During observation, the Atmospheric extinction coefficient is 0.17 mag per unit airmass at = 490 nm. If the outer atmosphere monochromatic magnitude star is m(490 nm) = 18.0 evaluate : a) Star's observed (inner atmosphere) magnitude, m(490 nm), at an altitude of 20 deg over the horizon?

b) If the hour angle is 0 and Declination is -5 degrees south at latitude 40 degrees determine the star observed within the atmosphere m=490nm