The 23.4-degree tilt in the Earth’s axis means night and day are different lengths. The side tilted closer to the sun experiences longer days during the summer, while the side tilted away experiences shorter winter days.

However, for two days of the year, during the equinox, night and day are almost equal in length. Equinoxes only occur two times a year and signify a change in season-- once in March (Spring Equinox), and once in September (Fall Equinox).

This year’s Spring Equinox fell on March 20th at 2:58 pm Pacific Time. NASA captured a photo of this happening, with the earth half swathed in light and half swathed in darkness.

Each year, the Spring Equinox indicates the coming of spring, and many important cultural and religious festivals are centered around this date.

Sources: LiveScience, TimeandDate, National Geographic