What is the Vernal Equinox? It is the first day of spring!
Spring officially began in the Valley during the vernal equinox, which took place at 5:29 a.m. on March 20, 2017.
The word “equinox” hearkens back to Medieval Latin, meaning “equal day and night”.
However, it’s not exactly true that the Valley will experience an equal amount of daylight and darkness on the equinox.
This is due to the refraction of the sun’s light as it travels through the atmosphere — and the earth’s atmosphere is thicker at the equator than it is at the poles. As a result, the date of “equal daylight/darkness” is earlier near the equator and later towards the poles. Huntsville experiences 11 hours, 57 minutes and 40 seconds of daylight on March 16, or about 4 days before the official equinox.
Nevertheless, the Tennessee Valley will gradually gain daylight until the Summer Solstice, which takes place on June 20 at 11:24 p.m.
Seasonal change from winter to spring
Recall that the earth orbits around the sun in an elliptical path that lasts approximately 365.25 days (every 4 years there is an extra day — February 29 — and the year is called “Leap Year”). As the earth travels around the sun, it is also spinning on its axis, causing half the earth to spend a period in daylight with the other half experiencing darkness, or night.
But the earth does not “sit” perfectly vertical on its axis. Instead, it is tilted 23.5 degrees from the vertical.
As a result, sunlight is concentrated on the hemisphere “tilted” towards the sun, which increases temperatures. We call this season “summer”, and in the case of the Northern Hemisphere (where the United States is located), this half of the earth is tilted towards the sun. The opposite is true for winter: the Northern Hemisphere is tilted away from the sun, resulting in less sunlight and colder temperatures.
During the vernal (spring) and autumnal equinoxes, the earth remains tilted 23.5 degrees from its vertical, but the earth’s orbit puts in it in place such that an equal amount of sunlight reaches both the Northern and Southern Hemispheres despite the tilt. For those located near the equator, they would observe the sun reaching straight overhead at noon.
From this point forward until the Summer Solstice, daylight would continue to increase the further north you go in latitude (for example, the further north you travel in the Northern Hemisphere). An extreme example of this is Anchorage, Alaska: For approximately 3 months between March 19 and June 23, Anchorage experiences an extended amount of daylight known as the “midnight sun” (22 hours, with the sun briefly dipping below the horizon for the remaining hours, though twilight is still visible).