Physics Photo of the Week

 Physics Photo of the Week

March 27, 2026

Equinox Sunrise - Photo and Discussion by Donald Collins

This year's equinox photo was taken the day before last week's equinox of March 20, 2026.  On the equinox, the Sun rises in the direct east as opposed to the winter solstice when the Sun rises in the southeast - way off the right side of the photo.

The equinox literally means "equal nights (and days).  The length of nighttime and daytime are equal.  "Nighttime" is defined as the time between sunset and sunrise, and "daytime" is the time between sunrise and sunset.  At the equinox, the daytime length equals the nighttime length.

An interesting effect in the picture above is the long dark streak radiating away from the Sun's position - made more visible by the zoomed-in image below:

The dark streak is a long shadow cast by a cloud about 1/3 the way between the Sun's position and the upper left corner of the zoomed-in image.  The reddish color of the clouds are red because the Sun-light coming to the Earth is essentially tangent to the Earth; the rays of sunlight must travel through a much longer distancce through the atmosphere at sunrise than at mid-day.  Because long wavelengths of red light can travel further through the atmosphere and aerosols than blue light, the resulting sunlight that strikes the cloud layers is red.  In this image we can see the small cloud light up by the Sun, and its long shadow.  The shadow line is basically horizontal.  It looks like it is directed upwards, but that is caused by the geometric perspective - similar to the apparent convergence of parallel railroad tracks as we look along the path of the rails from an overpass.  Looking closely at the above photo we can also see faint bright streaks radiating away from the Sun's position slightly to the right of the Sun.  These are called "crepuscular" rays caused by gaps in the clouds near the Sun's apparent position.  "Crepuscular" means the twilight time before dawn or soon after sunset.  The crepuscular rays and the long cloud shadow are all parallel and essentially horizontal.  The three-dimensional perspective causes them to appear to diverge and fan-out.

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Physics Photo of the Week is published periodically during the academic year on Fridays by Donald F. Collins, professor emeritus of Warren Wilson College. These photos feature interesting phenomena in the world around us.  Students, faculty, and others are invited to submit digital (or film) photographs for publication and explanation. Atmospheric phenomena are especially welcome. Please send any photos to dcollins@warren-wilson.edu.

All photos and discussions are copyright by Donald Collins or by the person credited for the photo and/or discussion.  These photos and discussions may be used for private individual use or educational use.  Any commercial use without written permission of the photo-provider is forbidden.

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