© NASA/JPL/MESSENGER

Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its Sep–Oct 2023 morning apparition. It will be shining brightly at mag -0.5.

From Fairfield , this apparition will be exceptionally well placed but tricky to observe, reaching a peak altitude of 17° above the horizon at sunrise on 25 Sep 2023.

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Sep–Oct 2023 morning apparition of Mercury

06 Sep 2023 – Mercury at inferior solar conjunction
21 Sep 2023 – Mercury at highest altitude in morning sky
22 Sep 2023 – Mercury at greatest elongation west
22 Sep 2023 – Mercury at dichotomy
20 Oct 2023 – Mercury at superior solar conjunction

The table below lists the altitude of Mercury at sunrise over the course of the apparition. All times are given in Fairfield local time.

Date Sun
rises at
Mercury
rises at
Altitude
at sunrise
Direction
at sunrise
Mag Phase
13 Sep 202306:2905:36east2.310%
16 Sep 202306:3205:1913°east1.020%
19 Sep 202306:3405:1015°east0.133%
22 Sep 202306:4105:0617°east-0.447%
25 Sep 202306:4405:1117°east-0.861%
28 Sep 202306:4705:2015°east-0.972%
01 Oct 202306:5005:3414°east-1.082%
04 Oct 202306:5205:4712°east-1.189%
07 Oct 202306:5506:01east-1.294%
10 Oct 202306:5806:15east-1.397%

Mercury will brighten rapidly at the start of its morning apparition as it emerges from inferior conjunction. Prior to its apparition, it passed between the Earth and Sun, at which time it had its unilluminated side turned towards the Earth and so appeared as a thin, barely illuminated crescent. As the apparition proceeds, this crescent waxes and becomes gibbous.

Since Mercury can only ever be observed in twilight, it is particularly difficult to find when it is in a thin crescent phase. Thus, it will be significantly easier to see in the days after it reaches its highest point in the sky – when it will show a gibbous phase – than in the days beforehand.

Altitude of Mercury at sunrise

A graph of the phase of Mercury is available here.

Apparitions of Mercury

11 Apr 2023 – Evening apparition
29 May 2023 – Morning apparition
09 Aug 2023 – Evening apparition
22 Sep 2023 – Morning apparition
04 Dec 2023 – Evening apparition
12 Jan 2024 – Morning apparition
24 Mar 2024 – Evening apparition

Observing Mercury

Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is lost in the Sun's glare much of the time.

It is observable for only a few weeks each time it reaches greatest separation from the Sun – moments referred to as greatest elongation. These apparitions repeat roughly once every 3–4 months.

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 days, only because Mercury's orbit is not quite perfectly aligned with the ecliptic.

Mercury's position

The coordinates of Mercury when it reaches dichotomy will be:

Object Right Ascension Declination Constellation Angular Size
Mercury 10h52m30s 8°07'N Leo 7.0"
Sun 11h57m +00°18' Virgo 31'52"

The coordinates above are given in J2000.0.

The sky on 22 September 2023
Sunrise
06:41
Sunset
18:53
Twilight ends
20:25
Twilight begins
05:05

7-day old moon
Waxing Gibbous

55%

7 days old

Planets
Rise Culm. Set
Mercury 05:10 11:42 18:13
Venus 03:21 10:05 16:49
Moon 14:32 18:55 23:12
Mars 08:08 13:50 19:31
Jupiter 20:46 03:43 10:40
Saturn 17:46 23:04 04:22
All times shown in EDT.

Warning

Never attempt to point a pair of binoculars or a telescope at an object close to the Sun. Doing so may result in immediate and permanent blindness.

Source

The circumstances of this event were computed using the DE430 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

22 Sep 2023  –  Mercury at greatest elongation west
04 Dec 2023  –  Mercury at greatest elongation east
08 Dec 2023  –  Mercury at highest altitude in evening sky
08 Jan 2024  –  Mercury at highest altitude in morning sky

Image credit

© NASA/JPL/MESSENGER

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41.14°N
73.26°W
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