def _at_time(self, m):
### J. Meeus, p. 103
loc = self.loc
rts = self.rts
### sid: sidereal time at Greenwich, in degrees,
### at time `m` (expressed as fraction of a day)
self.sid = sid = Angle_D.normalized \
(rts.sid.degrees + 360.985647 * m)
### n: `m` corrected by difference in
### Terrestrial Dynamical Time and UT
self.n = n = m + self.day.delta_T / 86400.0
self.alpha = alpha = Angle_R(rts.interpolator_a(n))
self.delta = delta = Angle_R(rts.interpolator_d(n))
self.ha = ha = hour_angle(sid, loc, alpha)
self.altitude = alt = altitude(delta, ha, loc)
return ha, delta, alt
