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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 2013 morning apparition. It will be shining brightly at mag -0.3.

From Cambridge , this apparition will be well placed but tricky to observe, reaching a peak altitude of 15° above the horizon at sunrise on 1 Aug 2013.

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 Cambridge local time.

Date Sun
sets at
rises at
at sunrise
at sunrise
17 Jul 201305:2604:47north-west
20 Jul 201305:2804:31north-west
23 Jul 201305:3104:1912°north-west
26 Jul 201305:3404:1013°north-west
29 Jul 201305:3704:0615°north-west
01 Aug 201305:4004:0515°north-west
04 Aug 201305:4304:1015°north-west
07 Aug 201305:4604:1813°north-west
10 Aug 201305:4904:3112°north-west
13 Aug 201305:5304:4710°north-west
16 Aug 201305:5605:05north-west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2013 morning apparition of Mercury
09 Jul 2013 – Mercury at inferior solar conjunction
30 Jul 2013 – Mercury at greatest elongation west
02 Aug 2013 – Mercury at dichotomy
03 Aug 2013 – 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 07h30m40s +20°40' Gemini 7.0"
Sun 08h50m +17°39' Cancer 31'31"

The coordinates above are given in J2000.0.

The sky on 02 August 2013
Twilight ends
Twilight begins

25-day old moon
Waning Crescent


25 days old

Rise Culm. Set
Mercury 04:06 11:30 18:54
Venus 08:28 14:59 21:31
Moon 02:06 09:27 16:48
Mars 03:20 10:57 18:35
Jupiter 03:00 10:35 18:10
Saturn 12:51 18:13 23:35
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 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

02 Aug 2013  –  Mercury at dichotomy
03 Aug 2013  –  Mercury reaches highest point in morning sky
11 Aug 2013  –  Mercury at perihelion
24 Aug 2013  –  Mercury at superior solar conjunction

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