The vernal equinox, marking the first day of spring, occurs today at 11:33 A.M. Eastern Time. (Conversely, in the southern hemisphere, today is the autumnal equinox, the first day of fall.) At that moment the sun's direct rays hit the equator, and on this day the entire earth will have daylight and darkness of approximately equal duration. As we progress toward summer, because of the tilt of the planet's axis, the sun's direct rays will move northward. In the northern hemisphere our days will grow longer until we reach the summer solstice in June, when the sun's direct rays hit the Tropic of Cancer, about 23 degrees north of the equator. While we enjoy longer days, in the southern hemisphere the days grow shorter.
While doing a bit of research on the equinoxes and solstices, I learned that the exact location of the Tropic of Cancer varies a bit from year to year due to the earth's wobble. On Wikipedia I found this photograph of a highway in Mexico where road signs were placed showing the shifting location of the Tropic of Cancer between 2005 and 2010.
Ages ago, back when I was in school, we learned that the equinoxes and solstices always occurred on the 21st of the month. I don't know if they were just teaching us an approximation, or if astronomical calculations have improved since then, but the first day of our seasons vary between the 20th and the 23th of the month.
In Mexico City the arrival of spring is being celebrated with a Spring Festival this weekend. More than fifty free cultural and sports events are being held in twenty locations in the Historic Center of the city. This image of the Zócalo, the central plaza, which I captured yesterday from the "Webcams de México" website, shows the large concert stage which has been set up on the plaza in front of the Cathedral.
Happy Spring to everyone!
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