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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 2017 morning apparition. It will be shining brightly at mag -0.1.

From Ashburn , this apparition will not be one of the most prominent and tricky to observe, reaching a peak altitude of 15° above the horizon at sunrise on 10 Jan 2017.

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 Ashburn local time.

Date Sun
sets at
rises at
at sunrise
at sunrise
01 Jan 201707:3206:41west
04 Jan 201707:3206:1812°west
07 Jan 201707:3206:0214°west
10 Jan 201707:3105:5315°west
13 Jan 201707:3005:4915°west
16 Jan 201707:2905:4815°west
19 Jan 201707:2805:5014°west
22 Jan 201707:2605:5413°west
25 Jan 201707:2405:5813°west
28 Jan 201707:2206:0311°west
31 Jan 201707:1906:0910°west
03 Feb 201707:1606:14west
06 Feb 201707:1306:18west
09 Feb 201707:1006:23west
12 Feb 201707:0706:27west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2017 morning apparition of Mercury
28 Dec 2016 – Mercury at inferior solar conjunction
13 Jan 2017 – Mercury at dichotomy
15 Jan 2017 – Mercury reaches highest point in morning sky
19 Jan 2017 – Mercury at greatest elongation west

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 18h02m10s -21°11' Sagittarius 7.4"
Sun 19h42m -21°21' Sagittarius 32'31"

The coordinates above are given in J2000.0.

The sky on 13 January 2017
Twilight ends
Twilight begins

15-day old moon
Waning Gibbous


15 days old

Rise Culm. Set
Mercury 05:49 10:39 15:29
Venus 09:49 15:26 21:03
Moon 19:03 00:58 07:59
Mars 10:10 15:57 21:44
Jupiter 00:23 06:02 11:41
Saturn 05:18 10:05 14:52
All times shown in EST.


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

13 Jan 2017  –  Mercury at dichotomy
15 Jan 2017  –  Mercury reaches highest point in morning sky
19 Jan 2017  –  Mercury at greatest elongation west
07 Feb 2017  –  Mercury at aphelion

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