Mercury at dichotomy

2019 Oct 25

In the southern hemisphere Mercury will be well placed for observation in the evening sky, shining brightly at mag -0.1.

From London however, it will not be observable – it will reach its highest point in the sky during daytime and will be 0° below the horizon at dusk.

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:

20 Oct 2019 01:41 BST  – Mercury at greatest elongation east
25 Oct 2019 12:55 BST  – Mercury at dichotomy
11 Nov 2019 15:16 GMT  – Mercury at inferior solar conjunction
11 Nov 2019 15:20 GMT  – Transit of Mercury

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 London local time.

Date Sun
sets at
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
18 Oct 2019 17:58 18:30 south-west
25 Oct 2019 17:44 18:16 south-west
01 Nov 2019 16:31 16:57 south-west
08 Nov 2019 16:18 16:29 south-west
15 Nov 2019 16:07 15:57 -1° south-west
22 Nov 2019 15:59 15:31 -4° west
29 Nov 2019 15:52 15:15 -6° west
06 Dec 2019 15:47 15:05 -6° west
13 Dec 2019 15:46 15:01 -6° south-west
20 Dec 2019 15:47 15:02 -6° south-west
27 Dec 2019 15:51 15:11 -5° south-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 15h28m00s -22°01' Libra 7.4"
Sun 13h57m -11°59' Virgo 32'09"

The coordinates above are given in J2000.0.

This entry in the observing calendar was provided by