Solar Altitude

Solar Altitude

Earth Rotation Around the Sun

As a result of the earth rotating around the sun, the sun appears to move between the Tropic of Cancer 23.45 degree North in the Northern Hemisphere and the Tropic of Capricorn 23.45 degree South in the Southern Hemisphere.

When the sun is over either of the two tropics, its is known as solstice and when the sun is over the equator is known as Equinox.

The Sun reaches the Tropic of Cancer at the Northern Solstice (June 21st) and the Tropic of Capricorn at the Southern Solstice (December 22nd). The Sun crosses the equator at the equinoxes in March 21st and September 23rd. This means that the altitude of the Sun at solar noon varies a total 46.9 degree during the calendar year.

The formula for calculating the altitude of the sun when its is over the equator (ỿ_e) for a specific altitude is:

ỿ_e=90 - Latitude (in degrees)

The formula for calculating the altitude of the when it is over the tropics (ỿt), i.e. either the Tropic of Cancer or Tropic of Capricorn, is:

                                    ỿt=90 - Latitude (in degrees) ± 23.45 degrees

Whether to use the + or - is dependent on the hemisphere (Northern or Southern) where you are located and over which "tropic" (either Cancer or Capricorn) you wan to determine the sun's altitude. when applying this formula it is assumed that you are facing the equator, for example, facing south in the Norther Hemisphere.

As a rule of thumb, when the latitude is in the same hemisphere as the tropics you add 23.45 degrees and when the latitude is in the opposite hemisphere as the tropic you subtract 23.45 degree.

For Delhi (which is outside if the tropics), the sun will never be directly overhead, as depicted in the figure below. its important to understand where the sun will actually be in the sky when positioning solar modules, as you need to determine if any obstacles will shade the modules.

In the tropics it is important that the sun can be in both the northern and the southern skies as shown in the figure below. If you are in the tropics it is important to determine the amount of time, and at what time of the year, the sun is in the northern and southern directions. This is to ensure that objects like trees and buildings will not shade the solar modules.

In the tropics, the sun is in the southern part of the sky at solar noon during Summer. In Mumbai for example, for the period from May 16th to July 28th, the sun will actually be in the northern part of the sky. Therefore you willneed to look for potential obstacles that could cause shading in both the Southern and Northern directions.

Table below provides information on altitude of the sun for various latitude in the northern hemisphere tropics (i.e. India). This table also provides the altitude of the two equinoxes and also indicated when the sin is in the northern sky with respect to the altitude. This can help locate possible obstacles that will shade the modules at various times of the year.