OK this may be the quickest blog you've ever read. Imagine you are about 20,000 miles in space at the altitude of the geostationary satellites. Geostationary in that the satellites rotate around the earth above the equator at the same speed that the earth rotates. Thus they remain (with some adjustments) in one spot above the earth's equator . Here is the view from the GOES weather satellite late yesterday.
See the equinox? OK I've circled it. It is the glint (or reflection) of the sun in the afternoon.
In this afternoon GOES image, imagine you are flying in space with the sun behind you and the earth in front of you. The plane of the equator and the orbital plane of the earth's movement around the sun are all aligned and you see the reflection of the sun in the Pacific Ocean. The reflection is right at the equator. It's the equinox. See it? :>)