Autumnal Equinox 2024: Fall Begins in the Northern Hemisphere
On September 22, 2024, at 8:44 a.m. Eastern Daylight Time (5:44 a.m. Pacific Daylight Time), autumn will officially begin in the Northern Hemisphere. This astronomical event marks the moment when the sun shines directly overhead as seen from a point in the equatorial Atlantic Ocean, 461 miles (743 km) south-southwest of Monrovia, Liberia.
The Autumnal Equinox is a time when day and night are said to be of equal length worldwide, hence the Latin term “equinox” which means “equal night.” However, this notion of equal day and night is actually a simplification. Atmospheric refraction plays a significant role in making daylight longer than night during an equinox. The sun’s disk is raised by more than its own apparent diameter due to refraction, especially during sunrise and sunset. This phenomenon, along with the definition of sunrise and sunset as the times when the first or last speck of the sun’s upper limb is visible above the horizon, results in daylight lasting slightly longer than 12 hours on the day of the equinox.
In Indianapolis, for example, sunrise on the Autumnal Equinox is at 7:32 a.m., and sunset occurs at 7:40 p.m., totaling 12 hours and 8 minutes of daylight. It’s not until September 25 that days and nights are truly equal, with sunrise and sunset occurring exactly 12 hours apart.
At the North Pole, the sun will be tracing out a 360-degree circle around the entire sky during the Autumnal Equinox, appearing to skim just above the horizon. Despite theoretically disappearing from view at the moment of the equinox, the sun’s disk will still be hovering just above the horizon. It will take nearly 51 hours for the last speck of the sun’s upper limb to drop completely out of sight due to strong refraction effects that cause the sun’s disk to appear oval near the horizon.
The misconception that the arctic region experiences six months of daylight and darkness stems from ignoring twilight. There are three types of twilight: astronomical, nautical, and civil. Civil twilight exists when the sun is less than 6 degrees below the horizon, while nautical twilight occurs when the sun drops to 12 degrees below the horizon. At the North Pole, civil twilight doesn’t end until October 8, and nautical twilight lasts until October 24, before astronomical twilight, when the sky becomes completely dark, ends on November 13.
Overall, the duration of 24-hour darkness at the North Pole lasts almost 11 weeks, not the commonly believed six months. Twilight cycles begin anew on January 28, marking the end of perpetual darkness until the next equinox.
In conclusion, the Autumnal Equinox in 2024 is a fascinating astronomical event that highlights the complexities of daylight and darkness during this transition period in the Northern Hemisphere. It serves as a reminder of the intricate workings of our planet and the impact of atmospheric phenomena on the perception of time and light.
