A MATLAB Script for Predicting Equinoxes and Solstices

Predict the calendar date & UTC time of the equinoxes and solstices for a user-defined calendar year

PDF document and a MATLAB script named equsol.m which determines the time of the equinoxes and solstices of the Earth. These events are the times when the apparent geocentric longitude of the Sun is an exact multiple of 90 degrees. This script uses Brent’s root-finder and a precision solar ephemeris to calculate these events.

Brent’s method requires an objective function that defines the nonlinear equation to be solved. The objective function for the spring and fall equinoxes is the geocentric declination of the Sun. The spring and fall equinoxes occur whenever the geocentric declination of the Sun is less than or equal to a user-defined convergence criterion.