Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.3.

From Fairfield, it will be difficult to observe as it will appear no higher than 11° above the horizon. It will rise at 05:19 (EST) – 1 hour and 48 minutes before the Sun – and reach an altitude of 11° above the south-eastern horizon before fading from view as dawn breaks at around 06:38.

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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 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
15 Dec 2018 10:17 EST – Mercury at greatest elongation west

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 Fairfield local time.

Date Sun
sets at
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
04 Dec 201807:0005:4111°west
11 Dec 201807:0605:2015°west
18 Dec 201807:1105:2815°west
25 Dec 201807:1505:4712°west
01 Jan 201907:1706:10west
08 Jan 201907:1706:31south-west
15 Jan 201907:1506:51south-west
22 Jan 201907:1107:06south-west
29 Jan 201907:0607:17-2°south-west
05 Feb 201906:5907:23-4°south-west
12 Feb 201906:5107:25-6°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 15h48m00s -17°19' Libra 7.3"
Sun 17h12m -22°59' Ophiuchus 32'29"

The coordinates above are given in J2000.0.

The sky on 11 December 2018
Twilight ends
Twilight begins

4-day old moon
Waxing Crescent


4 days old

Rise Culm. Set
Mercury 05:20 10:20 15:20
Venus 03:23 08:48 14:12
Moon 10:28 15:21 20:14
Mars 12:00 17:41 23:23
Jupiter 06:07 10:53 15:39
Saturn 08:31 13:09 17:47
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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