J2000 datetime to MJD

[Marijn]

For my current project, I need to convert J2000 datetime's to MJD.

I've found the javascript funtion below. But it does not include a time, only the date. The math is confusing me. Can anybody help me to alter this code so it will output a full MJD?

function J2000_to_YMD(t) {
  // Convert seconds past J2000 to (Year,Month,Day)
  var j,dg,c,dc,db,a,da,m, YMD = [];
  j = Math.floor(2451545 + t/86400 + 0.5) + 32044;
  dg = j % 146097;
  c = Math.floor((Math.floor(dg/36524) + 1)*3/4);
  dc = dg - c*36524;
  db = dc % 1461;
  a = Math.floor((Math.floor(db/365) + 1)*3/4); 
  da = db - a*365;
  m = Math.floor((da*5 + 308)/153) - 2;
  YMD[0] = Math.floor(j/146097)*400 + c*100 + Math.floor(dc/1461)*4 + a - 4800 + Math.floor((m + 2)/12); 
  YMD[1] = (m + 2) % 12 + 1; 
  YMD[2] = da - Math.floor((m + 4)*153/5) + 123;// + JD%86400;
  return YMD;
}

 

[bcbarnes]

The Modified Julian Date is a continuous count of days since midnight UTC at the beginning of November 17, 1858 (in the Gregorian calendar).

The J2000.0 TT offset (ΔtJ2000) is a continuous count of days since noon TT on January 1, 2000.

So - to convert J2000 to MJD, all you need to do is find the MJD for 1/1/2000 at noon (which the date.exe tool supplied with orbiter can easily do - here's a hint: 51544.500000), then to convert j2000 to MJD simply add that value (51544.500000) to your J2000 value.

 

[Marijn]

Thanks. An example of the value I want to convert is 660787200. These are seconds after Jan 1st 2000 at noon.

(660787200/86400) + 51544.5 = 59093.5.

Does that seem right? The examples I found all look a lot more complicated.

 

[bcbarnes]

MJD 59093.5 according to the orbiter utility "date.exe" is Sept. 1, 2020. Is that what you where hoping for? If so, you got it right.

 

[Marijn]

660787200 should correspond with Dec-09-2020. That's the output of the FormatDate function posted above, which I took from this Nasa site: https://trajbrowser.arc.nasa.gov/javascript/traj_lib.js

So the formula does not work (yet). Any other suggestions are very welcome. I can break my head on these things..

 

[bcbarnes]

Actually, it was the math above that is the problem:

660787200/86400 = 7648 days

51544.5 + 7648 = 59192.5, not 59093.5

59192.5 = Dec 9, 2020 - as expected.

Sorry I didn't catch this before.

 

[Marijn]

59192.5 = Dec 9, 2020 - as expected.

Thanks! I did enter 59093.5 in the date.exe as well to check and missed my own error again. How stupid of me. Sorry for that.

 

[bcbarnes]

No worries. Just repaying all the help I've received here.