Physics Photo of the Week

 Physics Photo of the Week

Total Lunar Eclipse of March 14, 2025

Moon Photos by Donald Collins

The weather improved last week for an excellent viewing of the total lunar eclipse.  The Moon was totally within Earth's shadow between about 2:30 am to about 3:30 am.  The Moon takes on this reddish color when totally within Earth's shadow. 

The animation below consists of three frames: 1-Near the beginning; 2-near the middle of eclipse; 3-near the end of the hour-long totality.  If you look carefully you can see two stars in the right half of the images.  The stars are stationary relative to the tracked telescope.  In the hour-long totality the Moon has moved approximately one radius relative to the stars as the Moon orbits the Earth.  The Earth's shadow also moves due to the Earth moving in its orbit around the Sun, but the Earth's shadow is much slower than the Moon's motion.  Unfortunately, I did not get any partial phases of the Earth's shadow on the Moon.  Waking up at 2:00 am is not very pleasant!

The Earth's atmosphere is responsible for both the red color and the variation of the brightness of the different parts of the Moon during the total eclipse.  The phenomenon is the same as red sky in the west soon after Sunset - or the eastern sky soon before sunrise.   Red light penetrates further in the atmosphere than the blue light.  This is because the the different colors of the white sunlight are scattered different amounts depending on the wavelength of light.  Blue light (short wavelength) is scattered more than the longer wavelength red light.  At dusk and dawn when the Sun is below the horizon, the sun's rays had to pass through a much longer distance in the Earth's atmosphere, thus the resulting light that penetrated further is redder because the blue light was removed.   The drawing below shows this effect graphically.

The drawing shows the positions of the Moon (left), the Earth and its atmosphere (blue, middle), and the Sun (yellow, right).  The Moon is being eclipsed by the Earth.  The Moon is in the Earth's shadow, but closer to the top edge of the shadow than the bottom of the shadow.  The long slanted lines from the edges of the Sun show only the special rays that happen to graze the Earth.  If the Earth had no atmosphere, the shadow would have a "hard-edge" and be uniformly dark during totality.  However, because of the scattering of the light by the atmosphere, some light is scattered into the Earth's shadow and weakly illuminates the Moon.  Because the blue light is scattered away much more than the red light, the light that reaches the Moon is highly reddened.   Notice that the long rays are drawn as double: red and blue together representing two of the many continuous colors of sunlight.  The scattered light for both the red and the blue is indicated by the respectively colored arrows of dashed lines.

The dotted lines radiating outward from the sunlight's contact with the Earth's atmosphere represent the sunlight being scattered by the atmosphere.  The blue light is scattered more widely than the red light.  Light is a wave of electromagnetism.  Red light consists long wavelength.  Blue light consist of short wavelengths - about 2/3 the wavelength of red light.  The shorter wavelengths are scattered more strongly by tiny particles, or tiny variations of atmospheric density.   This explains why the daytime sky is blue and the sky toward the set Sun is red.  Red penetrates the atmosphere much further than the blue.  The diagram above the scattering of both red light (dotted red arrows) and the blue light (dotted blue arrows).  Since the blue light is removed by more scattering, the remaining light that reaches the Moon is red. 

This diagram (reproduced again above) also explains why the totally eclipsed Moon is not uniformly lit.  The part of the Moon that is closer to the edge of the shadow receives more of the scattered light that penetrates the Earth's atmosphere.

These diagrams are not drawn to scale.  The Moon is about 1/4 the diameter of the Earth - almost correct in the drawing.  However the Moon's distance is about 30 times the Earth's diameter.  The Sun's diameter is about 100 times the diameter of teh Earth - about the size of a single car garage door.  The distance of the Sun from the Earth is about 1000 Earth diameters - about two city blocks away from the Earth in the drawing above. 

Finally, the picture at left is a photograph of the Earth taken by a lunar lander on the Moon - a mission by Firefly Aerospace - immediately after or before totality.  Here we see the ring of light scattered by Earth's atmosphere and the very bright light by a part of the Sun that was not totally eclipsed by the Earth - a "diamond ring" effect.

 

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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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