4 ms·

Can someone explain why the claimed shadow of the moon is vastly more bumpy than it ever appears in the night sky? Or how to square this with the fact that alt

Can someone explain why the claimed shadow of the moon is vastly more bumpy than it ever appears in the night sky? Or how to square this with the fact that altitude of the highest and lowest points on the Moon differ by less than 20km, or less than 1% of the diameter? The shadow looks like it has bumps of about 5-10% of the diameter.

21 9y ago
The effect was probably exaggerated, otherwise you would just see a smooth circle.
ErrantAstro 9y ago
There are two principal factors at work here. The first is the eccentricity of the lunar orbit - 0.0549 which means that depending on where in the lunar orbit the eclipse occurs, the moon is closer or further away meaning the apparent size ratio between the Sun and Moon varies. In fact - it's even possible to have an eclipse where the Sun's limb is visible around the edge of the Moon at totality - this is called an annular eclipse. [1] The second factor is that the Moon is lumpy! The depressions caused by craters and valleys means that totality, as defined as when the Moon completely occults the Sun's disc, varies according to where in the path of totality you are. Parallax effects move the Sun with respect to the observed position of the Moon, so the Sun will peek through the valleys/craters earlier or later depending on your latitude. This lumpiness actually creates one of the most spectacular features of a total solar eclipse, the "diamond ring" effect - I had the rare privilege of seeing this in the 1999 eclipse that crossed south-west England and northern France - a mere teenager but the image is burned into my brain. [2] Other minor factors come into play which add to why the shape of the polygon changes across the track - the Earth is a spheroid, not a flat plane so the ratio of Sun:Moon coverage varies along the track; also the Moon is still gently turning as it orbits in a manner called libration - while tidally locked to face the Earth, the Moon still rotates marginally faster or slower along its orbit. [3] The effects of the compensation for totality variation are shown in the second half of the video past ~1:20. [1] https://en.wikipedia.org/wiki/Solar_eclipse#/media/File:Annular_Eclipse._Taken_from_Middlegate,_Nevada_on_May_20,_2012.jpg https://en.wikipedia.org/wiki/Solar_eclipse#/media/File:Annu... [2] https://youtu.be/WBIyYCdwX1k?t=254 https://youtu.be/WBIyYCdwX1k?t=254 [3] https://en.wikipedia.org/wiki/Libration#Lunar_libration https://en.wikipedia.org/wiki/Libration#Lunar_libration
- scott_s 9y ago
  To anyone who viewed the YouTube link above, reference [2], go back to about 2:45 and watch the start of the diamond ring effect. It's spectacular.
  - marvin 9y ago
    And even better in reality. Everyone science and nature-interested who live near the totality, should seizr the opportunity to go see this.
  - deleted 9y ago
    [deleted]
Dave_Rosenthal 9y ago
The diameter of the path of totality is only about 100 miles, because it's only the umbra of the shadow. This is 20x smaller than the moon diameter. But, a 1km moon bump is a 1km bump on the umbra as well, magnifying it's effect by the mentioned 20x and leading to a much different appearance.
- deleted 9y ago
  [deleted]
- jessriedel 9y ago
  I hadn't thought about this effect! You're absolutely right. Thanks very much. (For others:) A diagram of the umbra makes this clear: https://c.tadst.com/gfx/750x500/total-solar-eclipse-com.png?1 https://c.tadst.com/gfx/750x500/total-solar-eclipse-com.png?... If you draw a few triangles, you can get an analytic expression.
gmiller123456 9y ago
The shadow is pretty circular near the center of the eclipse. If you look at the map, the bumpiest parts are far from the center where the shadow is elongated due to it hitting more to the side of the Earth's sphere. You'd see an oval if the Earth and moon were perfect spheres. But any irregularity on the moon gets stretched out, and irregularities of the Earth's surface add to it even more.

21 9y ago

The effect was probably exaggerated, otherwise you would just see a smooth circle.

ErrantAstro 9y ago

There are two principal factors at work here. The first is the eccentricity of the lunar orbit - 0.0549 which means that depending on where in the lunar orbit the eclipse occurs, the moon is closer or further away meaning the apparent size ratio between the Sun and Moon varies. In fact - it's even possible to have an eclipse where the Sun's limb is visible around the edge of the Moon at totality - this is called an annular eclipse. [1] The second factor is that the Moon is lumpy! The depressions caused by craters and valleys means that totality, as defined as when the Moon completely occults the Sun's disc, varies according to where in the path of totality you are. Parallax effects move the Sun with respect to the observed position of the Moon, so the Sun will peek through the valleys/craters earlier or later depending on your latitude. This lumpiness actually creates one of the most spectacular features of a total solar eclipse, the "diamond ring" effect - I had the rare privilege of seeing this in the 1999 eclipse that crossed south-west England and northern France - a mere teenager but the image is burned into my brain. [2] Other minor factors come into play which add to why the shape of the polygon changes across the track - the Earth is a spheroid, not a flat plane so the ratio of Sun:Moon coverage varies along the track; also the Moon is still gently turning as it orbits in a manner called libration - while tidally locked to face the Earth, the Moon still rotates marginally faster or slower along its orbit. [3] The effects of the compensation for totality variation are shown in the second half of the video past ~1:20. [1] https://en.wikipedia.org/wiki/Solar_eclipse#/media/File:Annular_Eclipse._Taken_from_Middlegate,_Nevada_on_May_20,_2012.jpg https://en.wikipedia.org/wiki/Solar_eclipse#/media/File:Annu... [2] https://youtu.be/WBIyYCdwX1k?t=254 https://youtu.be/WBIyYCdwX1k?t=254 [3] https://en.wikipedia.org/wiki/Libration#Lunar_libration https://en.wikipedia.org/wiki/Libration#Lunar_libration

scott_s 9y ago
To anyone who viewed the YouTube link above, reference [2], go back to about 2:45 and watch the start of the diamond ring effect. It's spectacular.
- marvin 9y ago
  And even better in reality. Everyone science and nature-interested who live near the totality, should seizr the opportunity to go see this.
- deleted 9y ago
  [deleted]

Dave_Rosenthal 9y ago

The diameter of the path of totality is only about 100 miles, because it's only the umbra of the shadow. This is 20x smaller than the moon diameter. But, a 1km moon bump is a 1km bump on the umbra as well, magnifying it's effect by the mentioned 20x and leading to a much different appearance.

deleted 9y ago
[deleted]
jessriedel 9y ago
I hadn't thought about this effect! You're absolutely right. Thanks very much. (For others:) A diagram of the umbra makes this clear: https://c.tadst.com/gfx/750x500/total-solar-eclipse-com.png?1 https://c.tadst.com/gfx/750x500/total-solar-eclipse-com.png?... If you draw a few triangles, you can get an analytic expression.

gmiller123456 9y ago

The shadow is pretty circular near the center of the eclipse. If you look at the map, the bumpiest parts are far from the center where the shadow is elongated due to it hitting more to the side of the Earth's sphere. You'd see an oval if the Earth and moon were perfect spheres. But any irregularity on the moon gets stretched out, and irregularities of the Earth's surface add to it even more.