Mercury at dichotomy

2019 Aug 12

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

From London however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 8° above the horizon at dawn.

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:

21 Jul 2019 13:28 BST  – Mercury at inferior solar conjunction
10 Aug 2019 04:26 BST  – Mercury at greatest elongation west
12 Aug 2019 14:49 BST  – Mercury at dichotomy
04 Sep 2019 02:27 BST  – Mercury at superior 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 before sunrise Mercury will rise each night; all times are given in London local time.

Date Sun
sets at
Mercury
rises at
Altitude of Mercury
at sunrise
Direction of Mercury
at sunrise
05 Aug 2019 05:27 04:05 10° north-west
12 Aug 2019 05:38 03:59 13° north-west
19 Aug 2019 05:49 04:22 11° north-west
26 Aug 2019 06:00 05:07 north-west
02 Sep 2019 06:11 05:59 west
09 Sep 2019 06:23 06:49 -4° west
16 Sep 2019 06:34 07:33 -9° west
23 Sep 2019 06:45 08:13 -13° west
30 Sep 2019 06:56 08:49 -18° south-west
07 Oct 2019 07:08 09:22 -20° south-west
14 Oct 2019 07:19 09:49 -22° 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 08h10m30s +19°18' Cancer 7.0"
Sun 09h26m +15°01' Leo 31'33"

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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