Print

Print


The site given in my earlier post
http://www.fourmilab.ch/yoursky/
allows Universal time or Julian time defined as
"Universal time:

    This option permits you to enter any Universal (also known as
Greenwich Mean) date and time in the format year/month/day
hour/minute/second. You can omit the minutes and seconds of the time,
or omit the time entirely if you wish to specify 00:00 UTC.

    You can enter dates as far back as Julian day 0, January 1, −4712
and as far into the future as you wish. Note that astronomers and
historians use different conventions for years before A.D. 1. In
history books, the year that preceded A.D. 1 is called 1 B.C., zero
not having come into use in European culture at the time. Astronomers
denote the year before A.D. 1 as "year 0". Thus when an astronomer
talks about an eclipse having occurred in the year −412, that's the
year historians refer to as "413 B.C.". In converting historical dates
to Julian days, Your Sky assumes the canonical date for the adoption
of the Gregorian calendar, Friday, October 15th, 1582. Many countries
shifted to the Gregorian calendar much later; in Great Britain, not
until 1752. When investigating events in history, make sure you
express all dates after October 15th, 1582 in the Gregorian calendar.

Julian day:

    Astronomers often have to do arithmetic with dates and times. The
Gregorian calendar is sufficiently eccentric that answering a question
like "What is the date and time 295.03589 days (10 lunar months) from
now?" is a nontrivial exercise. To facilitate computation, astronomers
employ the Julian day calendar. The Julian day number for a given
moment in time is simply the number of days, whole and fractional,
elapsed since noon (12:00) Universal time on 1st January −4712; time
is expressed as a fraction of a day. This system allows assigning a
positive number to the date and time of any observation in recorded
history and arbitrarily far into the future, and permits ordinary
arithmetic with dates and times without having to worry about B.C. and
A.D., Julian and Gregorian calendars, leap years, and all that. Of
course, you need to be able to convert back and forth between Julian
days and the civil calendar, but that's what computers are for.

    To show a star map for a given Julian day, simply check the box
and enter the Julian day number in the box to the right, including a
fraction where appropriate. (Be sure to remember that Julian days
begin at noon—a Julian date representing midnight will end in ".5".)
Click "Update" to show the map for the designated observing site at
that day."
Ο Θεός σας ευλογεί πάντα, όλοι οι τρόποι,
(as Aristotle said to Plato, "It's all Greek to me.")
Paul

On Jan 12, 2008 2:55 PM, Mark J. Reed <[log in to unmask]> wrote:
>
> On Jan 12, 2008 3:10 PM, Lars Finsen <[log in to unmask]> wrote:
> > Den 12. jan. 2008 kl. 20.23 skreiv Mark J. Reed:
> >
> > > Exactly when that year was depends on what you're talking about.  We
> > > currently use the Gregorian calendar, and sometimes astronomers extend
> > > it backwards well before its inception in 1582 to refer to past dates.
> > >  But usually dates before then are given in the Julian calendar,
> >
> > Astronomers used to reckon time in Julian days, before and after
> > 1582. Has that changed in later years?
>
> Astronomers still use Julian days, but mostly for calculations.
> Descriptions like "the total solar eclipse of JD 2,454,680" are not
> very meaningful, even to astronomers, without a conversion to a more
> conventional calendar system.
>
> For the year 0 CE = 1 BCE = AUC 753, the Julian Day that begins at
> noon UTC on each January 1st is as follows:
>
> January 1, proleptic Julian calendar          = JD 1,721,058
> January 1, contemporary Julian calendar = JD 1,721,059
> January 1, proleptic Gregorian  calendar  = JD 1,721,060
>
> --
>
> Mark J. Reed <[log in to unmask]>
>