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 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.
|Altitude of Mercury
|Direction of Mercury
|18 Oct 2019||17:58||18:30||3°||south-west|
|25 Oct 2019||17:44||18:16||3°||south-west|
|01 Nov 2019||16:31||16:57||2°||south-west|
|08 Nov 2019||16:18||16:29||0°||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.
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