This year, astronomical autumn will officially begin on Sunday, September 22, at precisely 15:43 Bulgarian time.
The onset of the season will occur as the Sun crosses the celestial equator during the autumnal equinox, an astronomical event that this year takes place within the constellation of Virgo. The equinox marks the moment when day and night are nearly equal in length across the globe.
In 2024, the start of autumn will arrive approximately 18 hours earlier than last year, a result of this being a leap year.
In 2023, the season began on September 23 at 09:50. The leap year’s additional day in February slightly shifts the timing of seasonal transitions, causing this earlier arrival.
The Science Behind the Equinox
The autumnal equinox is one of two points in the year when the Sun’s path, or ecliptic, crosses the celestial equator, an imaginary projection of Earth’s equator into space.
This alignment of the Sun with the equator results in nearly equal hours of daylight and darkness for much of the world.
As the Sun moves southward after crossing the celestial equator, it ushers in the autumn season for the Northern Hemisphere and spring for the Southern Hemisphere.
After the equinox, the days in the Northern Hemisphere will grow shorter as the Sun continues its journey southward, while in the Southern Hemisphere, longer days signal the arrival of spring.
In Bulgaria, residents will notice the increasingly crisp air and shorter daylight hours, signaling the full onset of autumn.
The Impact of the Leap Year
Leap years occur every four years, adding an extra day to the calendar on February 29. This adjustment helps align the calendar year with Earth’s actual orbit around the Sun, which takes about 365.25 days.
Without the leap year correction, each calendar year would drift by about a quarter of a day, gradually causing the dates of equinoxes and solstices to shift.
Over time, this misalignment would become more noticeable, leading to a much earlier or later start to the seasons. In 2024, this adjustment means the autumnal equinox falls earlier than it would in a standard year.
Autumn in the Northern Hemisphere
With the official start of autumn, the Northern Hemisphere will soon experience cooler temperatures, changing foliage, and the harvest season.
Countries like Bulgaria will begin to prepare for the traditional activities associated with the season, including celebrations of the autumn harvest and the transition towards winter.
For many, the equinox also holds cultural significance, marking a time of balance between light and dark. Various festivals and traditions worldwide are tied to this moment of equal daylight, particularly in cultures that celebrate the natural cycles of the Earth.
Spring in the Southern Hemisphere
While the Northern Hemisphere embraces autumn, the Southern Hemisphere will enter spring.
Countries like Australia, Argentina, and South Africa will witness the blossoming of flowers, milder temperatures, and the lengthening of days, as the Sun moves higher in the southern sky.
As the Sun continues its southward path until the winter solstice in December, both hemispheres will fully embrace their respective seasonal shifts.
The autumnal equinox is a reminder of the intricate balance between Earth’s rotation and its orbit around the Sun, guiding the rhythms of life across the globe.