> Charles Gann wrote: > > Ponder this one! > > On the calendar day of an equinox, from the moment that day first pops > into existence somewhere on the earth, to the moment it winks out of > existence, how long does it exist? > > How long does it exist at the solstices?
In addition to the answer of Fer de Vries and others, I would say that it is also a matter of definition. If the day is defined as the interval between two moments of crossing of the meridian, I have nothing to add to Fer's answer. Things get a bit more complicated if the day is defined as the interval between two sunrises or sunsets as they then also depend on the latitude and the season of the year. However, if the day is taken as a calendar day the issue is more complicated as at a given moment two or even three calendar days can be simultaneously in progress on the terrestrial globe, all dated by the same (Gregorian) calendar. This of course has to do with the International Date Line (IDL) where the calendar date makes awkward jumps. A given calendar day, when measured in UT, starts at -12 hours when it is midnight in the time zone immediately to the West of the IDL (for instance Kiribati) and is already 12 hours old when the day starts at midnight in Greenwich. It lasts until 24 + 12 hours UT when it finally becomes midnight in the time zone immediately to the East of the IDL (for instance Samoa). So in ordinary circumstances each (Gregorian) calendar day in fact lasts 48 hours though during half of the time it is either a day earlier or later somewhere else on the globe. When Summer Time applies for countries bordering on the IDL, things get even more complicated. During the summer months on the southern hemisphere, the New Zealand Dependencies adopt a Summer Time arrangement that puts them 12 hours 45 minutes ahead of Greenwich. During that period a calendar day can thus last 48 hours and 45 minutes before it winks out of existence on the globe. And this still not the longest possible day! During the last winter, the island republic of Tonga adopted a time zone 13 hours ahead of Greenwich to which they added another hour of 'Summer Time', putting them 14 hours ahead of Greenwich. Thus the New Year's Day of 2000 (erroneously claimed by the media world wide as being the first day of the 3rd millennium) lasted from 10h UT on 31 December to 12h UT on 2 January, or 50 hours in total. ======================================================== * Robert H. van Gent * Tel/Fax: 00-31-30-2720269 * * Zaagmolenkade 50 * * * 3515 AE Utrecht * E-mail: [EMAIL PROTECTED] * * The Netherlands * * ******************************************************** * Home page: http://www.phys.uu.nl/~vgent/homepage.htm * ========================================================
