Solar zenith angle
The solar zenith angle is the angle between the zenith and the centre of the Sun's disc. The solar elevation angle is the altitude of the Sun, the angle between the horizon and the centre of the Sun's disc. Since these two angles are complementary, the cosine of either one of them equals the sine of the other. They can both be calculated with the same formula, using results from spherical trigonometry. 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.
- is the solar zenith angle
- is the solar elevation angle or solar altitude angle, = 90° –
- is the hour angle, in the local solar time.
- is the current declination of the Sun
- is the local latitude.
The calculated values are approximations due to the distinction between common/geodetic latitude and geocentric latitude. However, the two values differ by less than 12 minutes of arc, which is less than the apparent angular radius of the sun.
The formula also neglects the effect of atmospheric refraction.
Precise times of sunset and sunrise occur when the upper limb of the Sun appears, as refracted by the atmosphere, to be on the horizon.
A weighted daily average zenith angle, used in computing the local albedo of the Earth, is given by
where Q is the instantaneous irradiance.
Summary of special angles
For example, the solar elevation angle is :
- 90° if you are on the equator, a day of equinox, at a solar hour of twelve
- near 0° at the sunset or at the sunrise
- between -90° and 0° during the night (midnight)
An exact calculation is given in position of the Sun. Other approximations exist elsewhere.
- Jacobson, Mark Z. (2005). Fundamentals of Atmospheric Modeling (2nd ed.). Cambridge University Press. p. 317. ISBN 0521548659.
- Hartmann, Dennis L. (1994). Global Physical Climatology. Academic Press. p. 30. ISBN 0080571638.
- Bonan, Gordon (2005). Ecological climatology: concepts and applications. Cambridge University Press. p. 62. ISBN 9781316425190. Retrieved 13 November 2019.
- Woolf, Harold M. (1968). "On the computation of solar elevation angles and the determination of sunrise and sunset times". NASA technical memorandu, X-1646. Washington, D.C.: 3.
- livioflores-ga. "Equation to know where the Sun is at a given place at a given date-time". Google. Retrieved 9 March 2013.