|Wed, 21 Jun 2017 at||06:36 EDT||(210 days ago)|
Dominic Ford, Editor
From the Inner Planets feed
In the southern hemisphere Mercury will be well placed for observation in the dawn sky, shining brightly at mag -2.4.
From Ashburn (click to change) however, it will not be readily observable since it will be very close to the Sun, at a separation of only 1° from it.
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:
|23 May 2017 10:35 EDT||– Mercury at dichotomy|
|21 Jun 2017 06:36 EDT||– Mercury at greatest brightness|
|21 Jun 2017 10:02 EDT||– Mercury at superior 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 before sunrise Mercury will rise each night; all times are given in Ashburn local time.
|Altitude of Mercury
|Direction of Mercury
|14 Jun 2017||05:42||05:10||4°||north-west|
|21 Jun 2017||05:43||05:41||0°||north-west|
|28 Jun 2017||05:45||06:20||-6°||north-west|
|05 Jul 2017||05:48||07:01||-11°||west|
|12 Jul 2017||05:52||07:36||-17°||west|
|19 Jul 2017||05:58||08:04||-21°||west|
|26 Jul 2017||06:03||08:23||-24°||west|
|02 Aug 2017||06:09||08:30||-26°||west|
|09 Aug 2017||06:16||08:23||-24°||west|
|16 Aug 2017||06:22||07:56||-19°||west|
|23 Aug 2017||06:28||07:08||-8°||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 21 June 2017|
All times shown in EDT.
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.
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.
|17 May 2017, 18:58 EDT||– Mercury at greatest elongation west|
|29 Jul 2017, 20:24 EDT||– Mercury at greatest elongation east|
|12 Sep 2017, 05:11 EDT||– Mercury at greatest elongation west|
|23 Nov 2017, 21:22 EST||– Mercury at greatest elongation east|