Spring is getting shorter every year — by about 30 seconds, that is, researchers say.

Spring officially started at 6:45 p.m. ET on Friday, which is the exact moment when the Northern Hemisphere achieved its vernal equinox — i.e., when the axis of the Earth reaches the halfway point where it neither points toward the sun (summer solstice) or away from it (winter solstice), according to a NBC News report.

It seems like an unchanging event, but in reality spring has been losing time for thousands of years. Spring, in fact, is a little bit shorter than summer this year at 92.76 days compared to 93.65 days, with autumn coming in at 89.84 days and winter at 88.99 days.

Spring will continue to lose time to summer, as will winter to autumn. By the year 3000, summer will be two days longer than the spring and five days longer than winter, according to the report.

So why is spring getting shorter? It’s because of how the axis of the Earth moves — like a wobbling top. Known as precession, it means that the Earth’s orbit causes the summer solstice to shift slightly. It’s a cyclical phenomenon that will begin to reverse itself in the year 8680 when spring reaches its shortest at 88.5 days, four days shorter than the spring is today.

The equinox occurs twice each year, around March 20 and September 22, which has since olden times has meant the point when daytime and night are of almost equal duration. In fact, the word “equinox” is derived from the Latin “aequus,” for equal, and “nox” for night.

An equinox happens when the plane of the Earth’s equator passes the center of the sun, which means that the Earth’s axis doesn’t til away from or toward the sun.