Day Question

[tikual]

I am sorry for asking a stupid question. What is Julian Day? I have used 'Clay's hammer' to convert day and the result is 2452770. I think that the unit should be 'day'. So what is the first day it counted?

Thanks!

[Jeff Schussele]

Hi Kit,

The unit is seconds - Specifically from/before 1/1/70 GMT I believe. This is historically the birthday of UNIX as I understand it.

Rgds,
Jeff

[John Poff · Accepted solution]

Hi,

I found a web site that explains Julian Dates. Here is an excerpt:

"Julian dates (abbreviated JD) are simply a continuous count of days and fractions since noon Universal Time on January 1, 4713 BCE (on the Julian calendar). Almost 2.5 million days have transpired since this date. Julian dates are widely used as time variables within astronomical software. Typically, a 64-bit floating point (double precision) variable can represent an epoch expressed as a Julian date to about 1 millisecond precision. Note that the time scale that is the basis for Julian dates is Universal Time, and that 0h UT corresponds to a Julian date fraction of 0.5. "

Here is the URL for the site:

http://aa.usno.navy.mil/data/docs/JulianDate.html

I think my friend Jeff is thinking of the epoch time in seconds from Jan. 1, 1970, which is widely used in Unix.

JP

[tikual]

Thanks Jeff,

But, I don't think that is 'seconds' in my result. I have typed as 'caljd.sh 05 04 2003'. Do the format go wrong?

[tikual]

Thanks John,

You provided that URL is really useful to me. I can prepare my script now. Thank you so much. Thanks to Jeff too although you may misunderstand my poor english. :p

[Jeff Schussele]

OK, I plead ignorance.
I searched back..I was mistaken.
Julian days started 4713 BCE.
But the point is the units of measure IS seconds since/before.

Rgds,
Jeff

[A. Clay Stephenson]

Actually, if you had simply done a caljd.sh -u; it would have explained itself.