# What are the Mathematical Algorithms/Formulas behind hindu Sunrise and Sunset Calculations?

There are differences between Astronomical Sunrise/Sunset calculation and Hindu calendar Sunrise/Sunset calculations. The astronomical algorithms to calculate the sunrise/sunset can be found over the internet. Here is a calculation that I followed to correctly calculate the astronomical sunrise/sunset time.

Sunrise and Sunset algorithm

In the Hindu calendar, sunrise/sunset is considered when the center of the Sun disk is at the horizon, also the refraction effect must be considered.

Sunrise on the Hindu calendar = Astronomical Sunrise + Time taken by the Sun to rise half of its diameter + Time taken by the Sun to rise further to neutralize the refraction effect.

Similarly, Sunset = Astronomical Sunset - Time taken by the Sun to set half of its diameter - Time taken by the Sun to set further to neutralize the refraction effect.

I am able to derive the Astronomical Sunrise/Sunset, but how to do the calculation for the following?:

1. Time taken by the Sun to rise half of its diameter.
2. Time taken by the Sun to rise further to neutralize the refraction effect.
3. Time taken by the Sun to set half of its diameter.
4. Time taken by the Sun to set further to neutralize the refraction effect.
• There is one misconception. The effect of diffraction causes the sun to appear earlier, so you must subtract time to account for the refraction at sunrise, and add time at sunset. The calculations you link to already account for refraction. – James K Aug 2 '20 at 13:07