Dominic Ford, Editor
From the Inner Planets feed
Mercury will reach half phase in its 2017–2018 morning apparition. It will be shining brightly at mag -0.2.
The table below lists how high Mercury will appear at sunrise over the course of the apparition. All times are given in Fairfield local time.
|17 Dec 2017||07:15||06:25||8°||south-west|
|20 Dec 2017||07:17||06:00||11°||west|
|23 Dec 2017||07:18||05:43||14°||west|
|26 Dec 2017||07:19||05:34||15°||west|
|29 Dec 2017||07:20||05:32||16°||west|
|01 Jan 2018||07:21||05:33||15°||west|
|04 Jan 2018||07:21||05:37||14°||west|
|07 Jan 2018||07:21||05:42||14°||west|
|10 Jan 2018||07:20||05:49||13°||west|
|13 Jan 2018||07:20||05:57||11°||west|
|16 Jan 2018||07:18||06:04||10°||west|
|19 Jan 2018||07:17||06:12||9°||west|
|22 Jan 2018||07:15||06:19||7°||west|
|25 Jan 2018||07:13||06:25||6°||west|
|28 Jan 2018||07:11||06:32||5°||west|
A graph of the phase of Mercury is available here.
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 days 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 hours, 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 27 December 2017|
9 days old
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.
|27 Dec 2017||– Mercury at dichotomy|
|28 Dec 2017||– Mercury reaches highest point in morning sky|
|01 Jan 2018||– Mercury at greatest elongation west|
|25 Jan 2018||– Mercury at aphelion|