When we picture the Earth in space, we see the North Pole pointing straight up, and the South Pole pointing straight down. It turns out, though, that the poles don’t point straight up and down. Earth is tilted and spins on an angle. So, sometimes the top half of Earth, the northern hemisphere, points towards the sun, and sometimes it points away.

This is why we have seasons on Earth. When the northern hemisphere points towards the sun, it’s summer there. At the same time, it’s winter in the southern hemisphere—which is pointing away from the sun. Seasons change when the northern and southern hemispheres switch who is closest to the sun. When the northern hemisphere points away from the sun, it’s winter there and summer in southern hemisphere.

Earth’s tilt is also why the days are long in the summer and short in the winter. The longest day each year is when the tilt is most towards the sun. This is called the summer solstice, or the first day of summer. The shortest day is the winter solstice, or the first day of winter. Twice a year, Earth’s tilt doesn’t point towards or away from the sun. This is called the equinox.

March 20th is the “Vernal” equinox, or the first day of spring, in the northern hemisphere. It is the first day of fall, or “Autumnal” equinox, in the southern hemisphere. If you’re on the equator, the sun passes directly overhead on these days. If you’re at the North or South Pole, the equinox is even more exciting. At the South Pole, the sun rises on the autumnal equinox and sets on the vernal equinox. In between are six continuous months of sunshine!

During the six months of darkness at the South Pole, NASA scientists can study parts of the night sky they might not always be able to see. Earth’s tilt lets NASA do studies for several months without the sun getting in the way. That makes March 20th pretty special, not just for the planet, but also for the people living and learning on its surface.

Learn more about what causes the seasons! Visit the NASA Space Place: