# For an object in a geostationary orbit to have the same apparent diameter as the sun, how big would it have to be?

I know this is just a maths question, but sorry I can't work it out :p

Suppose you wanted to make something like Mr Burns' sun blocker:

But in orbit, not attached to the ground.

This would be a stoopid way to tackle global warming, but it's the era we're living in.

Geostationary orbits, as any kind repondent will know, have to be a specific distance from earth since the speed of the orbit is determined by its distance from the earth.

Given that we know the distance, what circumference would a sun-blocking sphere have to be?

• You should start with looking the required information, you need the distance from Earth(surface) to Sun, the diameter of the Sun and the distance of a geostationary satellite to ground. But you should think if a geostationary orbit is really what you need to shade the Sun.
– Uwe
Commented Jul 17, 2023 at 13:20
• It is probably better to place a sun blocker in the position L1 instead of geostationary orbit. See this for more information: space.stackexchange.com/questions/34654/… Commented Jul 17, 2023 at 13:27
• @TheRocketfan I'm not sure I'd use the adjective "better" for something that would cause almost all life on Earth to freeze to death. It would certainly be extremely effective at ending global warming, though. Commented Jul 18, 2023 at 22:28

If something is twice as far away, it has to be twice as wide to have the same apparent size.

If it's 10x farther away, it needs to be 10x bigger.

The Sun is 4250 times farther away than GEO, so the object is 4250 times smaller than the Sun, so very close to 1000 km in circumference

That said, GEO wouldn't be a good place to permanently block the Sun. In GEO, a satellite is orbiting at the same speed as the Earth is rotating, which means that seen from the surface of the Earth, the satellite is motionless in the sky. The Sun is not motionless in the sky, so the occultation doesn't last very long (roughly the same time as a total solar eclipse, which just uses a slightly bigger sphere, slightly further out).

Another consideration is that ~1000km is just enough to make the Sun dark on a single point of the Earth. If you just drive a short distance away, the object will only cover parts of the Sun, and further away still, the Sun is shining like normal.

To block all of the Sun on all of the Earth, you would need something at least the size of the Earth

• Sun subtends roughly 0.5 degree, question becomes what distance subtends 0.5 degree at a distance of 36,000 km. 36k * sin(0.5) comes out at about 360km, which is the same ballpark figure. Commented Jul 18, 2023 at 7:42
• Ha yeah I forgot that the sun isn't stationary in the sky. I should work for Elon. Commented Jul 18, 2023 at 9:51
• @RalphJ Unfortunately, Costco's business model implies that by the time aluminum foil comes around, you'll have bought enough toilet paper and kitchen rolls to cover lake Ontario! Commented Jul 18, 2023 at 16:47
• The right spot is the Earth-Sun L1 point. But then it needs to be bigger. Commented Jul 18, 2023 at 19:38
• @MarkFoskey Only if you make it heavy. If it's light material, like aluminium foil, radiation pressure will throw it off completely. See for instance the sunshade arrangement Paul Birch proposed for Venus: orionsarm.com/fm_store/TerraformingVenusQuickly.pdf Commented Jul 18, 2023 at 21:14