© 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 Oct–Nov 2021 morning apparition. It will be shining brightly at mag -0.5.

From Washington , this apparition will be well placed but tricky to observe, reaching a peak altitude of 17° above the horizon at sunrise on 28 Oct 2021.

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Oct–Nov 2021 morning apparition of Mercury

09 Oct 2021 – Mercury at inferior solar conjunction
23 Oct 2021 – Mercury at dichotomy
23 Oct 2021 – Mercury at highest altitude in morning sky
24 Oct 2021 – Mercury at greatest elongation west
28 Nov 2021 – 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 Washington local time.

Date Sun
rises at
Mercury
rises at
Altitude
at sunrise
Direction
at sunrise
Mag Phase
15 Oct 202107:4006:4610°east2.210%
18 Oct 202107:4306:2914°east0.722%
21 Oct 202107:4706:1816°east-0.137%
24 Oct 202107:5006:1617°east-0.551%
27 Oct 202107:5306:1917°east-0.764%
30 Oct 202107:5706:2916°east-0.874%
02 Nov 202108:0106:3915°east-0.882%
05 Nov 202108:0106:5113°east-0.988%
08 Nov 202107:0506:0111°south-east-0.992%
11 Nov 202107:0906:1610°south-east-0.995%
14 Nov 202107:1306:27south-east-0.997%
17 Nov 202107:1406:38south-east-1.098%

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

17 May 2021 – Evening apparition
04 Jul 2021 – Morning apparition
13 Sep 2021 – Evening apparition
24 Oct 2021 – Morning apparition
07 Jan 2022 – Evening apparition
16 Feb 2022 – Morning apparition
29 Apr 2022 – 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 12h47m50s -03°06' Virgo 7.1"
Sun 13h53m -11°37' Virgo 32'09"

The coordinates above are given in J2000.0.

The sky on 23 October 2021
Sunrise
07:50
Sunset
18:50
Twilight ends
20:14
Twilight begins
06:22

17-day old moon
Waning Gibbous

90%

17 days old

Planets
Rise Culm. Set
Mercury 06:17 12:11 18:05
Venus 11:48 16:24 21:00
Moon 20:40 03:22 10:43
Mars 07:22 12:57 18:32
Jupiter 15:47 21:02 02:18
Saturn 15:01 20:02 01:03
All times shown in MDT.

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

23 Oct 2021  –  Mercury at dichotomy
24 Oct 2021  –  Mercury at highest altitude in morning sky
24 Oct 2021  –  Mercury at greatest elongation west
28 Nov 2021  –  Mercury at superior solar conjunction

Image credit

© NASA/JPL/MESSENGER

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