Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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Mercury will reach half phase in its 2016 evening apparition. It will be shining brightly at mag 0.2.

From Cambridge , this apparition will not be one of the most prominent and very difficult to observe, reaching a peak altitude of 11° above the horizon at sunset on 4 Aug 2016.

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 Cambridge local time.

Date Sun
sets at
sets at
at sunset
at sunset
14 Jul 201620:1820:56north-west
17 Jul 201620:1621:02west
20 Jul 201620:1421:06west
23 Jul 201620:1121:08west
26 Jul 201620:0821:0810°west
29 Jul 201620:0521:0711°west
01 Aug 201620:0221:0411°west
04 Aug 201619:5821:0111°west
07 Aug 201619:5420:5611°west
10 Aug 201619:5020:5011°west
13 Aug 201619:4620:4410°west
16 Aug 201619:4220:3610°west
19 Aug 201619:3720:28west
22 Aug 201619:3320:18west
25 Aug 201619:2820:07west
28 Aug 201619:2319:56west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2016 evening apparition of Mercury
03 Aug 2016 – Mercury reaches highest point in evening sky
16 Aug 2016 – Mercury at greatest elongation east
17 Aug 2016 – Mercury at dichotomy

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 days 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 11h27m40s +01°16' Leo 7.5"
Sun 09h48m +13°16' Leo 31'35"

The coordinates above are given in J2000.0.

The sky on 17 August 2016
Twilight ends
Twilight begins

15-day old moon
Waxing Gibbous


15 days old

Rise Culm. Set
Mercury 08:19 14:26 20:34
Venus 07:33 14:03 20:33
Moon 19:17 00:34 04:46
Mars 14:46 19:13 23:40
Jupiter 08:27 14:41 20:55
Saturn 14:45 19:29 00:18
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

16 Aug 2016  –  Mercury at greatest elongation east
28 Sep 2016  –  Mercury at greatest elongation west
01 Oct 2016  –  Mercury reaches highest point in morning sky
11 Dec 2016  –  Mercury at greatest elongation east

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