Standing before the Pyramid of the Sun in Teotihuacan, Mexico, a woman embraces spring on the vernal equinox. Image Credit: Ronaldo Schemidt/AFP/Getty Images

Today you, I, and everyone in the world have something in common. Regardless of whether you live in Australia or Austria or Austin, day and night are approximately the same length the whole world over. Today is the vernal equinox—the first day of spring. If you live in the Northern Hemisphere, your days will be getting longer, your trees greener, and your animals friskier. We think it all happens because billions of years ago, a proto-planet collided with an embryonic Earth. Here's what's going on.


Winter and summer have nothing to do with the distance of the Earth from the Sun. Rather, seasons are the product of axial tilt and orbital dynamics. The Earth is tilted slightly at 23.5 degrees relative to its orbital plane. (That tilt is thought related to the aforementioned collision.) The orientation of the tilt never changes, and as Earth revolves around the Sun over the course of a year, different latitudes are thus in direct sunlight. When the Northern Hemisphere is in direct sun, it is summertime there and wintertime down under, and vice versa when the Southern Hemisphere is in direct sunlight.

If this is hard to visualize, take your cell phone and hold it upright next to the left side of your computer screen, tilted slightly toward the computer. The top-right corner should be closest to the screen. It is summertime in that corner (the screen being the Sun in this demonstration). Now keep everything the same, but bring your phone to the right side of your screen. Now the bottom-left of your phone is closer. It is summertime there. If your phone were a spinning sphere, those two corners would be the Tropics of Cancer and Capricorn, respectively. Over a full orbit of the Earth around the Sun, this means the center of the Earth—the Equator—is in direct sunlight twice.

Today is one of those times, marking the transition from winter to spring in the Northern Hemisphere. (The other, autumnal equinox, transitions fall to winter.)


Humans today have it pretty easy. We have coats and fires during the winter, and shorts, air conditioning, and Disney vacations in the summer. For many in the developed world, seasons are in some ways a neat way of marking time, but they don’t necessarily dictate the course of our lives. For plants and animals, though, the seasons are serious business. The availability of food and warmth are vital for reproduction and rearing young animals. During the fall and short winter days, for example, the testicles of Siberian hamsters change dramatically in size (and under no circumstances should you google that). That’s a pretty granular-level effect of the axial tilt of an entire planet.

The same goes for birds flying south for the winter. Studies suggest that the migration is in large measure simply birds following the food. (Starvation winters in the north can be summer feasts in the south. Go where the worms are.) There is some evidence that the migratory patterns are also wired into the DNA of some birds. We’ve discussed this previously at mental_floss:

Captive birds have been observed getting pretty fidgety and changing their sleep patterns right before their natural migration time. Ethologists—those who study animal behavior—call the birds' behavior zugunruhe ("migratory restlessness"). Captive birds display zugunruhe even if they're not exposed to natural light or to seasonal temperature changes.

It goes far beyond that, though. Even plants know what’s up. “Spring is sooner recognized by plants than by men,” says the Chinese proverb. Plants produce phytochromes, which are compounds sensitive to the light spectrum and used to regulate flowering and budding. In some regions, the vernal equinox and the longer days of direct sunlight it brings lead to an increased production of red phytochromes. (In the winter, the Sun’s position in the sky, and the sunlight often shining indirectly, leads to the increased production of far-red phytochromes.) As the ratios shift, you get flowering. No tilt, no bouquets.

So while today is an interesting day to mark for social reasons—a rare point of global harmony and equality imposed by the natural world, even if only concerning light and dark—it’s also a day marking a shift in the behaviors of the natural world itself. Today we are all equal, and for the next few months, the Northern Hemisphere begins anew.