Changes

Sun vector is the vector from the center of earth to center of sun. Actually it is from the satellite to the sun but for all practical purposes we can assume it is from earth to sun (distance between earth and sun 149,597,870 km and earth and satellite around 700 km + radius of the earth for LEO satellites). Before dirtying our hands for the calculation, let’s look at how the equatorial plane and ecliptic planes are defined and what is vernal equinox.

[[File:SunVector4.png|frame|center|Ecliptic plane and ECI frame. Image reproduced from [https://commons.wikimedia.org/wiki/File:Earths_orbit_and_ecliptic.PNG here]]]

* '''Equatorial Plane:''' It is the plane perpendicular to the axis of rotation of earth. It is midway between North and South pole and divides the earth into two Hemispheres – Northern and Southern.

* '''Ecliptic Plane:''' The plane which is coplanar with the Earth’s orbit around Sun. The axis of Earth is tilted by 23.4 degrees with respect to this ecliptic plane. This tilt is the reason we have seasons.<ref>https://www.windows2universe.org/earth/climate/cli_seasons.html</ref> Isn’t that Cool and Hot and Rainy?