Spring is right around the corner. The Spring Equinox will occur at 4:01 a.m. on Thursday, March 20, 2025. This marks the point in the Earth’s orbit around the Sun when sunlight shines directly on the Equator. The equal distribution of light results in more moderate temperatures. Day and night are also nearly equal at this time – but not exactly equal.

The true balance of day and night, referred to as the equilux, occurs a few days before the Spring Equinox. The separation between equilux and equinox happens for a few reasons. Two factors contribute to the discrepancy:

Definition of Sunrise/Sunset: Sunrise is defined as the point when the Sun’s upper edge becomes visible above the horizon, whereas sunset occurs when the last sliver of the Sun sinks below the horizon. In other words, as long as any portion of the Sun is visible, it counts as daylight. This adds a few minutes of daylight each day. Specifically on the equinox, the choice in definition tips the balance toward more daytime than nighttime.

Atmospheric Refraction: Earth’s atmosphere bends, or refracts, sunlight, causing the Sun to appear slightly higher in the sky than its “true” position. This makes the Sun visible before it truly rises and after it actually sets. In other words, atmospheric refraction extends the time spent in daylight by several minutes.

These factors result in slightly under eight minutes of extra daylight on this Spring Equinox. When did the Spring Equilux occur this year? That would be Sunday, March 17 when Baton Rouge saw 12 hours and 51 seconds of daylight.

The same effects listed above add daylight onto the Fall Equinox as well. The Autumnal Equilux occurs a few days afterward.

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