© NASA/JPL/MESSENGER

Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

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Ashburn
The sky at

In the southern hemisphere Mercury will be well placed for observation in the evening sky, shining brightly at mag 0.2.

From Ashburn however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 9° above the horizon at dusk.

Begin typing the name of a town near to you, and then select the town from the list of options which appear below.

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:

07 Jul 2018 10:56 EDT – Mercury at dichotomy
12 Jul 2018 00:00 EDT – Mercury at greatest elongation east

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 Ashburn local time.

Date Sun
sets at
Mercury
sets at
Altitude of Mercury
at sunset
Direction of Mercury
at sunset
30 Jun 201820:3522:0916°west
07 Jul 201820:3422:0616°west
14 Jul 201820:3121:5214°west
21 Jul 201820:2721:2911°west
28 Jul 201820:2120:56west
04 Aug 201820:1420:15west
11 Aug 201820:0619:34-6°west
18 Aug 201819:5719:05-9°north-west
25 Aug 201819:4818:55-10°north-west
01 Sep 201819:3718:59-7°north-west
08 Sep 201819:2619:07-4°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 08h54m40s +17°51' Cancer 7.4"
Sun 07h05m +22°34' Gemini 31'27"

The coordinates above are given in J2000.0.

The sky on 07 July 2018
Sunrise
05:50
Sunset
20:37
Twilight ends
22:34
Twilight begins
03:53

24-day old moon
Waning Crescent

36%

24 days old

Planets
Rise Culm. Set
Mercury 07:59 15:02 22:06
Venus 09:17 16:06 22:56
Moon 01:40 08:04 14:27
Mars 22:13 02:58 07:39
Jupiter 15:36 20:50 02:07
Saturn 19:41 00:30 05:15
All times shown in EDT.

Source

The circumstances of this event were computed using the DE405 planetary ephemeris published by the Jet Propulsion Laboratory (JPL).

This event was automatically generated by searching the ephemeris for planetary alignments which are of interest to amateur astronomers, and the text above was generated based on an estimate of your location.

Related news

29 Apr 2018, 11:06 EDT  –  Mercury at greatest elongation west
12 Jul 2018, 00:00 EDT  –  Mercury at greatest elongation east
26 Aug 2018, 17:48 EDT  –  Mercury at greatest elongation west
06 Nov 2018, 09:59 EST  –  Mercury at greatest elongation east

Image credit

© NASA/JPL/MESSENGER

Ashburn

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Longitude:
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39.04°N
77.49°W
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