A few test cases for making sure we are talking about the same thing when we set up the cusp angles and KE offset.
The agreed convention is to measure angles positive East of North. Blade starting position (at least conceptually) is with the KE covering the East side of the lunar disc and pointing North-South. After the angle is set the blade is positioned in offset (positive only by convention – the KE has to be advanced from disc center enough to over the bright parts and a little more of the DS. Note that the blade is FIRST rotated so that the BS is covered.
JD2466871 November 5 2011: The Moon is bright on the WEST side (the side towards DEcreasing RA). The phase is over half, i.e. more than half the Moon is bright. Judging from the display in ‘skychart’ the Cusp Angle is -195 degrees, i.e. to cover the BS the blade is rotated by negative 195 degrees. A solution would also be to rotate it by +165 degrees. Since the BS is still vissible the blade is now advanced at right angle to its edge by about 3/4 lunar radii – i.e. by 0.19 degrees.
JD2455866 October 31 2011: The Moon is less than half and is bright in the West side.
Judging by the display on skychart the blade should be rotated -186 or +174 degrees. The blade should be advanced although the DS is more than half the disc, since we want to get away from the cusps, so advance by 1/4 lunar radius or 0.12 degrees.
Remember – the blade advancement is always positive – never negative. It pushes FORWARD from its starting position at disc center and covers MORE of the disc. As the example on JD2455866 shows the advancement is always at least 1/8 degree and can reach as much as almost 1/4 degree.
