The solar zenith angle is the zenith angle of the sun, i.e., the angle between the sun’s rays and the vertical direction. It is the complement to the solar altitude or solar elevation, which is the altitude angle or elevation angle between the sun’s rays and a horizontal plane. At solar noon, the zenith angle is at a minimum and is equal to latitude minus solar declination angle. This is the basis by which ancient mariners navigated the oceans. Solar zenith angle is normally used in combination with the solar azimuth angle to determine the position of the Sun as observed from a given location on the surface of the Earth. Formula where is the solar zenith angle is the solar altitude angle, is the hour angle, in the local solar time. is the current declination of the Sun is the local latitude. Derivation of the formula using the subsolar point and vector analysis While the formula can be derived by applying the cosine law to the zenith-pole-Sun spherical triangle, the spherical trigonometry is a relatively esoteric subject. By introducing the coordinates of the subsolar point and using vector analysis, the formula can be obtained straightforward without incurring the use of spherical trigonometry. In the Earth-Centered Earth-Fixed (ECEF) geocentric Cartesian coordinate system, let and be the latitudes and longitudes, or coordinates, of the subsolar point and the observer's point, then the upward-pointing unit vectors at the two points, and , are where , and are the basis vectors in the ECEF coordinate system. Now the cosine of the solar zenith angle, , is simply the dot product of the above two vectors Note that is the same as , the declination of the Sun, and is equivalent to , where is the hour angle defined earlier. So the above format is mathematically identical to the one given earlier. Additionally, Ref. also derived the formula for solar azimuth angle in a similar fashion without using spherical trigonometry. Minimum and Maximum At any given location on any given day, the solar zenith angle, , reaches its minimum, , at local solar noon when the hour angle , or , namely, , or . If , it is polar night. And at any given location on any given day, the solar zenith angle, , reaches its maximum, , at local midnight when the hour angle , or , namely, , or . If