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 evening apparition. It will be shining brightly at mag -0.6.

From Ashburn , this apparition will be well placed but tricky to observe, reaching a peak altitude of 17° above the horizon at sunset on 15 Feb 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 sunset over the course of the apparition. All times are given in Ashburn local time.

Date Sun
sets at
sets at
at sunset
at sunset
28 Jan 201317:2617:59south-west
31 Jan 201317:3018:15south-west
03 Feb 201317:3318:3010°south-west
06 Feb 201317:3718:4512°south-west
09 Feb 201317:4018:5914°south-west
12 Feb 201317:4419:1015°south-west
15 Feb 201317:4719:1817°west
18 Feb 201317:5019:2117°west
21 Feb 201317:5419:1816°west
24 Feb 201317:5719:0713°west
27 Feb 201318:0018:48west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2013 evening apparition of Mercury
18 Jan 2013 – Mercury at superior solar conjunction
16 Feb 2013 – Mercury at greatest elongation east
16 Feb 2013 – Mercury at dichotomy
17 Feb 2013 – Mercury reaches highest point in evening 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 23h08m00s -04°22' Aquarius 7.2"
Sun 22h01m -12°04' Aquarius 32'22"

The coordinates above are given in J2000.0.

The sky on 16 February 2013
Twilight ends
Twilight begins

6-day old moon
Waxing Crescent


6 days old

Rise Culm. Set
Mercury 07:42 13:30 19:19
Venus 06:39 11:47 16:54
Moon 10:23 17:25 00:27
Mars 07:40 13:16 18:52
Jupiter 11:26 18:42 02:01
Saturn 23:39 05:03 10:24
All times shown in EST.


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

16 Feb 2013  –  Mercury at dichotomy
16 Feb 2013  –  Mercury at perihelion
17 Feb 2013  –  Mercury reaches highest point in evening sky
04 Mar 2013  –  Mercury at inferior solar conjunction

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