Derived from the Latin for “equal night,” the term “equinox” suggests a perfect balance of 12 hours of day and 12 hours of night. In reality, however, the day is slightly longer. For instance, on the September 22 equinox, London will experience 12 hours and 11 minutes of daylight, while Shetland will have 12 hours and 14 minutes.
This discrepancy is caused by two key factors. The first is atmospheric refraction; the Earth’s atmosphere bends sunlight, making the Sun appear above the horizon even when it is physically below it. This adds several minutes of light at both dawn and dusk. Secondly, the measurement of the equinox differs from that of a sunrise or sunset. The equinox is defined by the moment the Sun’s center crosses the celestial equator, whereas a day’s length is measured from the first appearance of the Sun’s upper edge to the final disappearance of its trailing edge.
The day when daylight and darkness are truly 12 hours long is known as the “equilux.” This event occurs a few days after the autumn equinox, falling this year on September 25.
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