Mercury at dichotomy

2019 Nov 25

Mercury will be well placed for observation in the dawn sky, shining brightly at mag -0.4.

From London, it will be difficult to observe as it will appear no higher than 10° above the horizon. It will rise at 05:35 (BST) – 1 hour and 59 minutes before the Sun – and reach an altitude of 10° above the south-eastern horizon before fading from view as dawn breaks at around 07:00.

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:

11 Nov 2019 15:16 GMT  – Mercury at inferior solar conjunction
11 Nov 2019 15:20 GMT  – Transit of Mercury
25 Nov 2019 10:24 GMT  – Mercury at dichotomy
28 Nov 2019 12:18 GMT  – Mercury at greatest elongation west

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 London local time.

Date Sun
sets at
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
18 Nov 2019 07:20 05:58 10° west
25 Nov 2019 07:32 05:35 14° west
02 Dec 2019 07:42 05:49 13° west
09 Dec 2019 07:51 06:17 10° west
16 Dec 2019 07:58 06:50 south-west
23 Dec 2019 08:03 07:21 south-west
30 Dec 2019 08:05 07:48 south-west
06 Jan 2020 08:04 08:10 -1° south-west
13 Jan 2020 08:00 08:23 -3° south-west
20 Jan 2020 07:54 08:28 -5° south-west
27 Jan 2020 07:46 08:26 -6° 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 14h45m30s -13°23' Libra 7.2"
Sun 16h02m -20°40' Scorpius 32'24"

The coordinates above are given in J2000.0.

This entry in the observing calendar was provided by