Solar Zenith Angle is a important parameter in radiance to reflectance conversion.Solar Position Calculator is a free online tool created by University of Oregon Solar Radiation Monitoring Laboratory
Calculator for solar zenith angle,
Declination,
Julian day,
Equation of time,
Hour angle,
Instantaneous and Daily Extraterrestrial radiation values,
Sunrise and Sunset times.
CALCULATOR LINK
mygeospatial 