There are two important ways of describing time when calculating sun angles. "Clock time" is the artificial time that we use in everyday life to standardize our time measurements. It allows people in different locations to use the same time or to easily convert time from one location to another.

"Local solar time" (or simply "solar time") is the time according to the position of the sun in the sky relative to one specific location on the ground. In solar time, the sun is always due south (or north) at exactly noon. This means that someone a few miles east or west of you will realize a slightly different solar time than you, although your clock time is probably the same.

For the purpose of calculating local solar time, clock time must modified to compensate for three things: (1) the relationship between the local time zone and the local longitude, (2) daylight savings time, and (3) the earth's slightly-irregular motion around the sun (corrected for using the equation of time).

Local solar time (LSoT) is calculated as follows:

LSoT = LST + 4 minutes * (LL - LSTM) + ET

Where:

LST (local standard time) = Clock time, adjusted for daylight savings time if necessary.

LL = The local longitude; positive = East, and negative = West.

LSTM = The local standard time meridian, measured in degrees, which runs through the center of each time zone. It can be calculated by multiplying the differences in hours from Greenwich Mean Time by 15 degrees per hour. Positive = East, and negative = West.

ET = The equation of time adjustment in minutes

Note that if the site is east of the LSTM, the (LL - LSTM) factor should be a positive number, and if it is west it should be negative.

The "4" in the equation is the quotient of 60 minutes of time and the 15 degrees of longitude that the earth rotates in that time (i.e., the earth rotates one degree every four minutes).

To convert from LSoT to clock time, perform the reverse of this formula.

In the input section of SunAngle, you should indicate whether the time you entered is based on clock time or solar time. The output section then includes both clock time and solar time for reference.