When I was growing up, Spring was always on March 21st. I have had a number of inquiries about why it is on March 20th. As we learned earlier with a Leap Day, years do not come in even numbers and neither do the seasons. The earth’s elliptical orbit is another reason. In winter, the earth travels around the sun more quickly than it does in the summer. Since the seasons are defined by precise positions of the earth and sun, it takes less time for the faster moving earth to go from winter solstice to the first day of spring.
Another complication centers around the length of the day on a vernal equinox (Spring). We were always taught that on the first day of spring and fall, there is an exact amount of daylight and darkness…. another not true. On the days of spring and fall equinoxes, the amount of daylight is actually greater than darkness by several minutes.
The reason can be attributed to our atmosphere. If earth did not have an atmosphere, then the length of day and night would be even on the equinox days. However, our atmosphere acts like a lens and bend light around the end of the horizon. This means that when you watch the sun, either coming up above the horizon at sunrise, or going down below the horizon at sunset, you are looking at an illusion – the sun is not really there, rather it is below the horizon. As a result, we end up seeing the sun a few minutes before its disc rises and for a few more minutes after it sets. Thus, the length of the day on any given day is increased by six or seven minutes.
Spring has arrived and yes it is March 20th. I for one welcome it!