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BAA Observing calendar

Mercury at dichotomy

Tuesday, 2019, February 26 - 15:49

Mercury will be well placed for observation in the evening sky, shining brightly at mag -0.6.

From London, it will be difficult to observe as it will appear no higher than 11° above the horizon. It will become visible at around 17:59 (BST) as the dusk sky fades, 11° above your western horizon. It will then sink towards the horizon, setting 1 hour and 46 minutes after the Sun at 19:18.

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:

30 Jan 2019 02:36 GMT  – Mercury at superior solar conjunction
26 Feb 2019 15:49 GMT  – Mercury at dichotomy
26 Feb 2019 21:29 GMT  – Mercury at greatest elongation east
15 Mar 2019 01:42 GMT  – Mercury at inferior solar conjunction

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
Mercury
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
19 Feb 2019 17:15 18:45 11° south-west
26 Feb 2019 17:28 19:17 15° south-west
05 Mar 2019 17:40 19:17 14° west
12 Mar 2019 17:52 18:34 west
19 Mar 2019 18:04 17:28 -5° west
26 Mar 2019 18:16 16:34 -16° west
02 Apr 2019 19:28 17:06 -22° west
09 Apr 2019 19:40 17:00 -24° north-west
16 Apr 2019 19:51 17:12 -24° north-west
23 Apr 2019 20:03 17:35 -22° north-west
30 Apr 2019 20:14 18:09 -19° north-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 23h41m30s -00°28' Pisces 7.1"
Sun 22h36m -08°45' Aquarius 32'18"

The coordinates above are given in J2000.0.

This entry in the observing calendar was provided by In-The-Sky.org

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