A dogma of the Christian churches is that spring begins on March 21st, too: so you may see some tension between their teaching and astronomy.
|UT date and time of solstices and equinoxes|
You may see that every non-leap year, the spring starts about 6 hours (5:53 or so in average) later than the previous year. However, on every leap year, such as 2012, it's 18 hours later and the three previous 6-hour jumps are undone (and spring actually begins about 40 minutes earlier than 4 years earlier). Consequently, the 2012 spring equinox is the earliest equinox since 1896 when it was even earlier because 1900 wasn't a leap year (and because of mathematical reasons you should think about). This record will keep on improving until 2096. The year 2100 won't be a leap year so the date of the equinox will jump by 1 again.
Yes, this blog entry officially appeared at 6:14 am, Prague Winter Time, which is when the spring started.
Someone has created a video on the origin of seasons for Brian Cox: Seasons are not caused by the changing distance from the Sun
A homework exercise for you: Did the folks try to keep the same date for equinox? And if they did, what is the reason that they failed and the date has shifted to another date? Although leap years have periodicity of 400 years, vernal equinox in 1612 occurred 2+ hours later than today. That makes a 12-hour difference in 2000 years. If you've forgotten almost everything, try to remind yourself about some leap year maths.
Jean Michel Jarre, Equinox, 1978
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