The Fall Equilux occurs Monday; here's how it's different than the equinox

WASHINGTON (7News) — Monday, Sept. 26, is “Fall Equilux” for the DMV. That’s different than the Fall Equinox.

During the equilux, the Washington, D.C. area will receive exactly 12 hours of daylight and 12 hours of darkness. The day falls a few days after the autumn equinox and a few days before the spring equinox.

The Autumn Equinox occurred Sept. 22 at 9:03 p.m. That is when the sun was directly over the equator giving the northern and southern hemispheres approximately 12 hours of daylight and darkness.

So, why is this the case? Two reasons:

1. The sun is a disk not a point of light
2. Atmospheric refraction

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First off, the sun is a disk -- not a point of light. Sunrise is defined as the moment the top of the sun crests over the horizon. Sunset occurs when the last part of the disk slips below the horizon. The equinox has to deal with the center of the sun hitting the equator.

Secondly, the Earth’s atmosphere refracts (bends) sunlight. This distortion makes the sun appear higher than its true position on the horizon. This means the sun may appear earlier and sunset may appear later than when the actual sun crosses the horizon.

These factors add up to provide an additional eight or so minutes of daylight between the equinox and equilux.