Calculate Crescent Width

Thanks.

I have the phase as the percentage and I am getting roughly the right
answer, but out by a few arcseconds.

I have read that just by knowing elongation it is possible to compute the
width of the cresecent. Is that so?


"Bill Owen" wrote in message
...
N wrote:

Hi,

To calculate crescent width of the moon would the following formula be
correct,

apparent diameter * phase ?

Does the arc length of the moon need to be taken into account ?

Depends on what you mean by "phase".

1) If it's expressed as a percentage, with 0 for new moon and 100% for
full moon, then yes, apparent diameter * phase is good enough.

2) If it's the phase *angle* in the sense used by planetary scientists,
then 0 degrees phase is full moon, 90 degrees is either first or last
quarter, 180 degrees is new moon, etc. In this case, the formula you
want is
apparent RADIUS * (1 + cos phase)
or
apparent DIAMETER * (1 + cos phase) / 2
or
apparent DIAMETER * cos^2 (phase/2)

Either way, you get the width of the thickest part of the crescent.
-- Bill Owen