I understand. Thank you for explaining this. It is really awesome to learn about all of this.
TIL about umbras and penumbras.
For those interested, I did some solar physics problems to back up what rbray89 said... And I only got Kerbin done in about 2 hours. It was truly a pain in the rear.
Essentially Kerbin has a Penumbra of 1.832° starting at the moon (12,000km away) and an umbra of 2.046° starting at a focal point 1,605km away from the planet's surface. The ratio of the umbra shadow to the perumbra shadow on the planet is about 1:20. The perumbra is a subtle gradient as a shadow, but is noticeable in shadows in real siruations.
With Duna, whose moon is 1/4 of the distance Mun is from Kerbin, and orbits almost twice as far from the Sun, the shadow casted will be sharper, with shallower angles than Kerbin's shadow. Therefore the shadow is bigger (relative to Ike's radius), and the perumbra is smaller (in this case hardly noticeable).
P.S. Physics can be very hard.