I seem to remember learning that Spring starts on March 21st, Summer on June 21st, Autumn on September 23rd, and Winter on December 21st as a general rule of thumb.
Do you know the last time spring actually started on March 21st at least here in Michigan? It was 1979. But before getting into the answer why, let's see why spring starts this month.
Ever since the winter solstice, when the sun stopped moving southward, back on December 21st of last year, it has been getting higher in the south each noon. This progress will end on June 21st, when the sun will stop at its farthest north at the summer solstice. Solstice, by the way means sun standstill. This solar north and south journey is due to the earth's axial tilt of 23.5 degrees. If the earth's latitude lines could be projected on the imaginary celestial sphere of the sky, we would see the sun range from 23.5 degrees north to 23.5 degrees south. On star maps we do see those lines, but we call them declination lines. Celestial latitude is something else again, not related to earthly latitude.
The celestial equator, lies above the earth's equator at 0 degrees declination. It runs near the upper right star of Orion's Belt. The sun's path on this celestial sphere is a great circle, called the ecliptic. The point at which the ecliptic crosses the celestial equator with the sun heading northward is called the Vernal Equinox. The instant the sun crosses that point spring begins. The Vernal Equinox was also chosen to serve as the Greenwich of the sky. It is the point from where all east west coordinates are measured. Greenwich on the earth lies at 0 degrees longitude. While the Vernal Equinox lies at 0 hours right ascension. The measurement of east and west in the sky as hours is actually natural enough because the relationship between Greenwich and the Vernal Equinox changes with time as the earth rotates. The right ascension of the sky crossing one's meridian is called sidereal time, a quantity that must be known in order to find an object in the sky using coordinates. The meridian, by the way is an imaginary line running through the north and south horizon points and the zenith or overhead point.
The basis for your local sidereal time is in finding the sidereal time at Greenwich for that instant. That's what the Vernal Equinox does for astronomer. What does it do for the rest everyone else? The calendars that have evolved over time attempt to keep track of the seasons for agricultural and religious purposes. The first modern calendar was put into effect by Julius Caesar in 44 BC in which the year averaged 365.25 days. However it actually takes 365.242 days for the sun to move around the sky from Vernal Equinox and back again. That's an error of only 11.5 minutes a year. At the end of a century that's 19.2 hours or 8 tenths of a day.
It is this slippage that has pushed the Vernal Equinox back for us to March 20th. At Greenwich the event occurs on the 21st every 4th year three more times, in 1999, 2003, and 2007. For us spring will actually start on the 19th in 2016.
The correction to this back sliding came for religious reasons in 1582 when Pope Gregory XIII instituted his calendar reform. It seemed that Easter was drifting toward summer. The Easter date was calculated as the first Sunday after the first full moon after the Vernal Equinox. The problem was, that for ecclesiastical purposes the Vernal Equinox was stated to always occur on March 21st. By 1582 the real Vernal Equinox had retreated to March 11th. The fix was in two parts. First there was the deletion of 10 days to get the Vernal Equinox back to the March 21st. This happened when the ten days between October 4th and 15th of 1582. Second, to take care of the 0.8 day creep per century, all century years not divisible by 400 become ordinary years.
Yes, the Vernal Equinox will again be seen on March 21st for us, but we will have to wait until 2103, the third year after the omitted intercalary day (the term for February 29th) of the year 2100.
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