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Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its 2018 evening apparition. It will be shining brightly at mag 0.2.

From Fairfield , this apparition will be well placed but tricky to observe, reaching a peak altitude of 15° above the horizon at sunset on 8 Jul 2018.

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

The table below lists how high Mercury will appear at sunset over the course of the apparition. All times are given in Fairfield local time.

Date Sun
sets at
sets at
at sunset
at sunset
14 Jun 201820:2621:13north-west
17 Jun 201820:2721:2910°north-west
20 Jun 201820:2821:4112°west
23 Jun 201820:2821:4913°west
26 Jun 201820:2921:5514°west
29 Jun 201820:2921:5915°west
02 Jul 201820:2921:5915°west
05 Jul 201820:2821:5815°west
08 Jul 201820:2721:5515°west
11 Jul 201820:2621:5015°west
14 Jul 201820:2521:4313°west
17 Jul 201820:2321:3413°west
20 Jul 201820:2121:2411°west
23 Jul 201820:1921:12west
26 Jul 201820:1620:58west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2018 evening apparition of Mercury
06 Jul 2018 – Mercury reaches highest point in evening sky
07 Jul 2018 – Mercury at dichotomy
11 Jul 2018 – Mercury at greatest elongation east

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 hours, 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 08h54m40s +17°51' Cancer 7.4"
Sun 07h05m +22°34' Gemini 31'27"

The coordinates above are given in J2000.0.

The sky on 07 July 2018
Twilight ends
Twilight begins

24-day old moon
Waning Crescent


24 days old

Rise Culm. Set
Mercury 07:38 14:46 21:55
Venus 08:57 15:51 22:44
Moon 01:25 07:48 14:11
Mars 22:04 02:42 07:16
Jupiter 15:24 20:34 01:48
Saturn 19:32 00:15 04:53
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

07 Jul 2018  –  Mercury at dichotomy
11 Jul 2018  –  Mercury at greatest elongation east
20 Jul 2018  –  Mercury at aphelion
08 Aug 2018  –  Mercury at inferior solar conjunction

Image credit

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