© NASA/JPL/MESSENGER

Mercury at dichotomy

Dominic Ford, Editor
From the Inner Planets feed

Objects: Mercury
Please wait
Loading 0/4
Click and drag to rotate
Mouse wheel to zoom in/out
Touch with mouse to dismiss
The sky at

Mercury will reach half phase in its Aug–Sep 2021 evening apparition. It will be shining brightly at mag 0.2.

From Fairfield , this apparition will not be one of the most prominent and very difficult to observe, reaching a peak altitude of 8° above the horizon at sunset on 3 Sep 2021.

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

Aug–Sep 2021 evening apparition of Mercury

01 Aug 2021 – Mercury at superior solar conjunction
02 Sep 2021 – Mercury at highest altitude in evening sky
13 Sep 2021 – Mercury at greatest elongation east
17 Sep 2021 – Mercury at dichotomy
09 Oct 2021 – Mercury at inferior solar conjunction

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

Date Sun
sets at
Mercury
sets at
Altitude
at sunset
Direction
at sunset
Mag Phase
13 Aug 202119:5420:28west-0.893%
16 Aug 202119:5020:28west-0.690%
19 Aug 202119:4520:27west-0.587%
22 Aug 202119:4120:25west-0.384%
25 Aug 202119:3620:22west-0.281%
28 Aug 202119:3220:18west-0.178%
31 Aug 202119:2720:14west-0.175%
03 Sep 202119:2220:10west-0.072%
06 Sep 202119:1720:05west0.068%
09 Sep 202119:1219:59west0.064%
12 Sep 202119:0719:53west0.160%
15 Sep 202119:0219:46west0.155%
18 Sep 202118:5619:38west0.250%
21 Sep 202118:5119:30south-west0.344%
24 Sep 202118:4619:20south-west0.437%

Mercury will fade rapidly towards the end of the apparition as it heads towards inferior conjunction, when it will pass between the Earth and Sun. At inferior conjunction, the planet turns its unilluminated side towards the Earth, and so appears as a thin, barely illuminated crescent.

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 before it reaches its highest point in the sky than in the days after.

Altitude of Mercury at sunset

A graph of the phase of Mercury is available here.

Apparitions of Mercury

06 Mar 2021 – Morning apparition
17 May 2021 – Evening apparition
05 Jul 2021 – Morning apparition
13 Sep 2021 – Evening apparition
24 Oct 2021 – Morning apparition
07 Jan 2022 – Evening apparition
16 Feb 2022 – Morning 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 13h14m00s 11°11'S Virgo 7.5"
Sun 11h42m 1°56'N Virgo 31'50"

The coordinates above are given in J2000.0.

The sky on 17 Sep 2021

The sky on 17 September 2021
Sunrise
06:33
Sunset
18:58
Twilight ends
20:32
Twilight begins
04:59

10-day old moon
Waxing Gibbous

93%

10 days old

Planets
Rise Culm. Set
Mercury 08:54 14:18 19:42
Venus 10:15 15:24 20:32
Moon 17:44 22:38 03:40
Mars 07:08 13:12 19:15
Jupiter 17:39 22:49 03:58
Saturn 16:50 21:42 02:34
All times shown in EDT.

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

13 Sep 2021  –  Mercury at greatest elongation east
25 Oct 2021  –  Mercury at highest altitude in morning sky
24 Oct 2021  –  Mercury at greatest elongation west
07 Jan 2022  –  Mercury at greatest elongation east

Image credit

© NASA/JPL/MESSENGER

Share

Fairfield

Latitude:
Longitude:
Timezone:

41.14°N
73.26°W
EDT

Color scheme