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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 2015 evening apparition. It will be shining brightly at mag -0.3.

From Fairfield , this apparition will be exceptionally well placed but tricky to observe, reaching a peak altitude of 19° above the horizon at sunset on 9 May 2015.

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
15 Apr 201519:3220:02west
18 Apr 201519:3520:24west
21 Apr 201519:3820:4512°west
24 Apr 201519:4221:0414°west
27 Apr 201519:4521:2017°west
30 Apr 201519:4821:3418°west
03 May 201519:5121:4419°west
06 May 201519:5421:4919°west
09 May 201519:5721:5019°west
12 May 201520:0021:4718°west
15 May 201520:0321:4016°west
18 May 201520:0621:2813°west
21 May 201520:0921:1210°west
24 May 201520:1220:53north-west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2015 evening apparition of Mercury
09 Apr 2015 – Mercury at superior solar conjunction
02 May 2015 – Mercury at dichotomy
07 May 2015 – Mercury at greatest elongation east
08 May 2015 – Mercury reaches highest point in evening sky
30 May 2015 – Mercury at inferior solar conjunction

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 03h58m00s +23°03' Taurus 7.1"
Sun 02h37m +15°23' Aries 31'44"

The coordinates above are given in J2000.0.

The sky on 02 May 2015
Twilight ends
Twilight begins

14-day old moon
Waxing Gibbous


14 days old

Rise Culm. Set
Mercury 06:39 14:10 21:41
Venus 08:03 15:46 23:29
Moon 18:40 00:09 05:04
Mars 06:23 13:35 20:47
Jupiter 12:07 19:16 02:27
Saturn 21:20 02:20 07:15
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

02 May 2015  –  Mercury at dichotomy
07 May 2015  –  Mercury at greatest elongation east
08 May 2015  –  Mercury reaches highest point in evening sky
30 May 2015  –  Mercury at inferior solar conjunction

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