How often does a 'Dry Moon' happen with the horns of the Moon pointed straight up?

A 'Dry Moon' requires that the Moon's orbit be nearly perpendicular to the horizon above the setting Sun. On February 8, 1997 this condition was satisfied from some locations on the Earth. You can use the formula below to figure out the Position Angle of the terminator:


am = ra(moon)         as = ra(sun)
dm = dec(moon)        ds = dec(sun)

        -1             cos(ds) sin( ra(sun) - ra(moon) )
PA = tan   (--------------------------------------------------------- )
             cos(dm)sin(ds) - sin(dm)cos(ds)cos( ra(sun) - ra(moon))


We want a PA near 180 degrees by this convention of measuring angles. This will come about if the argument of the arcTangent = 0. That means that the right ascension of the Sun and the Moon are the same. But to be a 'Dry' crescent Moon, this must happen within a few days before or after New Moon.

The Moon visits the same RA as the Sun once each month. To be there within a few days after New Moon happens about once every 29/2 or 29/4 months or once each 7-14 months I would estimate depending on whether you choose a 2 or 4 day old crescent phase.


Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.