Fall Equinox: Why The Days Aren't Exactly 12 Hours Long

Michael

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Post on Sep 23, 2025

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Fall Equinox: Why the Days Aren't Exactly 12 Hours Long

The autumnal equinox, marking the official start of fall in the Northern Hemisphere, is often described as the day with exactly 12 hours of daylight and 12 hours of darkness. While this is a convenient simplification, it's not entirely accurate. So, why the discrepancy? Let's delve into the astronomical reasons behind this fascinating phenomenon.

The Illusion of Equal Day and Night

The term "equinox" itself comes from Latin, meaning "equal night." On the equinoxes (around September 22nd or 23rd in the Northern Hemisphere and March 20th or 21st), the sun crosses the celestial equator – the imaginary line directly above Earth's equator. This seemingly equal distribution of sunlight across the globe should result in 12 hours of daylight and 12 hours of darkness everywhere, right? Not quite.

Atmospheric Refraction: Bending Light's Path

One key factor is atmospheric refraction. As sunlight enters Earth's atmosphere, it bends slightly. This refraction effect makes the sun appear higher in the sky than it actually is, effectively lengthening the period of daylight. This is particularly noticeable near the horizon, meaning sunrise occurs slightly earlier and sunset slightly later than what we'd calculate without considering refraction.

The Sun's Size Matters

Another important point often overlooked is the sun's apparent size. The sun isn't a point of light; it's a disc. Sunrise is considered to begin when the top edge of the sun appears above the horizon, and sunset when the top edge disappears. Because of the sun's size, the actual moment of transition takes a few minutes. This adds a few more minutes of daylight to the equation.

Geographical Location: It's Not Just About Latitude

Even with atmospheric refraction considered, the exact duration of daylight on the equinox varies slightly depending on your location. This is due to several factors:

• Latitude: While the equinox brings near-equal daylight and darkness, locations further from the equator will experience a slightly longer day than those closer to the equator.
• Altitude: Higher altitudes can experience slightly longer periods of daylight.
• Local Topography: Mountains and hills can cast shadows, affecting sunrise and sunset times.

So, How Much Longer is the Day?

The difference isn't drastic; it's typically a matter of minutes, not hours. In many locations, the daylight period on the equinox might be closer to 12 hours and 7 minutes or 12 hours and 8 minutes rather than exactly 12 hours. However, this small deviation is enough to dispel the myth of perfectly equal day and night on the equinox.

Beyond the Equinox: Understanding the Seasons

The equinoxes are just two key points in Earth's yearly journey around the sun. Understanding these astronomical events provides a valuable insight into the changing seasons and the fascinating interplay between our planet and its star. Learn more about the and their impact on day length!

Key Takeaways:

• The autumnal equinox doesn't result in precisely 12 hours of daylight and 12 hours of darkness.
• Atmospheric refraction and the sun's apparent size extend daylight.
• Geographical location also plays a role in the actual duration of daylight.

This year's autumnal equinox serves as a great reminder that even seemingly simple astronomical events have intricate details waiting to be discovered. Keep looking up!