At the vernal equinox, the earth, which tilts on its axis so that one of its hemispheres is usually closer to the sun than the other, stands up straight. From our viewpoint here on earth, the sun rests directly over the equator. It has been moving gradually northward in the three months since the winter solstice (southward if we are in the southern hemisphere), and has reached the halfway point of its journey. Everywhere on the planet, day and night are of perfectly equal length.
There are two equinoxes every year: the vernal equinox, which is the official first day of spring, and the autumnal equinox, which is the official first day of fall. They come in March and September, between the twentieth and the twenty-third of the month. The vernal equinox, the first day of spring, comes in March in the northern hemisphere and September in the southern.
In the three months from the vernal equinox to the summer solstice, days grow longer and nights grow shorter. Throughout the season of spring, the sun moves farther into our hemisphere, until it reaches the farthest point.
The sun moves, that is, only from our point of view. If we could look at our planet from outer space, we would see the earth tilting a little more every day, until one of its hemispheres leans as close as it can toward the sun. It is springtime in the hemisphere that the earth is tilting closer and closer to the sun.
From March to June, the northern hemisphere tilts toward the sun, more and more every day. From September to December, the southern hemisphere does. In June and December, the earth gradually begins to straighten again, until the next equinox, when it begins to tilt the other way. As our hemisphere tilts toward the sun, daylight hours begin to lengthen. How much they lengthen depends on where on the globe we are.
In the tropics, there is little or no change. Outside the tropics, days grow longer after the vernal equinox, but how much change there is depends on how close to the pole we are. Vancouver, Canada sees more dramatically lengthening days in the spring than El Paso, Texas, because it is much closer to the North Pole. In polar regions, the change is drastic. There is no sunrise in the winter and no sunset in the summer. In the spring, each day may be noticeably longer than the day before.
Lengthening daylight results in changes in the weather. The longer hours of sun and the (usually) warmer weather, in turn, trigger new growing or flowering cycles in many plants and the awakening of animals that hibernate. For animals that migrate, mate, or give birth in the spring, these behaviors are often triggered by the changes in weather and light. New growing cycles for plants also result in new foods being available for herbivores, which themselves may become new food for carnivores.
In ways that it is beyond the scope of this article to describe, lengthening daylight hours help create atmospheric patterns that lead to storms in some parts of the globe and dry spells in others. Where winter has brought snow, the warmer weather melts it. Storms, and sometimes floods, are part of spring for many of us.
As a result, we associate many changes in the world around us with the equinox. We consider the vernal equinox to be the first day of spring, even though springlike weather may begin earlier or later. On the vernal equinox, there may be continuing winter storms, or spring flowers may have already been blooming for a month. Changes in the weather and daylight hours happen gradually.
Just as we age all the time but only count ourselves older on our birthdays, seasons progress all the time, even though we give each one an official first day. The vernal equinox is the birthday of spring.