Physics Photo of the Week

 Physics Photo of the Week

January 23, 2026  - Solstice Sunrise - Photos by Donald Collins

   

Within two days of the 2025 winter solstice, when the weather permitted, I snapped a photo of the Sun rising over the Swannanoa Mountains looking southeast from my home near Warren Wilson College.  I was away on the actual December 21 solstice, but this photo was taken at 8:11 AM on December 19, 2025 very close to the true solstice.  The "solstice" literally means the "standstill of the Sun".  The Sun on the on the winter solstice stops its apparent southerly fall migration of its rising point, and gradually begins its northerly migration of the rising point.  The solstice day is the day of the year with the shortest amount of daylight and marks the beginning of the calendar season of winter.

After the solstice, the Sun's rising position gradually moves northward along the horizon.  The image below shows the position and time of sunrise for each of 3 days taken during the following month.  Each frame of the sequence shows the calendar date and the time of sunrise.  Notice that on the solstice - the shortest daylighted day of the year, the Sun does not rise at the latest.  The latest sunrise in the sequence is on January 4, 2026 at about 8:15 am - about 4 minutes later than the Sun's rise on the solstice.  This delay is caused by the elliptical orbit (as opposed to circular) of the Earth's motion around the Sun.  This non-intuitive result is explained below the animation.

 

The Earth's orbit about the Sun is elliptical, not circular.  It happens that the earth's position is closest to the Sun at about the time of New Year's Day.  Because of the laws of planetary motion (Kepler's laws and Newton's laws of gravity and forces) a planet's speed in its orbit is faster when it is closer to the Sun where the Sun's gravity is stronger - an "inverse-square" force.  The seasons are due to the inclination of the Earth's axis relative to the Earth's orbit around the Sun.  During the northern hemispheric winter the Sun's position relative the the Earth is south of the Earth's equator.  The north pole of the Earth leans away from the Sun.  In the summer, the the Earth has moved halfway around the Sun, but the Earth's axis always points in the same direction in space, so that the Earth's north pole is then leaning toward teh Sun.  The sequence of sunrise position's in the animation above shows part of this seasonal apparent migration .  

The latest time of sunrise, however, occurs a bit later than the sunrise on the solstice.  This happens because the Earth is moving faster in the winter when it is closer to the Sun in its elliptical orbit. The Earth advances further in its orbit each day in winter than the advance rate in the summer.  The time of sunrise in the winter lags the progress through the Earth's orbit.  If the Earth's orbit were exactly circular - the Sun-Earth distance would be the same throughout the year - and there would be no delay between the solstice date and the latest sunrise date.  

In the final frame of the sequence, where the Sun rises over a noticeably lower elevation of the mountains, the Sun's rising advances even more minutes.

In some of the photos of the sequence, we can see a second, fainter image of the Sun displaced from the primary image of the bright Sun.  This is an artifact caused by partial multiple reflections from the lens surfaces in the camera (a smart phone). 

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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.

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This is the first Physics Photo of the Week post for the 2025-2026 academic year for Warren Wilson College.   PPOW took an extended "vacation" for the last half of 2025.