7
$\begingroup$

I'm currently trying to determine the position of the sun and the moon from jpl ephemeris DE200 which is referred to the dynamical equator and equinox of 2000 and uses tdb time. I am using the j2000 system ECI.The things that trouble me are the following:

1.Is the moon position given with respect to Earth or with respect to the barycenter? Also when we find the sun position by using the earth-moon barycenter position and lunar coordinate , will the sun position be with respect to Earth?

2.If I'm using UTC time , how exactly to obtain the TDB or TAi, since it's oscillating?( After that I want to obtain the JD for TDB)

3.If I'm using J2000 ECI coordinate system , do I need to take in acount the effect of nutation and precession? And how to do that?

$\endgroup$
  • 1
    $\begingroup$ DE200 is fairly old, you may want to use DE431 instead. Arguably, the "best" way to use these files is through SPICE: naif.jpl.nasa.gov/naif $\endgroup$ – barrycarter Jun 13 '18 at 15:54
  • 1
    $\begingroup$ If I remember correctly, Jean Meeus' book Astronomical Algorithms has chapters on this with descriptions of how to do this with different levels of precision, depending on your needs. $\endgroup$ – Mick Jun 20 '18 at 6:56
2
$\begingroup$

You don't mention what you are trying to access this with but I would strongly recommend AstroPy for handling the ephemeris and handling the co-ordinates and time systems. For your questions:

  1. The Moon and the Earth are both given relative to the Earth-Moon barycenter (center of mass). Separately you can get the vectors from the Solar System Barycenter to the Earth-Moon Barycenter. So if you want SSB->Moon you will need to do two calls to the ephemeris and do vector addition.

  2. It depends how accurately you want things and how fast the thing you want is moving. You can probably use Terrestrial Time (TT) with the ephemeris and you want be horribly off for anything other than Moon. The offset TT-UTC (currently 67.184 seconds) consists of two parts; a variable part from UTC to TAI, currently 37 seconds, which consists (mostly) of the number of leapseconds and a fixed part (32.184 seconds) from TAI to TT. TDB varies around TT by about 2 milliseconds and it's somewhat periodic (dominated by Earth's orbit ellipticity and Jupiter and Saturn) but doing it properly requires a 791 term expansion (see e.g. http://star-www.rl.ac.uk/docs/sun67.htx/sun67ss154.html#Q1-158-946)

  3. Yes, if you want to do anything related from the Earth such as predicting positions of something in the sky, you will need to take the frame bias, precession and nutation into account along with Earth rotation/sidereal time to align the celestial reference frame (ICRS/J2000) with that of the Earth (ITRS).

I recommend USNO Circular 179 (http://aa.usno.navy.mil/publications/docs/Circular_179.php) which has a lot of details of how to do this in the new IAU framework. Meeus's book is very good (I have it also) but is based on the old IAU1984 equator and equinox of date methods.

$\endgroup$
  • $\begingroup$ I'm trying to implement it using matlab.The ideea is that I'm trying to build an orbit propagator in order to determine the position of 2 satellites in the ECI frame,using as initial values the position and velocity vectors of the satellites.The propagator takes account of SRP, drag, j2, and the third body(sun , moon).So ,what I understand from what you say is that if I want to take account of precession and nutation, I will first need to transform the position and velocities of the satellites in ECEF and after that to take account of precession ,nutation and after that back to ECI. $\endgroup$ – Alexandru Lapusneanu Jul 26 '18 at 23:03
  • 1
    $\begingroup$ You're brave... ;-) I think you will need some form of precession and nutation code to get from one frame to another. From what I've seen of satellite code such as Bill Gray's, all calculations are mean equator and equinox of date so you need to go one way or another through the precession/nutation matrix or its transpose to compare/add the satellite position vector to Sun/Moon vectors which will in be J2000 mean equator/equinox (ICRS). (Or convert the Sun/Moon pos to mean equator of date and ECI with a change or origin) $\endgroup$ – astrosnapper Jul 27 '18 at 1:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.