Moon/Sun Size Comparison Overlay Info
The purpose of the Current Moon/Sun Size Comparison Overlay program is to do as implied and compute the current relative geocentric sizes of the moon and sun and then overlay their relative disks for size comparison. 

The moon and the sun in the sky appear to be almost the same size, about 1/2 degree wide on the celestial sphere.  However, due to their continuous motion and changing distances, their sizes vary slightly.  If compared side-by-side at any given moment, sometimes the lunar disk would be larger than the sun and sometimes the solar disk would be larger than the moon.  It is also possible for them to be the same apparent size.

Below are some demonstrative examples.


EXAMPLE: 1

Lunar Disk Larger Than Solar Disk


In the diagram below, the outer black ring represents the moon.  The shaded yellow inner circle represents the smaller solar disk.  If there was a solar eclipse at that moment, then it would be a total eclipse, since the lunar disk is larger than the solar disk by the thickness of the black outer circle.




EXAMPLE: 2

Solar Disk Larger Than Lunar Disk


In the diagram below, the outer bright yellow ring represents the sun.  The black inner circle represents the smaller lunar disk.  If there was a solar eclipse at that moment, then it would be an annular eclipse, since the lunar disk is smaller than the solar disk by the thickness of the bright yellow outer circle and would not completely cover the sun enough for a total eclipse.

Jay Tanner - $cYear