# Thread: Block out the sun.

1. If I want to block out the sun by casting a shadow over the entire earth, where in space would I position the disk/shield and how large the disk please?
ie. optimum size and distance of disk please.

2.

3. is this homework ?

4. No, this is not homework. I'm 71 and the only homework I do is take out the garbage. Thank you.

5.

6. Originally Posted by flantoons
If I want to block out the sun by casting a shadow over the entire earth, where in space would I position the disk/shield and how large the disk please?
ie. optimum size and distance of disk please.
The two are directly related (distance and size.) If you want to put it at L1 (location where it would be relatively stable) then it would have to be at least the size of the Earth.

Do you want to just "cast a shadow" (i.e partial eclipse) or completely shade the earth (total eclipse everywhere?)

7. Depending on the time of year, the Sun varies is angular size from ~31.5 to 32.5 sec of a degree (just a tad over 1/2 of a degree). To work out how large a object would need to be to shadow the whole Earth, first you need to work out how far "behind" the Earth you would have to be in order for the Earth just block the entire Sun. In other words, how far would you have to be from the Earth to have it appear to be the same size as the Sun, as seen from the Earth? The radius of an object, divided by its distance will give you the tangent of 1/2 its angular size. ( for instance, the Moon has a radius of ~1738 km and is ~384,000 km away. 1738/384,000 = ~0.004526, which is the tangent of ~0.26 degree, which is 1/2 of 0.52 degrees, which is a bit over 1/2 of degree, which is why the Moon can be such a close fit over the sun during total eclipses (though the Moon's distance from the Earth varies a bit, so depending on the circumstances it can appear either slightly larger or smaller than the Sun.)

You can use the fact that the Moon is just about the right size and distance to cover the Sun as seen from the Earth to your advantage. We know how much larger the Earth is than the Moon, and from this we can work out how much far we would have to be from the Earth in order for the Earth to appear the same size as the Moon appears to us. This puts us at the distance from the Earth where the Earth would just cover the Sun.

Now, for our sun-shield to just shade the whole Earth, this means that from this new viewing position, the Earth should just about hide it. If we know its distance from the Earth in the Sun direction (for instance, at the suggested L1 point), we know how how much further away it is from us at our viewing position than the Earth is, and thus how much bigger than the Earth it would have to be to shade the Earth entirely, at least to a fair estimate.

8. Originally Posted by flantoons
If I want to block out the sun by casting a shadow over the entire earth, where in space would I position the disk/shield and how large the disk please?
ie. optimum size and distance of disk please.
The two are directly related (distance and size.) If you want to put it at L1 (location where it would be relatively stable) then it would have to be at least the size of the Earth.

Do you want to just "cast a shadow" (i.e partial eclipse) or completely shade the earth (total eclipse everywhere?) To block the sun completely everywhere, if the shade was at L1, it would need to be 16,500 miles in diameter.

9. Thanks for all your advice. I have make a sketch of a possible solution. Any thoughts on this one please?
I tried to attach it but it refused, why?

Hope this works....

10. not sure if the upload facility works - try uploading it elsewhere and then refer to its URL inside the IMG tag

11.