There’s more than spring in the air these days. But let’s set aside the virus for a moment and take note of the new season. Spring begins at 10:49 p.m. (Central Time) tonight. That’s the moment the center of the sun will shine directly over the equator. There’s also a celestial equator, a projection of Earth’s equator into space. Like Earth’s axis it’s imaginary but a useful concept.
The first day of spring occurs when the center of the sun crosses the celestial equator moving north (higher up in the sky) for those living in the northern hemisphere. If you live in the southern hemisphere, where the sun shines in the northern sky, the sun’s northward movement means it gets lower and lower until the first day of winter on June 20.
Earth’s tipped axis is responsible for the seasons. When the north polar axis is angled toward the sun we experience summer (see diagram below) and long days. When it’s tilted away from the sun, winter comes and brings long nights. On the first day of spring and fall Earth’s axis lies midway between those extremes, tilted neither toward nor away from the sun. As a result, day and night are almost exactly equal across the planet — 12 hours apiece.
Because the celestial equator intersects the horizon at the east and west points of the compass the sun will set almost exactly due west tonight and rise due east tomorrow morning. The sun continues moving higher in the sky after today until June 20th when it reaches the end of its northern journey at the first day of summer. This yearly yo-yo-like motion of the sun is a reflection of our planet’s tilted axis in conjunction with its revolution around the sun. Were the tilt 0° instead of 23½° the sun would follow the exact same track across the sky every day of the year.
Spring can occur on March 19, 20 or 21. If March 19 seems early you’re right. We haven’t had a March 19th vernal equinox date for the entire U.S. since 1896 (spring started for part of the U.S. on the same date in 2016). Typically, time zone differences cause the date to change. For instance, if spring begins on March 21 at 2 a.m. in the UK, that’s computes to March 20 at 9 p.m. in the Central Time Zone. Outside of the time zone variation, the March equinox would occur on the same day every day if the Earth revolved around the sun in exactly 365 days. But it takes 365.25 days instead. This makes the time of equinox arrive about 6 hours later than the previous year. Over time the hours add up and cause the equinox date to vary.
Then, every 4 years at Leap Year when we add an extra day to the calendar — 4 years x 6 hours = 24 hours — to keep the date of spring in line with the time the sun crosses the equator. If we didn’t, that date would slip past March 21.
But that’s not the end of it. A year really last 365.24 days or 11 minutes and 14 seconds less than a quarter-day. This slight discrepancy adds up. To account for it we omit the leap day on century years that are not divisible by both 4 and 400. We last dropped a leap day in the year 1900. As a result, the equinox date got pushed to later than normal, changing the start of spring to March 20 or 21 for much of the 20th century.
But thanks to that 11 minutes and 14 seconds the equinox was getting earlier by about 45 minutes every four years. Then, in 2000, we observed a leap day (2000 is evenly divisible by both 4 and 400), which turned the clock back a full day. That’s why in 2020 the entire country — even with time zone differences — celebrates spring on March 19.
Did you even understand any of that? No matter. Spring begins tonight, the sun will shine and no one can stop either.