In the southern hemisphere Mercury will be well placed for observation in the evening sky, shining brightly at mag -0.4.
From Ashburn (click to change) however, it will not be observable – it will reach its highest point in the sky during daytime and will be no higher than 6° above the horizon at dusk.
Mercury's orbit lies closer to the Sun than the Earth's, meaning that it always appears close to the Sun and is very difficult to observe most of the time.
It is observable only for a few days each time it reaches greatest separation from the Sun – moments referred to as greatest elongation.
Mercury's brightness depends on two factors: its closeness to the Earth, and its phase. Its 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 reaches its brightest when it is still a crescent – with less than half of its disk illuminated. This is because it is much closer to the Earth during its crescent phases than at other times.
As a result, during evening apparitions, Mercury reaches maximum brightness a few days after it is at greatest separation from the Sun, which always coincides with it showing half-phase (dichotomy).
Conversely, during morning apparitions, Mercury reaches maximum brightness a few days before it is at greatest separation from the Sun.
Mercury in coming weeks
The key moments in this apparition of Mercury are as follows:
|22 Nov 2017 13:20 EST||– Mercury at greatest brightness|
|23 Nov 2017 21:22 EST||– Mercury at greatest elongation east|
|28 Nov 2017 04:03 EST||– Mercury at dichotomy|
|12 Dec 2017 20:43 EST||– Mercury at inferior solar conjunction|
Over coming weeks, the distance between Mercury and the Sun will decrease each night as it sinks back into the Sun's glare. The table below lists how long Mercury will remain up after sunset each night; all times are given in Ashburn local time.
|Altitude of Mercury
|Direction of Mercury
|15 Nov 2017||16:52||17:52||8°||south-west|
|22 Nov 2017||16:47||17:59||10°||south-west|
|29 Nov 2017||16:44||17:59||10°||south-west|
|06 Dec 2017||16:42||17:37||8°||south-west|
|13 Dec 2017||16:43||16:47||0°||south-west|
|20 Dec 2017||16:45||15:57||-9°||west|
|27 Dec 2017||16:49||15:32||-14°||west|
|03 Jan 2018||16:54||15:24||-17°||west|
|10 Jan 2018||17:01||15:28||-17°||west|
|17 Jan 2018||17:08||15:40||-16°||west|
|24 Jan 2018||17:16||15:59||-14°||west|
A graph of the brightness of Mercury is available here.
The coordinates of Mercury when it reaches greatest brightness will be:
|Object||Right Ascension||Declination||Constellation||Angular Size|
The coordinates above are given in J2000.0.
|The sky on 22 November 2017|
All times shown in EST.
The circumstances of this event were computed using the DE405 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.
|12 Sep 2017, 05:11 EDT||– Mercury at greatest elongation west|
|23 Nov 2017, 21:22 EST||– Mercury at greatest elongation east|
|01 Jan 2018, 19:40 EST||– Mercury at greatest elongation west|
|15 Mar 2018, 06:18 EDT||– Mercury at greatest elongation east|