This gif is the perfect way to understand why it gets cold in winterS

See that slant in the Earth's terminator line as day turns to night? It's a product of the Earth's tilt — and a beautiful illustration of why the Northern Hemisphere gets so much less sunlight in December.

These full-disk pictures of Earth were taken by the EUMETSAT weather satellite a couple of days ago on December 21 — the Winter Solstice. It shows the 23.5-degree tilt of our planet's axis with respect to the sun, a principal factor behind our changing seasons.

[ h/ t Alan Boyle/NBC News ]