549 days ago
Dominic Ford, Editor
From the Inner Planets feed
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.
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.
|13 Aug 2021||19:54||20:28||5°||west||-0.8||93%|
|16 Aug 2021||19:50||20:28||6°||west||-0.6||90%|
|19 Aug 2021||19:45||20:27||7°||west||-0.5||87%|
|22 Aug 2021||19:41||20:25||7°||west||-0.3||84%|
|25 Aug 2021||19:36||20:22||8°||west||-0.2||81%|
|28 Aug 2021||19:32||20:18||8°||west||-0.1||78%|
|31 Aug 2021||19:27||20:14||8°||west||-0.1||75%|
|03 Sep 2021||19:22||20:10||8°||west||-0.0||72%|
|06 Sep 2021||19:17||20:05||8°||west||0.0||68%|
|09 Sep 2021||19:12||19:59||8°||west||0.0||64%|
|12 Sep 2021||19:07||19:53||8°||west||0.1||60%|
|15 Sep 2021||19:02||19:46||7°||west||0.1||55%|
|18 Sep 2021||18:56||19:38||7°||west||0.2||50%|
|21 Sep 2021||18:51||19:30||6°||south-west||0.3||44%|
|24 Sep 2021||18:46||19:20||5°||south-west||0.4||37%|
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|
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 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.
The coordinates of Mercury when it reaches dichotomy will be:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
The sky on 17 Sep 2021
|The sky on 17 September 2021|
10 days old
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.
|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|