approxECISunPositio​n

Calculates an approximate position of the Sun at a given UTC time (vector) in Earth-centered inertial (ECI) coordinates.

Given Gregorian year(s), day(s), and time(s) in UTC, calculate Earth-centered inertial (ECI) coordinates (in meters) of the vector(s) pointing from Earth's center to the center of the Sun.
Encapsulated by Seth B. Wagenman, from:
stargazing.net/kepler/sun.html as corrected by:
https://astronomy.stackexchange.com/a/37199/34646, along with
other minor corrections based on recent revisions to the ecliptic plane. Obliquity of the ecliptic plane formula mostly derived from:
https://en.wikipedia.org/wiki/Ecliptic#Obliquity_of_the_ecliptic