Vernal Equinox: Spring is Here!

The signs of spring are all around us. Robins are back in the Midwest, daffodils are pushing out of the ground, and trees are budding. Longer days and shorter nights mean the northern hemisphere is warming and waking back up after the sleep of winter. In fact, the Vernal Equinox, the official beginning of spring, is this coming Saturday, March 19. 

Sun rising due east at Stonehenge on the Vernal Equinox. Image Credit

So what makes this day special? Why is it the first day of spring? It all has to do with the Sun and the tilt of the Earth.

Both spring and fall start with the equinoxes, the two days of the year when there is an equal amount of daylight and nighttime.  We have solstices and equinoxes (or in other words, seasons) because of the Earth's tilt on its axis.  As the Earth orbits the Sun, usually one hemisphere is tilted toward the Sun while the other is tilted away. When the Northern Hemisphere is tilted toward the Sun and is in summer, the Southern Hemisphere is tilted away and is in winter, and vice versa. The equinoxes occur when the Earth is at a point in its orbit where both hemispheres get equal amounts of sunlight - neither is tilted more towards the Sun than the other. The equinoxes are the only days of the year that the Sun rises due east and sets due west, giving everyone equal amounts of day and night.

Illumination of the Earth at different seasons. Notice that on the equinoxes, both hemispheres receive an equal amount of sunlight. Image credit: Wikipedia Commons.

The Sun's path across our sky gives us longer days in the summer and shorter days in the winter. On the Equinoxes, day and night are equal! Image credit: Daniel V. Schroeder

In fact, the exact time of our equinox in the central timezone is 11:30 PM, which is the time the Sun crosses the equator, so that it is shining just a tiny bit more on the northern hemisphere than the southern hemisphere. Because of time zones, some people in the US have their equinox on March 20th. 

From now until the Summer Solstice on June 20th, each day will be a little longer than the one before as the Sun takes a higher path through our sky. As the days lengthen, the total warming time in the northern hemisphere increases, and the temperatures rises. Soon green will be all around us and it will be gardening time again! Hello spring!

If this weather makes you feel like getting outside, why not run a race? Info here.