
04 Nov 201817:0218:03south-west
11 Nov 201816:5517:59south-west
18 Nov 201816:4917:42south-west
25 Nov 201816:4517:02south-west
02 Dec 201816:4316:15-5°west
09 Dec 201816:4215:46-11°west
16 Dec 201816:4415:34-13°west
23 Dec 201816:4615:34-14°west
30 Dec 201816:5115:40-13°west
06 Jan 201916:5715:53-11°west
13 Jan 201917:0416:13-10°south-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 16h37m20s -24°46' Ophiuchus 7.4"
Sun 15h06m -17°27' Libra 32'18"

The coordinates above are given in J2000.0.

The sky on 11 November 2018
Twilight ends
Twilight begins

4-day old moon
Waxing Crescent


4 days old

Rise Culm. Set
Mercury 08:48 13:24 17:59
Venus 04:54 10:17 15:40
Moon 10:31 15:20 20:10
Mars 13:30 18:46 00:03
Jupiter 07:44 12:40 17:36
Saturn 10:26 15:10 19:54
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

06 Nov 2018, 09:59 EST  –  Mercury at greatest elongation east
15 Dec 2018, 10:17 EST  –  Mercury at greatest elongation west
26 Feb 2019, 16:29 EST  –  Mercury at greatest elongation east
11 Apr 2019, 11:37 EDT  –  Mercury at greatest elongation west

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