import math


def axis_tilt_projection(longitude,tilt):
  """
  Project of Z-axis of a tilted frame

  The tilt is in the YZ plane towards -Y

  @param longitude : longitude in XY plane from the X-axis (Y = 0)
  @type  longitude : float

  @param tilt : tilt angle measured CCW about X-axis
  @type  tilt : flat

  @return: radians
  """
  tan_projected_tilt = - math.cos(longitude)*math.tan(tilt)
  projected_tilt = math.atan(tan_projected_tilt)
  return projected_tilt