wyatt@cfa.harvard.EDU (Bill Wyatt) (11/10/87)
I sent mail to the original author, and then decided there might be some general usefulness in posting the article... > ...the system base date and time, which is 00:00 o'clock [sic] > November 17, 1858 (the Smithsonian base data and time for the > astronomical calendar)... > > VAX/VMS System Services Reference Manual > page 9-2, Section 9.1 The System Time Format > > Unfortunately, no one at either the Smithsonian Museum in Washington, D.C. > or at the Smithsonian Astrophysical Observatory in Massachusetts had *ever* > heard of this ``astronomical calendar;'' not to mention that they were > supposed to have been responsible for it! Well, I work at the Smithsonian Astrophysical Observatory, and I have heard of it and used it. If you take Julian Day Number (zero point is GMT noon Jan 1, 4713 B.C.), and subtract 2,400,000.5 from it, the zero point becomes the one above (I assume, I haven't worked it out, but I know it's in the late 1850's). This new counting value we called `Smithsonian Day', sometimes `modified Julian Day Number'. The current SD is about 47,000 by this reckoning. The reason for the `new' zero point, as far as I knew, was that SAO used to (1957-1983) track satellites for geophysical reasons. Lots of ephemerides calculations, but the early computers had trouble handling lots of significant figures. Thus, we subtracted the `useless' most significant digits off the JDN. In addition, most routines using SD took the integer and fractional parts of the time as separate arguments. I myself had to take routines, originally written for some antique IBM (1109???), converted to a CDC 3200, then a CDC 6400 (where I also used them), to the Vax 11/780. Veritable computer archaeology; relics all over the place. The same group developed the SAO Catalog, (originally to be able to find lots of stars to measure satellite images against), and used the same time base. I have no idea how the people at DEC decided to use the same time base, but there is a lot of cross-fertilization going on in the Cambridge area (:-). Bill UUCP: {seismo|ihnp4}!harvard!cfa!wyatt Wyatt ARPA: wyatt@cfa.harvard.edu (or) wyatt%cfa@harvard.harvard.edu BITNET: wyatt@cfa2 SPAN: cfa2::wyatt P.S. Our first 11/780 was only purchased after DEC agreed to develop a new floating point format with more range. We thus were responsible for, and became the first site with: G-float arithmetic. -- Bill UUCP: {seismo|ihnp4}!harvard!cfa!wyatt Wyatt ARPA: wyatt@cfa.harvard.edu (or) wyatt%cfa@harvard.harvard.edu BITNET: wyatt@cfa2 SPAN: 17410::wyatt (this will change, sometime)