Equinox in Autumn 2025 Marks the Arrival of Fall in the Northern Hemisphere on September 22nd
On Monday, September 22, 2025, the first day of fall will arrive in the Northern Hemisphere, marking the astronomical start of autumn. However, the events surrounding this seasonal transition are not as straightforward as one might think.
In Atlanta, the amount of daylight on the first day of fall will be 12 hours and 8 minutes, not the usual 12 hours as one might expect. This slight discrepancy is due to the way daylight is measured, taking into account the exact moment the first or last speck of the sun's upper limb is visible above the horizon.
Meanwhile, at the North Pole, the sun's behaviour is even more intriguing. On the first day of fall in 2025, the sun's disk will still be hovering just above the horizon, even though it should theoretically disappear completely. This is because the strong refraction effect at the North Pole causes the sun's disk to appear oval when it is near the horizon, and its lower limb is lifted more than the upper, distorting the sun's disk noticeably.
The sun will trace out a 360-degree circle around the entire sky, appearing to skim just above the horizon. Not until 52 hours after the Autumnal Equinox at the North Pole will the last speck of the sun's upper limb finally drop completely out of sight.
It's important to note that the definition of the equinox as a time of equal day and night is a convenient oversimplification. In reality, the duration of daylight at an equinox is still more than 12 hours, and the first day of fall in 2025 will see cities north of the equator near or above the Arctic Circle, such as Nordkap in Norway, experiencing daylight length exceeding 12 hours. Cities farther south in Germany or Austria, like Hamburg or Oberedt, will have daylight shorter than 13 hours but close to around 12 hours around the equinox.
The sun crosses the equator on around March 20 and Sept. 22, which are called equinoxes. At an equinox, atmospheric refraction raises the sun's disk by more than its own apparent diameter while it is rising or setting, creating an optical illusion.
Interestingly, at the North Pole, astronomical twilight - when the sky becomes completely dark - ends on Nov. 13, and it remains perpetually dark until Jan. 28 when the twilight cycles begin anew. Civil twilight does not end until Oct. 8, even though the sun disappears from view for six months beginning on Sept. 24.
In conclusion, while the first day of fall in 2025 may seem like a simple event, the intricacies of the sun's behaviour at the North Pole and the way daylight is measured add a layer of complexity to this seasonal transition. The equinox, while traditionally thought of as a time of equal day and night, is in fact a fascinating time of year where the sun's behaviour is far from ordinary.
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