Solar Declination is Sinusoidal
To understand the calendar theoretically, we need to study the sun’s declination and its altitude over the course of a year.Solar declination is the angle between the sun’s rays and the plane of the earth’s equator. It is shown above as 23.4 degrees for summer solstice when the sun appears highest in the sky.
By agreeing on this definition of declination, and since the earth’s orbit around the sun is nearly circular, the combined effect of viewing the sun from earth over a linear time period of a year we obtain the sinusoidal graph below. Sinusoidal is an adjective meaning like a sine function, a sine curve, shown here.
When we calculate the declination of a theoretical earth-sun system from geometrical principles of earth in orbit, we get a sine curve for angle…
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