Wikipedia:Reference desk/Archives/Mathematics/2008 May 19

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May 19

chemical mathematics

what is chemical math? and specifically is graph energy a subcategory? also, what is the definition of both 'chemical math' and 'graph energy' and where are their applications? —Preceding unsigned comment added by 79.127.9.254 (talk) 04:00, 19 May 2008 (UTC)

Chemical math - do you mean stoichiometry? Neıl 龱 15:00, 19 May 2008 (UTC)

For a basic chemistry class I don't think you need to know any math beyond algebra. StuRat (talk) 19:31, 19 May 2008 (UTC)

Days of the Week for BC

I found a website that allows me to calculate the day of the week for any given AD date. (http://katzentier.de/_misc/perpetual_calendar.htm) However this doesn't include BC, and I don't know how to calculate it...should I just use the calendar worky-thingy as if it were BC or is there something/somewhere else I need to do/can go?

Lady BlahDeBlah (talk) 18:42, 19 May 2008 (UTC)

Try entering BC dates as negative numbers. X BC is the same as -X+1 AD (the +1 is because there was no year 0, it goes from 1 BC to 1 AD), so 20 BC gets entered as -19. --Tango (talk) 19:00, 19 May 2008 (UTC)

The thing on the website won't let me enter the '-'... Lady BlahDeBlah (talk) 19:16, 19 May 2008 (UTC)

Also note that the farther back in time we go, the fewer civilizations will be using the familiar 7-day week. I doubt if American pre-Columbian civilizations would have a 7-day week, unless by sheer coincidence. I wonder if we have an article on this ? StuRat (talk) 19:27, 19 May 2008 (UTC)

Yes, of course, but this is really more for my benefit, as in 'Just wondering'. I think the site only has 4 spaces for the year, so I can do something like 86 BC, but not like 2270 BC. Lady BlahDeBlah (talk) 19:50, 19 May 2008 (UTC) EDIT: Nope, I tried it with the minus and it doesn't like it. Blargh!!! Lady BlahDeBlah (talk) 19:53, 19 May 2008 (UTC)

The site explains the method - you'll just have to do it by hand (or google for another calculator). --Tango (talk) 20:17, 19 May 2008 (UTC)

Claus Tønderings' Calendar FAQ might provide some good information about this kind of thing. Note especially that while the seven-week cycle was strictly followed much before 1 AD, the Julian calendar was introduced only in 45 BC, so you can't reliably calculate when a date before that was. – b_jonas 21:00, 19 May 2008 (UTC)

This page will compute the Julian day for any date back to 4713 BC (using the Julian calendar for all dates prior to the October 1582 switchover). To get the day of the week from the Julian day, round to the nearest integer (rounding up if it's halfway between) and take the remainder modulo 7. 0 is Monday, 1 is Tuesday, and so on. This isn't quite as convenient as one might hope for, but it'll do in a pinch. -- BenRG (talk) 21:13, 19 May 2008 (UTC)

I believe you can probably just add a multiple of 400 years to get an equivalent AD date. --Prestidigitator (talk) 21:15, 19 May 2008 (UTC)

That works for Gregorian dates, but I think it's much more common to use the Julian calendar for very old dates, in which case you need to add a multiple of 28 instead, and use a Julian day-of-week calculator. -- BenRG (talk) 21:52, 19 May 2008 (UTC)

Note that the choice of Julian or Gregorian calendar depends not just on date but on location too. Only Catholic countries adopted the Gregorian calendar in 1582 - many Protestant countries were deeply suspicious of its Catholic origins. Britain only adopted it in 1752, and Greece continued to use the Julian calendar until 1923. Gandalf61 (talk) 08:46, 20 May 2008 (UTC)