Vernal Equinox- the First Day of Spring

Vernal Equinox- the First Day of Spring

According to the seasons, spring arrives on March 20 or 21. In the year 2009 there are some folks who would find that hard to believe. The upper mid-west has had over 20 inches of snow and people around Bismarck, ND are sandbagging the river banks and blasting ice that has dammed the Red River and forced the river to record levels. Although there wasn’t any snow, temperatures in north Texas got into the low 30’s on March 28. So defining spring by the change of season does not always seem to be accurate.

Astronomically the equinox officially occurs when the center of the sun passes directly over the equator. There is another important fact about this date. It is the only time that the sun rises exactly in the east and sets directly in the west. For a related article and pictures demonstrating this phenomenon, visit the link below.

Many people think that, as the name implies, spring is determined by the day that there is twelve hours of light and twelve hours of darkness. Although refraction of light and the Earth’s atmosphere may alter data slightly, for a person at the equator, the statement is generally correct. The further north or south from the equator the person goes, the situation is different. If a person is standing at the North Pole on March 21, he will see the sun low in the sky and the onset of six months of uninterrupted daylight. Six months of darkness begins for those at the South Pole.

Most people know about a leap year but did you know about a leap century? Julius Caesar developed the Julian calendar that became the generally accepted world calendar. In that calendar a year was 365.25 days long, so every fourth year was “leap year” and an extra day was added to February. This seemed satisfactory for a while but the numbers were not accurate enough. By the year 1500 the equinox was arriving on March 11. This discrepancy occurs because a year is 365.2425 years long- the actual year was 11 minutes shorter than the calendar year. In 1582 Pope Gregory created the Gregorian calendar that was basically the same except when it came to century years (1900, 2000, etc.). Only century years that were evenly divisible by 400 (1600, 2000, etc) would be a leap year. Other century years (1700, 1800, etc.) would not be a leap year. With this correction the discrepancy between the actual and calculated year is only 11 seconds. Because 2000 was divisible by 400 we didn’t notice the difference. We will not see the effect until the year 2100- I mean you won’t see the effect.

The equinox has played an interesting role in both developing the calendars we use and in understanding how we calculate direction. Easter is always the first Sunday after the first full moon after the equinox. The Great Sphinx is built so that it points to the rising sun on the vernal equinox. The equinox is an interesting time of the year.

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