Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Aug–Sep 2018 morning apparition. It will be shining brightly at mag -0.5.

From Ashburn , this apparition will be well placed but tricky to observe, reaching a peak altitude of 16° above the horizon at sunrise on 29 Aug 2018.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

Aug–Sep 2018 morning apparition of Mercury

08 Aug 2018 – Mercury at inferior solar conjunction
26 Aug 2018 – Mercury at greatest elongation west
28 Aug 2018 – Mercury at dichotomy
28 Aug 2018 – Mercury at highest altitude in morning sky
20 Sep 2018 – Mercury at superior solar conjunction

The table below lists the altitude of Mercury at sunrise over the course of the apparition. All times are given in Ashburn local time.

Date Sun
rises at
rises at
at sunrise
at sunrise
Mag Phase
14 Aug 201806:1905:47east3.75%
17 Aug 201806:2205:2710°east2.410%
20 Aug 201806:2505:1313°east1.318%
23 Aug 201806:2805:0315°east0.529%
26 Aug 201806:3005:0016°east-0.140%
29 Aug 201806:3305:0216°east-0.653%
01 Sep 201806:3605:1015°east-0.865%
04 Sep 201806:3905:2114°east-1.077%
07 Sep 201806:4105:3612°east-1.286%
10 Sep 201806:4405:52east-1.392%
13 Sep 201806:4706:08east-1.596%

Mercury will brighten rapidly at the start of its morning apparition as it emerges from inferior conjunction. Prior to its apparition, it passed between the Earth and Sun, at which time it had its unilluminated side turned towards the Earth and so appeared as a thin, barely illuminated crescent. As the apparition proceeds, this crescent waxes and becomes gibbous.

Since Mercury can only ever be observed in twilight, it is particularly difficult to find when it is in a thin crescent phase. Thus, it will be significantly easier to see in the days after it reaches its highest point in the sky – when it will show a gibbous phase – than in the days beforehand.

Altitude of Mercury at sunrise

A graph of the phase of Mercury is available here.

Apparitions of Mercury

15 Mar 2018 – Evening apparition
29 Apr 2018 – Morning apparition
12 Jul 2018 – Evening apparition
26 Aug 2018 – Morning apparition
06 Nov 2018 – Evening apparition
15 Dec 2018 – Morning apparition
26 Feb 2019 – Evening apparition

Observing Mercury

Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is lost in the Sun's glare much of the time.

It is observable for only a few weeks each time it reaches greatest separation from the Sun – moments referred to as greatest elongation. These apparitions repeat roughly once every 3–4 months.

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 days, only because Mercury's orbit is not quite perfectly aligned with the ecliptic.

Mercury's position

The coordinates of Mercury when it reaches dichotomy will be:

Object Right Ascension Declination Constellation Angular Size
Mercury 09h16m20s 15°46'N Cancer 7.0"
Sun 10h26m 9°43'N Leo 31'40"

The coordinates above are given in J2000.0.

The sky on 28 Aug 2018

The sky on 28 August 2018
Twilight ends
Twilight begins

17-day old moon
Waning Gibbous


17 days old

Rise Culm. Set
Mercury 05:03 12:00 18:56
Venus 10:24 15:53 21:21
Moon 20:42 02:30 08:25
Mars 18:18 22:47 03:17
Jupiter 12:29 17:38 22:48
Saturn 16:07 20:51 01:35
All times shown in EDT.


Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.


The circumstances of this event were computed using the DE430 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

26 Aug 2018  –  Mercury at greatest elongation west
28 Aug 2018  –  Mercury at highest altitude in morning sky
06 Nov 2018  –  Mercury at greatest elongation east
10 Nov 2018  –  Mercury at highest altitude in evening sky

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