On September 23rd at 8:49 a.m. Italian time this year, we will witness the occurrence of the autumnal equinox, deviating from the traditional date of September 21st. This shift is due to a multitude of astronomical factors and the discrepancy between the solar and calendar year.
The Interplay Between Earth's Movements and Our Calendar
Commonly, we perceive a year to be 365 days, but in reality, the Earth doesn't orbit the Sun in precisely 365 days. The actual time taken by our planet is 365.256 days, a sidereal year, which adds approximately six hours to "our" year.
To make up for this delay, we introduce a leap year every four years, which compensates for the lost 24 hours by adding an extra day to our calendar. However, the delay becomes significant during the autumnal equinox and other seasonal transitions.
Interestingly, the sidereal year doesn't equate to the tropical solar year, our reference for leap years. The former lasts 365d 6h 9m 9s, while the latter is around 20 minutes shorter, lasting 365d 5h 48m 46s.
The discrepancy arises from the "precession of the equinoxes," a phenomenon caused by the Earth's imperfect sphere shape and the gravitational pull of the Sun and Moon, which affect the Earth's tilted axis movement. This results in a yearly clockwise shift of 50″.26, equivalent to 1° every 71.6 years, leading to equinoxes and solstices occurring approximately 20 minutes earlier each year.
Nonetheless, the autumnal equinox usually experiences a delay due to Kepler's Law II. When the Earth is farthest from the sun (aphelion), its orbit slows down, typically occurring during our summer season.
The Autumnal Equinox and Geographic Latitude
The term equinox signifies the moment when daylight and darkness hours are equal. However, the day this event happens isn't necessarily the day of the equinox, although it's typically quite close. The precise day when daylight and darkness are of equal duration depends on one's geographical latitude.
In essence, the literal equinox varies based on our position on the planet relative to the equator.
So, while the tradition of the equinox on September 21st may not align with scientific reality, it's a custom we continue to acknowledge and celebrate. By understanding the complexities of astronomical phenomena like the autumnal equinox, we can appreciate the intricate workings of our universe.
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