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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:SunVector1.png|frame|center]]

* '''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?

*'''Vernal Equinox:''' The Vernal (Spring – around 21st March) Equinox in the Northern Hemisphere is the Autumnal (Fall- around 23rd September) Equinox in the Southern Hemisphere and vice versa. Since most of the countries are in the Northern Hemisphere, so in general we define the Vernal and Autumnal equinox with respect to Northern hemisphere. <br \>

The vernal equinox is one of two days in a year, when the earth has equal length of day and night. Why only two days? Why not all days? Blame it on the tilt of Earth’s axis. It’s a bit hard to imagine but at spring and fall the (in the Fig alongside) the northern and southern hemispheres receive the same amount of sunlight.

Enough of the preparatory material! Let’s now calculate the Sun Vector. The figure below is the top view of the ecliptic plane. In this plane the sun vector is from earth to sun and is making an angle ϑ from vernal equinox.

[[File:SunVector3.png|frame|center]]