Where to get rate of change for calculating ephemeris from JPL Horizons I have a working implementation of Schlyter's method for calculating the Sun's position.   I would like to know how Schlyter got the 3.24587E-5_deg in the following calculation for Mercury.

N =  48.3313_deg + 3.24587E-5_deg   * d    (Long of asc. node)


I could not find this number in JPL Horizons.   I'm guessing it's the difference between the N at two epochs divided by number of days between the two epochs, but I'm not sure.
My email to Paul Schlyter and his reply, posted with permission:


  On http://www.stjarnhimlen.se/comp/tutorial.html you note that
  Mercury's ascending node is given as:
  
  N =  48.3313_deg + 3.24587E-5_deg   * d
  
  According to https://ssd.jpl.nasa.gov/txt/p_elem_t1.txt Mercury's
  ascending node precesses in the negative direction.
  
  As Where to get rate of change for calculating ephemeris from JPL Horizons notes, NASA's
  value matches the negative of your value almost exactly.
  
  Are you using a different sign convention here, or is this an error?


Reply:


  It's neither - instead I use a somewhat different reference system.
  
  I use the "epoch of the day" instead of the fixed epoch of, say,
  J2000.0. So add the rate of precession to the negative rate of change
  you found at NASA's site, and you should get my rate of change.
  
  Since I use the "epoch of the day", the positions I get are suitable
  to compute e.g. the rise, transit, and set times for the planets. Of
  course they are also good for computing the positions of several
  planets relative to one another. But if you need the positions to plot
  on a star map drawn for some fixed epoch, you need to apply a
  correction for precession to these positions.
  
  Yes, it can be confusing to distinguish those different astronomical
  coordinate systems from one another. But that's a consequence of
  living on the planet Earth which orbits the Sun, rotates, and wobbles.