Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
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Mercury will reach half phase in its 2021 evening apparition. It will be shining brightly at mag -0.2.

From Fairfield , this apparition will be exceptionally well placed but tricky to observe, reaching a peak altitude of 19° above the horizon at sunset on 13 May 2021.

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

The table below lists how high Mercury will appear at sunset over the course of the apparition. All times are given in Fairfield local time.

Date Sun
sets at
sets at
at sunset
at sunset
25 Apr 202119:4320:25west
28 Apr 202119:4620:4710°west
01 May 202119:4921:0713°west
04 May 202119:5221:2515°west
07 May 202119:5621:4017°west
10 May 202119:5921:5119°west
13 May 202120:0221:5919°west
16 May 202120:0522:0219°west
19 May 202120:0722:0219°west
22 May 202120:1021:5818°west
25 May 202120:1321:4915°west
28 May 202120:1521:3713°west
31 May 202120:1821:2110°west
03 Jun 202120:2021:03north-west

A graph of the phase of Mercury is available here.

Observing Mercury

The 2021 evening apparition of Mercury
18 Apr 2021 – Mercury at superior solar conjunction
12 May 2021 – Mercury at dichotomy
15 May 2021 – Mercury reaches highest point in evening sky
17 May 2021 – Mercury at greatest elongation east
10 Jun 2021 – Mercury at inferior solar conjunction

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 hours, 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 04h42m20s +24°43' Taurus 7.1"
Sun 03h15m +18°06' Aries 31'39"

The coordinates above are given in J2000.0.

The sky on 12 May 2021
Twilight ends
Twilight begins

1-day old moon
Waxing Crescent


1 day old

Rise Culm. Set
Mercury 06:37 14:16 21:55
Venus 06:18 13:40 21:02
Moon 06:16 13:31 20:46
Mars 08:44 16:21 23:58
Jupiter 02:21 07:40 12:59
Saturn 01:36 06:35 11:35
All times shown in EDT.


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

12 May 2021  –  Mercury at dichotomy
15 May 2021  –  Mercury reaches highest point in evening sky
17 May 2021  –  Mercury at greatest elongation east
09 Jun 2021  –  Mercury at aphelion

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