The Autumnal Equinox: Exploring the Shift in Sunlight and Day Length
Astronomy and the Changing Seasons
Astronomically, autumn begins in the Northern Hemisphere on Saturday, September 23, at 06:50 GMT, marking the start of spring in the southern hemisphere.
The Sun’s Migration
Currently, the sun is moving south after shining directly on the northern half of our planet for the past six months.
The Equinox and Daylight
The autumnal equinox is the day when day and night are considered equal worldwide. However, this notion may be slightly misleading as many only consider night when the sun is below the horizon and disregard twilight.
Daylight is longer than night during the equinox due to how sunrise and sunset are defined. They are considered when the first or last point of the sun’s upper limb is visible above the horizon, not the center of its disk. This causes a slight difference in the length of daylight, which is usually reported as exactly 12 hours.
After the autumn equinox, sunset in the Northern Hemisphere will occur earlier, while sunrise will happen later. This means that the days will gradually become shorter as we approach the winter solstice.