This chapter introduces basic terminology and concepts needed to understand the measurement of solar and infrared radiation. The Sun is considered by astronomers to be a yellow dwarf star somewhere in the middle of its life cycle. Solar radiation incident just the Earth’s atmosphere is called extraterrestrial radiation. The solar zenith angle is defined as the angle between the zenith and the Sun. The hour angle and declination can be used to point an equatorial-mounted instrument and the zenith angle and azimuth angle can be used to point an alt-azimuth-mounted solar device. Algorithms are easier to formulate and use in solar time, but it is important to relate the solar position and descriptions to the local standard time. The calculation of sunrise and sunset times provides an easy exercise to test understanding of the information presented so far. The ultimate source of the Sun’s radiative energy is the fusion of hydrogen atoms in the Sun’s core.