Mercury at greatest brightness

Thu, 19 Jan 2017 at15:43 EST(363 days ago)
20:43 UTC

Dominic Ford, Editor
From the Inner Planets feed

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The sky at

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.2.

From Ashburn (click to change), it will be difficult to observe as it will appear no higher than 9° above the horizon. It will rise at 05:50 (EST) – 1 hour and 36 minutes before the Sun – and reach an altitude of 9° above the south-eastern horizon before fading from view as dawn breaks at around 06:58.

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 brightness

Mercury's brightness depends on two factors: its closeness to the Earth, and its phase. Its 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 reaches its brightest when it is still a crescent – with less than half of its disk illuminated. This is because it is much closer to the Earth during its crescent phases than at other times.

As a result, during evening apparitions, Mercury reaches maximum brightness a few days after it is at greatest separation from the Sun, which always coincides with it showing half-phase (dichotomy).

Conversely, during morning apparitions, Mercury reaches maximum brightness a few days before it is at greatest separation from the Sun.

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
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
12 Jan 201707:2705:4914°west
19 Jan 201707:2405:4914°west
26 Jan 201707:2005:5912°west
02 Feb 201707:1406:11west
09 Feb 201707:0706:22west
16 Feb 201706:5906:30west
23 Feb 201706:4906:37west
02 Mar 201706:3906:41west
09 Mar 201706:2906:43-3°west
16 Mar 201707:1807:44-5°west
23 Mar 201707:0707:41-6°west

A graph of the brightness of Mercury is available here.

Mercury's position

The coordinates of Mercury when it reaches greatest brightness will be:

Object Right Ascension Declination Constellation Angular Size
Mercury 18h24m20s -22°02' Sagittarius 6.6"
Sun 20h07m -20°12' Capricornus 32'30"

The coordinates above are given in J2000.0.

The sky on 19 January 2017
Sunrise 07:24
Sunset 17:14
Twilight ends
Twilight begins

21-day old moon
Age of Moon
21 days

All times shown in EST.
Rise Culm. Set
Mercury 05:49 10:36 15:23
Venus 09:36 15:22 21:09
Moon 00:02 05:48 11:34
Mars 09:56 15:49 21:41
Jupiter 23:56 05:38 11:17
Saturn 04:56 09:43 14:30


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

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
29 Jul 2017, 20:24 EDT  –  Mercury at greatest elongation east

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