It’s Time Again for the Change of Seasons—the Vernal or March Equinox 2020!
This marks the beginning of spring for the Northern Hemisphere and the beginning of fall for the Southern Hemisphere. The change happens at 11:49 p.m. EDT (0349 UTC/GMT on March 20.)
This will be the earliest that the vernal equinox will occur across the contiguous United States in 124 years. In 1896, the vernal equinox arrived on March 19 at 9:29 p.m. EST (0229 GMT on March 20). But don’t worry, we won’t have to wait long for this to happen again—it’s now synched with leap years!
There are two specific reasons for this variation of the date: leap years and daylight saving time:
- The leap year that we observed in the year 2000 is an extra one added in a century year, evenly divisible by 400.
- The occurrence of the equinox comes about 47 minutes earlier (on average) every four years because the solar year is not exactly one-quarter of a day longer than the 365-day calendar year.
- Find out more about the timing at Space.com.
But what is the equinox exactly?
The astronomical event, known as an equinox, occurs twice each year around March 20th and September 23rd, when the plane of Earth’s equator passes through the center of the sun.
Not Entirely Equal Day & Night
On an equinox, the Sun shines directly on the equator and the length of day and night is nearly equal – but not quite. They are not exactly equal because the angular size of the sun and atmospheric refraction. The word equilux is used to mean a day in which the durations of light and darkness are equal to distinguish the day from an equinox.
The March equinox marks the moment the Sun crosses the celestial equator – the imaginary line in the sky above the Earth’s equator – from south to north and vice versa in September. The equinoxes are the only times when the solar terminator (the dividing line between night and day) is perpendicular to the equator. Then the northern and southern hemispheres are equally illuminated.
Equinoxes are the only times when the subsolar point is on the equator, i.e. the Sun is exactly overhead at a point on the equatorial line. The subsolar point crosses the equator moving northward at the March equinox and southward at the September equinox.
= The Sun’s position directly overhead (zenith) in relation to an observer.
= The Moon’s position at its zenith in relation to an observer (Moon phase is not shown).
Civil Twilight = lightest shade
Night, no twilight = darkest shade
The equinoxes and solstices, are directly related to the seasons of the year. In the northern hemisphere, the vernal equinox (March) conventionally marks the beginning of spring while the autumnal equinox (September) marks the beginning of autumn. In the southern hemisphere, the vernal equinox occurs in September and the autumnal equinox in March.
We can even see how the sun illuminates Earth during equinoxes and solstices thanks to NASA satellites.