The autumnal equinox, a day when the night and day were nearly equal in most locations, commenced the start of autumn in the Northern Hemisphere on 22nd September 2020. A similar event called the Vernal equinox occurs around March 21, indicating the onset of spring. In the Southern Hemisphere, the seasons are reversed, as evident by Christmas celebrations in the summer season in countries like Australia and New Zealand.
Understanding Equinoxes
The term “equinox” stems from two Latin words – aequus (equal) and nox (night). An equinox is one of the two times a year when the Earth’s axis is tilted neither toward nor away from the sun, resulting in approximately equal daylight and darkness across all latitudes. The Sun stands exactly above the Equator on these days, making day and night of nearly equal duration. Interestingly, because of how the length of the day is defined and the atmospheric refraction of sunlight, most locations on Earth receive over 12 hours of daylight on equinoxes.
Equinoxes, which typically take place in March and September, are also significant for Northern Lights sightings as geomagnetic activities are twice as likely to occur during these times compared to summer or winter.
Date Variations of Equinoxes
While the September equinox usually falls on September 22 or 23, it can occasionally occur on September 21 or 24. This anomaly arises due to the discrepancy between the Gregorian calendar’s definition of a year (365 days) and the actual time taken for Earth to complete its orbit around the Sun (approximately 365 and 1/4 days). As a result, each equinox arrives about six hours later than the preceding year’s equinox, gradually shifting the date.
Significance of Equinoxes in Season Change
Equinoxes and solstices are crucial markers of the changing seasons on Earth. These changes occur because Earth’s axis is slightly tilted as it orbits the Sun at an angle of 23.5 degrees to its normal and 66.5 degrees with the orbital plane. Without this tilt, the Sun would constantly appear directly over the Equator, resulting in no seasonal changes nor a need for equinoxes or solstices.
Understanding Solstices
The two solstices occur in June and December. They represent the days when the Sun’s path in the sky is the farthest from the Equator. In the Northern Hemisphere, the June solstice signifies the start of summer, when the North Pole tilts closest to the Sun, and the Sun’s rays directly overhead at the Tropic of Cancer. Conversely, the December solstice represents the start of winter when the South Pole tilts closest to the Sun, and the Sun’s rays are directly overhead at the Tropic of Capricorn.