© NASA/JPL/MESSENGER

Mercury at dichotomy

Fri, 13 Jan 2017 at17:02 EST(369 days ago)
22:02 UTC

Dominic Ford, Editor
From the Inner Planets feed

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Ashburn
The sky at

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.1.

From Ashburn (click to change), it will be difficult to observe as it will appear no higher than 10° above the horizon. It will rise at 05:49 (EST) – 1 hour and 40 minutes before the Sun – and reach an altitude of 10° above the south-eastern horizon before fading from view as dawn breaks at around 06:59.

Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is very difficult to observe most of the time.

It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.

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 in coming weeks

The key moments in this apparition of Mercury are as follows:

28 Dec 2016 13:41 EST – Mercury at inferior solar conjunction
13 Jan 2017 17:02 EST – Mercury at dichotomy
19 Jan 2017 08:45 EST – Mercury at greatest elongation west
19 Jan 2017 15:43 EST – Mercury at greatest brightness

Over coming weeks, the distance between Mercury and the Sun will decrease each night as it sinks back into the Sun's glare. The table below lists how long before sunrise Mercury will rise each night; all times are given in Ashburn local time.

Date Sun
sets at
Mercury
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
06 Jan 201707:2806:0512°west
13 Jan 201707:2605:4814°west
20 Jan 201707:2305:5014°west
27 Jan 201707:1906:0111°west
03 Feb 201707:1306:12west
10 Feb 201707:0606:23west
17 Feb 201706:5706:31west
24 Feb 201706:4806:37west
03 Mar 201706:3806:41west
10 Mar 201706:2706:43-3°west
17 Mar 201707:1707:43-5°west

A graph of the phase of Mercury is available here.

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
Sunrise 07:27
Sunset 17:08
Twilight ends
18:42
Twilight begins
05:52

15-day old moon
Age of Moon
15 days

All times shown in EST.
Rise Culm. Set
Mercury 05:48 10:38 15:28
Venus 09:48 15:25 21:02
Moon 19:02 00:57 07:58
Mars 10:09 15:56 21:43
Jupiter 00:21 06:00 11:39
Saturn 05:17 10:04 14:51

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, 02:45 EST  –  Mercury at greatest elongation east
19 Jan 2017, 08:45 EST  –  Mercury at greatest elongation west
01 Apr 2017, 01:52 EDT  –  Mercury at greatest elongation east
17 May 2017, 18:58 EDT  –  Mercury at greatest elongation west

Image credit

© NASA/JPL/MESSENGER

Ashburn

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39.04°N
77.49°W
EST

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