What Is the Science Behind the Autumn Equinox’s Magic?

Autumn arrives in a cascade of golden leaves and crisp, cool air, awakening a sense of nostalgia and anticipation. It is a season of transformation, when nature quietly prepares for the stillness of winter. As the days shorten and the sun’s warmth fades, the world embraces the spirit of the harvest—a time to gather, reflect, and celebrate the abundance of the year. Fields are ripe, trees heavy with fruit, and communities come together in gratitude for the earth’s bounty.

Beyond the harvest, autumn signals the approach of winter’s festive celebrations, a period where light and warmth are cherished against the growing chill. It is a season of balance and beauty, where nature’s final burst of colour and energy reminds us of the cyclical rhythms of life, and the quiet joy found in preparation, reflection, and togetherness.

Autumn, a season known for its vibrant foliage and cooler temperatures, signals a transition from the long, sun-soaked days of summer to the shorter, crisper days of winter. But when exactly does autumn start in 2024, and what is the science behind this shift in seasons?

Let’s explore the science behind the autumn equinox and the fascinating factors that define this transformational season.

Autumn 2024: When Does It Start?

In 2024, autumn in the Northern Hemisphere begins on Sunday, September 22, at precisely 18:43 UTC (Coordinated Universal Time). This moment is marked by the autumn equinox, an astronomical event when day and night are nearly equal in length.

However, the start of autumn can vary depending on how it’s defined. There are two primary ways to determine the beginning of autumn: astronomical autumn and meteorological autumn.

Astronomical Autumn

This is based on the Earth’s position in its orbit around the Sun and the equinoxes and solstices. The autumn equinox, which occurs around September 22 or 23 each year, marks the start of astronomical autumn. As Earth continues its orbit, the tilt of its axis causes the Northern Hemisphere to receive progressively less sunlight, leading to cooler temperatures and the characteristic shift in seasons.

Meteorological Autumn

Meteorologists, on the other hand, divide the year into four seasons based on the calendar months and annual temperature cycles. Meteorological autumn begins on September 1 and ends on November 30. This system simplifies data collection and forecasting because each season lasts exactly three months.

What Is the Autumn Equinox?

The word “equinox” is derived from the Latin words aequus (equal) and nox (night), which refers to the time when day and night are approximately equal in length. The autumn equinox occurs when the Sun is directly above the equator, resulting in nearly equal amounts of daylight and darkness across the planet.

This astronomical event happens twice a year: once in March (the spring equinox) and once in September (the autumn equinox). The 2024 autumn equinox will take place on September 22.

The Science Behind the Equinox

The equinox occurs due to the Earth’s 23.5-degree axial tilt. Throughout the year, different parts of the planet receive varying amounts of sunlight, leading to the changing seasons. During the equinoxes, however, the Earth’s axis is not tilted toward or away from the Sun, so sunlight shines equally on both the Northern and Southern Hemispheres.

After the autumn equinox, the Northern Hemisphere begins to tilt away from the Sun. This results in shorter days and cooler temperatures. Conversely, in the Southern Hemisphere, the opposite occurs, and spring begins.

Autumn by the Numbers

One of the most noticeable changes during autumn is the shortening of daylight hours. In mid-September, cities like New York receive about 12 hours and 10 minutes of daylight, but by late December, this drops to about 9 hours and 15 minutes.

In terms of temperatures, autumn is characterized by a gradual cooling. According to the National Oceanic and Atmospheric Administration (NOAA), average global surface temperatures drop by 0.5°C (0.9°F) from September to November. The U.S. Midwest, for example, often experiences a dramatic transition, with September highs around 75°F (24°C) dropping to 50°F (10°C) by November.

Fascinating Facts: The Autumn Equinox in Different Cultures

Throughout history, the autumn equinox has held cultural and spiritual significance across the globe:

Stonehenge in England is aligned with both the solstices and equinoxes and many people still gather there during these events to observe the Sun’s movements.

In Japan, the autumn equinox marks a national holiday called Shūbun no Hi, a time to honour deceased family members and reflect on nature.

In Greek mythology, autumn is linked to the tale of Persephone, whose return to the underworld marks the end of summer and the onset of autumn and winter.

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How Climate Change is Affecting Autumn

In recent years, scientists have noted that autumn has been starting later in the year and lasting longer due to the effects of climate change. A study published in the journal Global Change Biology in 2020 showed that, across the Northern Hemisphere, the average length of autumn has increased by around 0.5 to 1.5 days per decade over the last century. This shift is primarily due to warmer temperatures persisting later into the fall, delaying the onset of cooler weather and the traditional leaf changes.

Leaf Colour and Climate

One of the most iconic features of autumn is the vibrant display of leaves turning red, orange and yellow. The timing of this change is influenced by temperature and daylight. Warmer autumns can delay the process, and in some cases, the intensity of the colours may diminish due to prolonged heat stress or drought.

The Magic of Autumn 2024

Autumn in 2024 begins with the equinox on September 22, a moment when the Sun’s rays fall evenly on both hemispheres. This marks the official start of cooler days, falling leaves, and shorter daylight hours. However, with climate change gradually reshaping the seasons, the future of autumn could look quite different. As we savour the crisp air and beautiful foliage, it’s a reminder of nature’s delicate balance and the scientific forces that govern our world.

Whether you’re watching the leaves change in Japan or Greece, celebrating new cultural traditions, or simply being at home enjoying the cool breeze, autumn offers a unique blend of natural beauty and scientific wonder.

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