© NASA/JPL/MESSENGER

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 -0.4.

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 14 Dec 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
Mercury
sets at
Altitude
at sunset
Direction
at sunset
20 Nov 201616:1616:52south-west
23 Nov 201616:1416:56south-west
26 Nov 201616:1317:01south-west
29 Nov 201616:1117:07south-west
02 Dec 201616:1117:13south-west
05 Dec 201616:1017:1910°south-west
08 Dec 201616:1017:2610°south-west
11 Dec 201616:1017:3011°south-west
14 Dec 201616:1017:3311°south-west
17 Dec 201616:1117:3111°south-west
20 Dec 201616:1317:2310°south-west
23 Dec 201616:1417:07south-west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2016 evening apparition of Mercury
11 Dec 2016 – Mercury at greatest elongation east
14 Dec 2016 – Mercury at dichotomy
16 Dec 2016 – 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 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 18h57m30s -24°16' Sagittarius 7.3"
Sun 17h29m -23°14' Ophiuchus 32'30"

The coordinates above are given in J2000.0.

The sky on 14 December 2016
Sunrise
07:05
Sunset
16:11
Twilight ends
17:52
Twilight begins
05:23

15-day old moon
Waning Gibbous

99%

15 days old

Planets
Rise Culm. Set
Mercury 08:40 13:06 17:33
Venus 10:09 14:53 19:37
Moon 17:35 00:52 07:05
Mars 10:53 16:04 21:15
Jupiter 01:40 07:20 13:01
Saturn 06:44 11:22 16:00
All times shown in EST.

Source

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

11 Dec 2016  –  Mercury at greatest elongation east
16 Dec 2016  –  Mercury reaches highest point in evening sky
12 Jan 2017  –  Mercury reaches highest point in morning sky
19 Jan 2017  –  Mercury at greatest elongation west

Image credit

© NASA/JPL/MESSENGER

Cambridge

Latitude:
Longitude:
Timezone:

42.38°N
71.11°W
EST

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