Spring has sprung at its earliest in more than a century. At 1:14am EDT today, the sun's path across the sky intersected with the celestial equator, marking the spring equinox and the start of springtime—in an astronomical sense—in the northern hemisphere, and the beginning of fall in the southern hemisphere. March 21 is the official first day of spring, but the equinox has only fallen on that day for Americans on 36 out of the last 100 years, none of them since 1981, and it won't again until the year 2102.
The time and date of the equinox keeps changing because of quirks in the Gregorian calendar and slight changes in the Earth's orbit, Scientific American explains. These changes are affecting the length of the astronomical seasons themselves, with summer gaining around a minute every year from spring, and autumn gaining around 30 seconds a year at the expense of winter. (Read more equinox stories.)