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  • The Sun and the Moon are about the same size when you look at them in the sky, though that’s

  • just thanks to the coincidence that the Sun is about 400 times farther away than the Moon

  • and also about 400 times bigger. Another fun coincidence is that the radius of the Sun

  • is about twice the distance to the moon, so if you put the sun where the Earth is,

  • the Moon would be just over halfway to the surface of the Sun.

  • The Sun is also so big that it takes light over 4 and a half seconds to travel across

  • its facethough I don't know whether that says more about how big the Sun is, or how

  • slow light is, or both.

  • What about the scale of other solar system objects? Well, here are Pluto and Charon compared

  • to the Earth and the Moon.

  • And Jupiterwhich, as a rough rule of thumb, has about 10 times the radius of the earth

  • and one tenth the radius of the Sun.

  • Saturn is slightly smaller than Jupiter, though its rings go out twice as farstill only

  • a third of the way to the Moon, though.

  • Marsmoons are very close and very small, which gives rise to another moon-sized coincidence:

  • Phobos is almost the same size in the martian sky as the Sun, and can cause annular eclipses

  • just like our Moon here on Earth.

  • If two objects have the same apparent size in the sky, like the Moon and the Sun, it

  • means that their radii divided by their distances are about the same value. This also implies

  • that the strengths of the objectstidal forces, which are roughly proportional to

  • the radius over the distance cubed, will also be about the same. In fact, ANY two objects

  • of roughly the same density that look about the same size in the sky will have tidal forces

  • of similar strengths.

  • So while the apparent equal size of the moon and sun in our sky is a coincidence, the fact

  • that they have similar apparent sizes AND roughly similar effects on the tides is a

  • consequence - of geometry and physics.

The Sun and the Moon are about the same size when you look at them in the sky, though that’s

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B1 INT moon sun coincidence radius sky size

How Big Is The Sun?

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    林宜悉   posted on 2020/03/28
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