NavList:

Francis, you wrote:
"There are rare occasions when the moon /sun altitudes are such that the LDsd is the same as the LDo.(no clearance required) Is there any way of telling this from eyeballing the sky? Or predicting from the almanac?"

I'm not quite sure what you're asking here, but I have a guess. There are cases where the observed limb-to-limb lunar --the actual angle taken off the sextant-- nearly equals the cleared lunar after all the work is done. Is that what you're talking about? This happens because the pre-clearing step adds the sum of the semi-diameters which is close to half a degree. And the effect of the Moon's parallax, which is the primary component of the clearing, can easily be around half a degree subtracted from the distance. There's no special significance to these cases. The Moon's parallax along the lunar arc is HP*cos(alt_moon)*cos(alpha) where alpha is the angle between the lunar arc at the Moon and the vertical at the Moon, and since HP is nearly a degree, if the product of those two cosine factors is about equal to one-half, then the clearing correction will be about the same as the sum of the semi-diameters.

-FER
----------------------------------------------------------------
NavList message boards and member settings: www.fer3.com/NavList
Members may optionally receive posts by email.
To cancel email delivery, send a message to NoMail[at]fer3.com
----------------------------------------------------------------