Mercury at greatest brightness

Sun, 08 Oct 2017 at06:14 EDT(101 days ago)
10:14 UTC

Dominic Ford, Editor
From the Inner Planets feed

Please wait
Loading 0/4
Click and drag to rotate
Mouse wheel to zoom in/out
Touch with mouse to dismiss
The sky at

In the southern hemisphere Mercury will be well placed for observation in the evening sky, shining brightly at mag -1.7.

From Ashburn (click to change) however, it will not be readily observable since it will be very close to the Sun, at a separation of only 1° from it.

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:

12 Sep 2017 05:11 EDT – Mercury at greatest elongation west
12 Sep 2017 23:34 EDT – Mercury at dichotomy
08 Oct 2017 06:14 EDT – Mercury at greatest brightness
08 Oct 2017 16:38 EDT – Mercury at superior solar conjunction

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 Mercury will remain up after sunset each night; all times are given in Ashburn local time.

Date Sun
sets at
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
01 Oct 201718:4918:44-1°west
08 Oct 201718:3818:44west
15 Oct 201718:2818:43west
22 Oct 201718:1818:42west
29 Oct 201718:0918:42south-west
05 Nov 201717:0117:44south-west
12 Nov 201716:5417:49south-west
19 Nov 201716:4917:56south-west
26 Nov 201716:4518:0010°south-west
03 Dec 201716:4317:5010°south-west
10 Dec 201716:4317:11south-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 12h56m00s -04°45' Virgo 4.8"
Sun 12h55m -05°56' Virgo 32'01"

The coordinates above are given in J2000.0.

The sky on 08 October 2017
Sunrise 07:11
Sunset 18:41
Twilight ends
Twilight begins

18-day old moon
Age of Moon
18 days

All times shown in EDT.
Rise Culm. Set
Mercury 07:10 12:57 18:44
Venus 05:18 11:35 17:52
Moon 21:11 03:11 09:54
Mars 05:09 11:28 17:46
Jupiter 08:21 13:50 19:19
Saturn 12:40 17:26 22:12


Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.


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

12 Sep 2017, 05:11 EDT  –  Mercury at greatest elongation west
23 Nov 2017, 21:22 EST  –  Mercury at greatest elongation east
01 Jan 2018, 19:40 EST  –  Mercury at greatest elongation west
15 Mar 2018, 06:18 EDT  –  Mercury at greatest elongation east

Image credit





Color scheme