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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 2017 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 12 Sep 2017.

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 sunrise over the course of the apparition. All times are given in Ashburn local time.

Date Sun
sets at
rises at
at sunrise
at sunrise
31 Aug 201706:4106:02west
03 Sep 201706:4305:4211°west
06 Sep 201706:4605:2814°west
09 Sep 201706:4905:2015°west
12 Sep 201706:5105:1916°west
15 Sep 201706:5405:2416°west
18 Sep 201706:5705:3414°west
21 Sep 201706:5905:4613°west
24 Sep 201707:0206:0111°west
27 Sep 201707:0506:16west
30 Sep 201707:0806:31west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2017 morning apparition of Mercury
26 Aug 2017 – Mercury at inferior solar conjunction
12 Sep 2017 – Mercury at greatest elongation west
12 Sep 2017 – Mercury at dichotomy
14 Sep 2017 – Mercury reaches highest point in morning sky

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 10h18m20s +11°07' Leo 7.0"
Sun 11h24m +03°50' Leo 31'47"

The coordinates above are given in J2000.0.

The sky on 12 September 2017
Twilight ends
Twilight begins

22-day old moon
Waning Gibbous


22 days old

Rise Culm. Set
Mercury 05:19 11:59 18:40
Venus 04:23 11:17 18:11
Moon 23:15 06:14 13:13
Mars 05:31 12:10 18:49
Jupiter 09:37 15:13 20:49
Saturn 14:17 19:04 23:51
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 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

12 Sep 2017  –  Mercury at dichotomy
14 Sep 2017  –  Mercury reaches highest point in morning sky
15 Sep 2017  –  Mercury at perihelion
08 Oct 2017  –  Mercury at superior solar conjunction

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