Physics Photo of theWeek

Physics Photo of the Week

November 3, 2023

Moon's Shadow - Images from NASA GOES*

This color satellite image (from NASA GOES image viewer) was taken on October 14, 2023 during the partial solar eclipse (see PPOW for October 17, 2023).  Notice that there is no land visible in the bottom left part of the image covering parts of the American southwest.  The clouds in the upper right of the image are bright white, while the clouds near the edge of the darkest regions are tinted red.  From parts of the lower left part of the image the Moon was covering the maximum amount of the Sun's apparent disk as possible.  For this eclipse the Moon was too far away from Earth to completely cover the Sun's apparent disk, but left a narrow ring of Sun exposed.  This was an annular eclipse.  Some thin clouds are barely visible in southeast Colorado illuminated by the narrow annulus of the maximally-eclipsed Sun.

Why do the clouds close to the darkest regions of the photograph have a red tinge to them?  The answer is due to "limb darkening" of the Sun.  The limb of the Sun is the edge of the Sun's apparent disk.  However, since the Sun is a sphere - not a disk - the light leaving the perimeter of the Sun's disk must travel through a much longer path through the Sun's atmosphere than light that leaves the center of the Sun's disk.  When light travels through an atmosphere - be it the Sun's atmosphere or the Earth's atmosphere - the blue light is absorbed or scattered away from direct view much more than red light.  Thus the edge of the Sun's disk becomes "reddened" - the blue portion of white light has been removed, and the clouds likewise look red.  The Earth's atmosphere also reddens the Sun's image at sunset and sunrise.  Click on the image above to view a short sequence of three satellite images spaced 30 minutes apart.  The darkest part of the shadow moves from middle left down to lower right part of the image.  The numbers on each frame of the animation indicate each frame's sequence number.

The total track of the Moon's shadow as it passed from the west coast of the USA until the shadow disappeared at sunset in South America can be seen in the animaed image below.  The time lapsed for the shadow to pass from the west coast of California to the East coast of Brazil is 4 hrs to cover a distance of about 7,000 miles.  We leave it to the interested reader to calculate the speed of the Moon's shadow in miles/hr.  The satellite images were downloaded from the current satellite views (https://weather.ndc.nasa.gov/GOES/) the same day as the eclipse.  Readers are also challenged to download similar images from the above website very soon (same day) after the April 8, 2024 total solar eclipse.  They are only available at the above website for a maximum of 8 hours each day.

 

 

 

*GOES = "Geostationary Operational Environmental Satellites"

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Physics Photo of the Week is published weekly during the academic year on Fridays by the Warren Wilson College Physics Department. 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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