the planet mercury and wednesday -- 12/27/18 End of Year Selections: Space

Today's encore selection -- from Planet Mercury by David A. Rothery. The planet Mercury is the closest to the Sun, reaches 700 degrees centigrade, is only slightly larger than the Earth's Moon, and, of the planets known to the ancients, is the most difficult to locate in the sky. Mercury has roughly 3 "days" for every 2 of its "years." It is closely associated with Wednesday:

"Unless you are in the tropics (or witness a total solar eclipse), you will never see Mercury in a completely dark sky, because when the Sun is far enough below the horizon for the sky to be dark, Mercury will be below the horizon too. You have to be in the right place at the right time to see it at all. Discounting total solar eclipses, which are extremely rare, the right place is anywhere with a clear, low horizon in the direction of either the sunset or the sunrise as appropriate. The right time is after sunset as the sky darkens, or, for early risers, before dawn. Even that will do you no good unless you have also chosen a date to coincide with Mercury's brief excur­sions far enough from the Sun for it to be sufficiently high above the horizon for it show itself in the brief interlude between daylight and full darkness. I have often glanced unawares at the sky and seen Venus shining high and bright in the evening or morning sky. You can hardly miss Venus when it's around, but I've never seen Mercury without deliberately setting out to look for it.

Mercury in enhanced color, imaged by MESSENGER (2008)

"So what does Mercury look like? A pink star, that shows as no more than a pale pink dot through binoculars or any telescope to which you or I are ever likely to have access. Disappointing in a way, but to see Mercury with your own eyes is to see our Solar System's smallest planet, and the one closest to the Sun. Congratulations if you have achieved this -- you have seen the trickiest to locate of the five planets known in the ancient word: Mercury, Venus, Mars, Jupiter and Saturn. The great Copernicus himself, working for most of his life near the shores of the Baltic Sea, is reputed never to have managed to see it.

"The earliest known observations of Mercury are recorded on Assyrian clay tablets dated to 687 BC, ... [and] the name given to Mercury by the Assyrians translates as 'the jumping planet', perhaps a reflection of its rapidly changing position in the sky, and of how it can appear east and west of the Sun in fairly rapid succession. The Babylonians identified Mercury with Nabu and the Egyptians with Thoth, their deities associated with writing and who sometimes acted as the messenger of the gods. That was also the role of the ancient Greek Hermes and his Roman equivalent Mercury, though it took the Greeks a while to realise that morning and evening apparitions of Mercury were the same object. They were similarly confused over Venus.

"To the imperial Romans, Wednesday was dies Mercurii (Mercury's day) and many languages descended from Latin preserve the name Mercury for the corresponding day in various forms: mercredi, mercoledi, miércoles, miercuri (French, Italian, Spanish, Romanian) and even Dydd Mercher (Welsh). The Norse/Germanic personification of Mercury was Odin/Wotan, from which English derives Wednesday (Woden's day), Dutch woensdag, and Danish, Norwegian and Swedish Onsdag. Although German preserves the names of gods in other days of the week, Wotansdag was replaced by the more prosaic Mittwoch ('Midweek') more than a 1,000 years ago, a convention also followed by Icelandic and most Slavic languages.

"Oriental writings more than 2,000 years old recognise Mercury as Chen Xing ('the hour star') in China, and as Budha (god of merchandise) in Hindu mythology. In modern Chinese, Japanese, Korean and Vietnamese Mercury is literally 'the water star'.

"The principal modern languages of the Indian subcontinent preserve the Budha element in their word for Wednesday, whereas in Chinese, Japanese and Korean Wednesday translates as 'water day': so all these preserve a link between the planet Mercury and Wednesday. ...

"The next transit of Mercury [across the sun] will happen ... 11 Nov 2019 starting 12:35 and ending 18:04. There will then be a 13 year gap until the next transit on 13 November 2032, followed by 7 Nov 2039 and 7 May 2049."



David A Rothery


Planet Mercury


Springer International Publishing Switzerland 2015


1-7, 12-13
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