Mercury at greatest brightness

Sun, 18 Feb 2018 at12:35 EST(4 hours ago)
17:35 UTC

Dominic Ford, Editor
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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 2° 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:

17 Feb 2018 07:12 EST – Mercury at superior solar conjunction
18 Feb 2018 12:35 EST – Mercury at greatest brightness
14 Mar 2018 04:59 EDT – Mercury at dichotomy
15 Mar 2018 06:18 EDT – Mercury at greatest elongation east

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
11 Feb 201817:3717:12-5°west
18 Feb 201817:4517:49west
25 Feb 201817:5318:28west
04 Mar 201818:0019:0711°west
11 Mar 201819:0720:3816°west
18 Mar 201819:1420:4817°west
25 Mar 201819:2120:2612°west
01 Apr 201819:2819:35west
08 Apr 201819:3518:39-10°west
15 Apr 201819:4217:59-19°west
22 Apr 201819:4817:40-25°north-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 22h14m00s -12°59' Aquarius 4.9"
Sun 22h07m -11°33' Aquarius 32'21"

The coordinates above are given in J2000.0.

The sky on 18 February 2018
Sunrise 06:56
Sunset 17:48
Twilight ends
Twilight begins

3-day old moon
Age of Moon
3 days

All times shown in EST.
Rise Culm. Set
Mercury 07:09 12:29 17:49
Venus 07:29 13:01 18:34
Moon 08:40 14:34 20:29
Mars 02:19 07:05 11:52
Jupiter 00:33 05:37 10:42
Saturn 03:57 08:43 13:28


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

01 Jan 2018, 19:40 EST  –  Mercury at greatest elongation west
15 Mar 2018, 06:18 EDT  –  Mercury at greatest elongation east
29 Apr 2018, 11:06 EDT  –  Mercury at greatest elongation west
12 Jul 2018, 00:00 EDT  –  Mercury at greatest elongation east

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