WOW!!!! What a great answer! I was curious to see if anyone would take the zig-zag international date line into account. It did not occur to me to take summer time at various locales into account.
Again, a great answer Robert!! Thanks!!! Charlles ----- Original Message ----- From: "R.H. van Gent" <[EMAIL PROTECTED]> To: "Charles Gann" <[EMAIL PROTECTED]> Cc: "Sundial List" <sundial@rrz.uni-koeln.de> Sent: Tuesday, July 04, 2000 5:59 PM Subject: Re: How long is a day.....really? > > 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 * > ======================================================== > >
