Twice every year, something remarkable happens. Day and night are almost exactly equal in length everywhere on Earth. This event is called an equinox, and one of them falls around March 20. The word equinox comes from Latin words meaning equal night. On this day, the sun rises due east and sets due west, no matter where you live. The equinox happens because of Earth's axis, an imaginary line running from the North Pole to the South Pole. Earth does not spin straight up and down. It is tilted at about 23.5 degrees. As Earth orbits the sun over the course of a year, this tilt causes different parts of the planet to receive more or less sunlight. During the spring equinox, neither the North Pole nor the South Pole is tilted toward the sun. Both hemispheres receive roughly equal amounts of sunlight. After March 20, the Northern Hemisphere begins tilting toward the sun. Days grow longer and nights grow shorter until the summer solstice in June, the longest day of the year. Meanwhile, the Southern Hemisphere experiences the opposite: their days shorten as autumn begins.