Each year we mark events that recur every year like birthdays, anniversaries, income tax day, Christmas, Hanukkah, Kwanzaa, and New Years Day. The event I am referring to comes around at roughly the same time each year and is celebrated by people all over the world. What could it be? We’ve already moved our clocks ahead as the Spring Forward rule was followed so that’s not it. If you pay attention to your calendar you probably noticed that the Vernal Equinox is fast approaching. It marks the beginning of Spring in the Northern Hemisphere. This year it falls on Monday March 20th at 3:29 AM Pacific Daylight Time.
We all remember being taught about Equinox and Solstice events in school, but how many of us actually remember the details today? That’s why I am going to explain it in detail now.
Here is the definition of the Vernal Equinox according to an article in Astronomy Essentials by Deborah Byrd posted on earth sky.com: The Vernal Equinox “signals the beginning of spring in the Northern Hemisphere and autumn in the Southern Hemisphere. It marks the special moment when the sun crosses the celestial equator going from south to north.” Equinox translates as “equal night” which means the length of day and night is nearly equal all over the world during the equinox.
It all has to do with the earth’s axis. That’s the reason for the seasons. As you might remember the earth’s axis is tilted and not parallel to the earth’s orbit. According to the meteorology text book The Atmosphere by Anthes, Panofsky, Cahir, and Rango “There is an angle between the plane of the equator and the plane of the earth’s orbit (also called the ecliptic). This angle, which has the impressive name obliquity of the ecliptic, is now 23 1/2 degrees.” “As the earth revolves about the sun, it’s axis points in the same direction in space.”
Both the northern and southern hemispheres get exactly the same amount of sunshine during the two equinoxes, March 20-21 and September 22-23. The authors explain that over tens of thousands of years this angle has changed, and, as a result, the severity of the seasons has also changed. The seasons are less harsh when the angle is small and conversely they are more harsh when the angle is large. Over the last 100,000 years or so the angle has varied between 22 and 25 degrees because the earth actually rocks back and forth a bit as it continues it’s march around the sun.
At the Vernal Equinox the rays of the sun are directed straight at the equator and then move northward continuing the spring warming and then bringing on summer, the warmest time of the year.
One would think that the direct straight-line rays of the sun when the distance between the earth and the sun are at their closest would make the area under them see the warmest time of the year but that is not the case. There is a space of about 3 months between the Vernal Equinox and the warmest days of summer.
With the below normal temperatures we have seen this winter most people yearn for the sunshine and warmer temperatures that will increase both as the season of spring gets under way.
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