Re: [Orekit Users] Time question - julian date

[beowulf zhang <beowulf.zhang@gmail.com>]

The absolute class not have the constructor:
AbsoluteDate t = new AbsoluteDate(DateComponents.JULIAN_EPOCH, my_jd, utc);

I use the code:

DateComponents dc = new DateComponents(DateComponents.JULIAN_EPOCH, 2456413);
TimeComponents tc = new TimeComponents(0.5*86400);
AbsoluteDate jdEpoch = new AbsoluteDate(dc, tc, utc);

but I get:

2013-04-30T12:00:00.000

not

01-May-2013 00:00:00

could you tell me the reason?

2011/5/28, MAISONOBE Luc <luc.maisonobe@c-s.fr>:
> Petrus Hyvönen <petrus.hyvonen@gmail.com> a écrit :
>
>> Hi,
>>
>> Haven't managed to try that algorithm, but downloaded the SPG4 package
>> from
>> Vallado, and I get same Julian Date there as my previous algoritm.
>>
>> If I create a own date reference on:
>> ad_jd = AbsoluteDate(-4712,01,01,12,00,00.000,utc)
>>
>> and use;
>> epoch_date=AbsoluteDate(ad_jd,my_jd,utc)
>>
>> I get conversions from julian date to .. time ... same as the Vallado
>> routines.
>>
>> Why is orekit using a reference point at -4712-01-01T11:59:27.816 ? Is it
>> different timescales?
>
> Yes, Orekit uses the TT time scale for this referene date.
> In fact, julian day is mainly used only as a number of day reference,
> and the time is added later, and can theoretically be expressed in
> different time scales, mostly TT or UTC. This means that simply saying
> a date is 2455709.29166667 is ambiguous, one should say in which time
> scales the fractional part is considered.
>
> As the other reference epoch are precisely defined in TT (J2000 is the
> best example), and as TT is the most widely used time scale for
> defining references in IAU recommendations, we have chosen in Orekit
> to use the same TT time scale for all these references. This is
> specified in the javadoc for J2000 (see
> <https://www.orekit.org/static/apidocs/org/orekit/time/AbsoluteDate.html#J2000_EPOCH>)
> but obviously it is not explained in the javadoc for JULIAN_EPOCH, my
> bad.
>
> We could perhaps provide two reference epoch, one JULIAN_EPOCH_TT and
> one JULIAN_EPOCH_UTC, but this may also confuse users.
>
> Another way to achieve what you want could be something like:
>
> AbsoluteDate t = new AbsoluteDate(DateComponents.JULIAN_EPOCH, my_jd, utc);
>
> Luc
>
>>
>> Best regards
>> /Petrus
>>
>>
>>
>> On Fri, May 20, 2011 at 5:53 PM, beowulf zhang
>> <beowulf.zhang@gmail.com>wrote:
>>
>>> This algorithm is in the JAT. link is http://jat.sourceforge.net/
>>>
>>> 2011/5/20, beowulf zhang <beowulf.zhang@gmail.com>:
>>> > I can't find the reason of this question, but I give you a piece of
>>> > program that could give correct answer.
>>> >
>>> >     /** Create a CalDate object using MJD. From Montenbruck C++ code.
>>> >      * @param mjd Modified Julian Date.
>>> >      */
>>> >     public CalDate(double mjd){
>>> >         long    a,b,c,d,e,f;
>>> >         double  Hours,x;
>>> >
>>> >         // Convert Julian day number to calendar date
>>> >         a = (long)(mjd+2400001.0);
>>> >
>>> >         if ( a < 2299161 ) {  // Julian calendar
>>> >             b = 0;
>>> >             c = a + 1524;
>>> >         }
>>> >         else {                // Gregorian calendar
>>> >             b = (long)((a-1867216.25)/36524.25);
>>> >             c = a +  b - (b/4) + 1525;
>>> >         }
>>> >
>>> >         d     = (long)( (c-122.1)/365.25 );
>>> >         e     = 365*d + d/4;
>>> >         f     = (long)( (c-e)/30.6001 );
>>> >
>>> >         long temp = (long)(30.6001*f);
>>> >         this.Day   = (int)(c - e - temp);
>>> >         temp = (long)(f/14);
>>> >         this.Month = (int)(f - 1 - 12*temp);
>>> >         temp = (long)((7+Month)/10);
>>> >         this.Year  = (int)(d - 4715 - temp);
>>> >
>>> >         Hours = 24.0*(mjd-Math.floor(mjd));
>>> >
>>> >         this.Hour = (int)Hours;
>>> >         x = (Hours-Hour)*60.0;
>>> >         this.Min = (int) x;
>>> >         this.Sec = (x-Min)*60.0;
>>> >         this.DOY = day2doy(this.Year, this.Month, this.Day);
>>> >     }
>>> >
>>> >
>>> > 2011/5/19, Petrus Hyvönen <petrus.hyvonen@gmail.com>:
>>> >> Hi,
>>> >>
>>> >> This maybe is a FAQ but anyway...
>>> >>
>>> >> I'm trying to create an absolute date based on a julian date created
>>> >> in
>>> >> another software.
>>> >>
>>> >> My julian date is 2.456413500000000e+006, corresponding to 01-May-2013
>>> >> 00:00:00 UTC according to that software and some web conversion i
>>> checked
>>> >> it
>>> >> with.
>>> >>
>>> >> with orekit i do:
>>> >> ad_jd = AbsoluteDate.JULIAN_EPOCH
>>> >> ad_jd =
>>> >> -4712-01-01T11:59:27.816
>>> >>
>>> >> AbsoluteDate(ad_jd,t*24*3600)
>>> >> ans =
>>> >> 2013-04-30T23:58:53.816
>>> >>
>>> >> There must be some kind of time scale or different references here I
>>> >> suppose, how can convert this properly?
>>> >>
>>> >> Best regards
>>> >> /Petrus
>>> >>
>>> >
>>>
>>
>>
>>
>> --
>> _____________________________________________
>> Petrus Hyvönen, Uppsala, Sweden
>> Mobile Phone/SMS:+46 73 803 19 00
>>
>
>
>
> ----------------------------------------------------------------
> This message was sent using IMP, the Internet Messaging Program.
>
>
>