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 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.
|Altitude of Mercury
|Direction of Mercury
|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||7°||south-west|
|23 Dec 2019||08:03||07:21||4°||south-west|
|30 Dec 2019||08:05||07:48||1°||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.
The coordinates of Mercury when it reaches dichotomy will be:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
This entry in the observing calendar was provided by In-The-Sky.org