Can you feel a change in the air? You should. Last night, Sunday, March 20, 2011 marked the vernal equinox, or spring equinox, and the official first day of spring.
At 7:21 p.m. EST, astronomical spring began in the Northern Hemisphere as defined by International Astronomical Union (IAU). In the world’s Southern Hemisphere, it was the start of the fall season.
On the vernal equinox, day and night are almost in length, 12 hours each, at Earth’s equatorial plane.
An equinox occurs twice a year, once in March and again in September, when the center of the Sun crosses directly over the Earth’s equator. At this point, the Earth is tilted neither away nor toward the Sun.
Although the word equinox is often understood to mean “equal [day and] night,” this is not strictly true. For most locations on earth, there are two distinct identifiable days per year when the length of day and night are closest to being equal. Those days are referred to as the “equiluxes” to distinguish them from the equinoxes.
Equinoxes are points in time, but equiluxes are days. By convention, equiluxes are the days where sunrise and sunset are closest to being exactly 12 hours apart.
While most mark the vernal equinox as the first official day of spring, many around the world actually think of it as the mid-point of spring.
According to the World Meteorological Organization, the first day of climatological spring was on March 1, 2011.
You can learn more about the vernal equinox in the video below. Happy Spring!
- Spring Forth with Tonight’s Vernal Equinox (newsfeed.time.com)
- First Day Of Spring 2011: Vernal Equinox DATE, TIME (PHOTOS) (huffingtonpost.com)
- Today’s the vernal equinox (cbsnews.com)