Celestia Forums

I'm not sure if this is the correct thread to post this, but...

I started to create a document for my own personal reference, a listing of the Beginning and Ending dates and times in UTC (Zulu) for each of the addon satellites I now have (12, plus the GPS constellation). I've run into a bit of a problem though.

Some of the addons state their visual date/times, others don't... so I started prodding through the SSC files to pull out the data, which is in Julian format for Celestia, and then plugged the values into one of the calandars I got from the documentation (http://www.calendarhome.com/converter/)... but I'm getting dates in return which are different.

I'll use the astp & astpmodel (Apollo-Soyuz Test Program) for example -

Epoch = 2442615.1872048 (08 Jul 1975 per the calculator)
Beginning = 2442608.97222222 (02 Jul 1975 per the calculator)

In Celestia though, the satellite appears at 15 Jul 1975 11:20UTC precisely. I've not checked every one of my addons yet, but so far I find this of the first two (the other is explorer1 & explorer1model).

So, before I go any further... is there something I'm missing here (calculating incorrectly) having to do with the Epoch? I've been browsing the documentation but I've either missed it or not found it yet.

Could someone point me in the right direction?

Thanks!

You seem to be using the wrong calculator and looking at the wrong SSC file.

The alphabetic dates that you calculated would be correct if we still used the Julian calendar, but it hasn't been used for centuries. As that web page says, the world is currently using the Gregorian calendar: the calculator display in the top left of the page.

Also, you seem to be specifying the wrong Beginning and Ending dates. The ones specified in the SSC file for Jack's Addon are


which translate into 1975 July 15 11:19:35
and 1975 July 24 20:18:23

You can also use strings to specify times in a more human readable format. For example to set the beginning date to 1975 July 15 11:19:35 you can either use this:

Beginning 2442608.97222222

or this:

Beginning "1975 7 15 11:19:35"

--Chris

chris wrote:You can also use strings to specify times in a more human readable format. For example to set the beginning date to 1975 July 15 11:19:35 you can either use this:

Beginning 2442608.97222222

or this:

Beginning "1975 7 15 11:19:35"

--Chris

OOOH ! I didn't knew that ! Tis is VERY usefull, thanks Chris !

My apologies for the mixup in thinking Celestia used Julian dates rather than Gregorian, I was under the impression it did. Still, I'm finding this most disturbing, because the calculator seems to not work for me.

If I enter Beginning date "2442608.97222222" into the Gregorian calculator Greg. serial day: field and click the Calculate button below, it tells me in the fields above that the date is 19 August 6688 Leap year 23 : 19 : 59 Sunday, and Ending date "2442618.34611111" yields 29 August 6688 Leap year 8 : 18 : 23 Wednesday. Also, entering the Gregorian date/time of 1975 July 15 11:19:35 yields "721184.4719328703".

Interestingly enough, using this calandar calculator with "2442608.97222222" as the Given Julian Date yields 1975-07-15 11:19:60 (correct date though 20 seconds off from what the time should be).

I think you are confusing Astronomical Julian day numbers (number of days since Noon, January 1st, 4713 BC) with Christian Julian dates. Celestia uses both Astronomical Julian day numbers (fractional numbers) and Astronomical Gregorian dates (year month day).

On the calendar calculator page, you should enter the Astronomical Julian day number in the Julian Day entry field (middle box, 2nd row) click on its "Calculate" button, and read the Astronomical Gregorian date from the top left display. (Note that this is *not* the Christian Gregorian date: the Astronomical calendar includes a year 0, the Christian calendar does not. The calendar page you're using is one of a very few that use the Astronomical Gregorian calendar.)

The Web page http://www.hermetic.ch/cal_stud/jdn.htm
has a good description of the definition of Astronomical Julian day numbers. It links to pages with details about the differences among the various Western calendar systems.

[edit]
The difference between the two times is due to the inclusion of leap seconds in one of them. I'm leaving it as "an exercise for the student" to determine which does it and why the other one does not .
[/edit]

Thanks Selden. I remember some of this, but off the top of my head the why escapes me. I've opened that link and will be reading it after this.

First, I have to laugh a bit here, seeing the bottom of that link's page... another link entitled "Why Is Wednesday November 17, 1858 The Base Time For VAX/VMS?", which I've opened. Back in the day (1989-1992 in particular) I worked with VAX systems, and VMS (and Dibol lol). I remember seeing that very same technical brief about time in VMS, the lot of us sitting around a conference room table laughing about it! So, thanks for the memory LOL!