Speed of the umbra during eclipse vs rate of the Earth's rotation

In summary, the speed of the umbra, or the shadow cast by the moon during a solar eclipse, is significantly faster than the rate of the Earth's rotation. While the Earth rotates at about 1,670 kilometers per hour at the equator, the umbra can move across the surface at speeds exceeding 7,000 kilometers per hour, allowing for a brief totality experienced in specific locations. This difference in speed explains why total solar eclipses are relatively rare events at any given location on Earth.
  • #1
imsmooth
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The earth rotates faster than the moon orbits. Thus, the moon appears to move from East to West each day. The shadow of the moon during an eclipse as it moves along the curved earth makes it move faster and we see it move West to East.

My question is If a solar eclipse happens near noon and one is directly under the sun-moon alignment, wouldn't the shadow move East to West for a brief moment since the earth rotation is faster at this point? Or is it never faster?
 
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  • #2
It is never faster. The Earth spin makes the equator move about 500 m/s. The Moon’s orbital speed is about 1 km/s.
 
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  • #3
To add to that, it is easy to draw this conclusion knowing only the distance to the Moon (ca 384000 km), the radius of the Earth (ca 6300 km) and the periods (28 days and 1 day). As 6300x28 is about 180000, the corresponding equator speed is about half of the Moon’s orbital speed.
 
  • #4
Thank you. I should have done the calculation first.
 

FAQ: Speed of the umbra during eclipse vs rate of the Earth's rotation

What is the umbra during an eclipse?

The umbra is the darkest part of a shadow, specifically the region where the light source is completely obscured. During a solar eclipse, the umbra is the area on the Earth's surface where the Sun is completely blocked by the Moon.

How fast does the umbra move across the Earth during a solar eclipse?

The speed of the umbra can vary depending on the specific geometry of the eclipse, but it typically moves at speeds ranging from about 1,000 to 2,500 miles per hour (1,600 to 4,000 kilometers per hour) across the Earth's surface.

How does the speed of the umbra compare to the Earth's rotation?

The Earth's rotation at the equator is approximately 1,670 kilometers per hour (about 1,040 miles per hour). During a solar eclipse, the speed of the umbra can exceed this rotational speed, allowing it to cross the surface of the Earth more quickly than the Earth rotates.

Why does the umbra move faster than the Earth's rotation during an eclipse?

The umbra moves faster than the Earth's rotation due to the relative motion between the Earth and the Moon. The orbital speed of the Moon around the Earth is significant, and when combined with the Earth's rotation, it results in the umbra traveling at a speed that can surpass the rotational speed of the Earth.

What factors influence the speed of the umbra during an eclipse?

Several factors influence the speed of the umbra, including the distance between the Earth and the Moon, the angle of the Sun's rays, and the specific path of the eclipse. The geometry of the alignment between the Earth, Moon, and Sun determines how quickly the umbra will move across the Earth's surface.

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