Today is the autumnal equinox, when the Earth receives sunlight at its most direct angle relative to its equator and poles. As Earth orbits around the Sun over the course of a year, its axial tilt causes the angle of solar illumination to change – a predictable and regular change, but a change nonetheless. This is what gives us our seasons and affects our climate zones and weather and basically how life on our planet has evolved to be what it is, where it is! Fascinating stuff, although admittedly a little hard to envision.

This helps:

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The video was taken with the European METEOSAT-9 Earth-observing satellite – or rather, an animation made up of images of our planet taken over a single year. You don’t see the Earth itself move, but the terminator line – the edge of the shadow between the day side and the night side – clearly changes angle through the animation.

Since the angle of the sunlight isn’t changing, the realization is that the Earth itself is what’s moving! Very cool.

Originally posted by Phil Plait at the BadAstronomy blog… read more on Universe Today here.