# Small eclipse on earth

Could one make a smaller eclipse kept stationary on the surface of the earth in order that one may be able to see the stars 24/7 from one site?

How could this be achieved?

Geostationary satellite with a heap of fuel to steer it keeping the shadow path on the one point?

• It's your world, if you want it to happen, make it happen. Or are you looking for a reality check or science to back this up? And welcome to world building. – Trevor Jan 4 '19 at 5:12
• I've seen this concept before..on The Simpsons: "But, sir, every plant and tree will die. Owls will deafen us with incessant hooting. The town's sundial will be useless." -- Waylon Smithers – user535733 Jan 4 '19 at 5:29
• Welcome to worldbuilding, please take the tour and visit the help center to make yourself familiar with our standards. – L.Dutch - Reinstate Monica Jan 4 '19 at 6:39
• A stationary eclipse requires an object which sits always on the line between the Sun and Earth. Such an object revolves around the Earth with a period of one year. The radius of an Earth orbit with a period of one year is exactly the same as the radius of the orbit of the Earth; the problem is that there already is another object there, namely, the Sun. – AlexP Jan 4 '19 at 8:39
• @AlexP Nope, the diameter of an orbit with one year cycle is much closer, because earth is much lighter than the sun. It's actually just five times the distance to the moon, and the position is called L1. (see my answer below) – Karl Jan 4 '19 at 20:21

A geostationary satellite can only be placed above the equator, and therefore could create an eclipse only on and around the equator (assuming it has the right size). Not very useful to observe circumpolar stars, together with the major complication that the atmosphere around the equator is usually really humid and therefore lowers the quality of the observations.

To generate an eclipse the satellite should have at least the same angular size of the Sun, which is about 0.5 degree. The relation between dimension, distance and angular size are expressed according to the formula $$\alpha=2 \cdot arctgD \over L$$.

Considering that the geostationary orbit is at 35786 km above the surface of Earth, this gives us a transverse dimension for the satellite of at least 333 km.

A structure of that size could be built with something similar to a solar sail, a thin and opaque foil kept in position by a frame, and it would be a challenge to balance it against the pressure of the solar wind and solar light. Not even mentioning the need to repair it against any hole created by micrometeorites.

Last but not least, the Sun is not at the same azimuth around the year. I am not a pro in orbital mechanics, but I am pretty sure that there is no way to keep a satellite geostationary while chasing the Sun along its walk across the ecliptic around the year.

Summarizing, a geostationary satellite would not be feasible: too large for our current technology level and not able to chase the Sun year round. To quote what AlexP stated in a comment to the question:

A stationary eclipse requires an object which sits always on the line between the Sun and Earth. Such an object revolves around the Earth with a period of one year. The radius of an Earth orbit with a period of one year is exactly the same as the radius of the orbit of the Earth; the problem is that there already is another object there, namely, the Sun

• A geostationary satellite would eclipse the Sun exactly two short times per year, wouldn't it? The question wants a stationary eclipse. – AlexP Jan 4 '19 at 8:34
• @AlexP, it was the (convoluted) message of my answer. I made it more explicit now – L.Dutch - Reinstate Monica Jan 4 '19 at 8:41
• Ok, your point is taken. But what if the area that I need to eclipse is smaller than what happens from the moon ( about 250 km dia) maybe I can get away with one just , say, 80 km dia and still get a view of the heavens during daytime. If I used a coronagraph type sail and as I said gave the spacecraft heaps of fuel for maneuvering, could it keep a spot on the surface covered all day. Maybe if it could be placed closer to earth and orbiting in the same direction as the spin of the earth?This is just a thought experiment to ask if it is in any way feasible or possible. – Cfrancisco Jan 6 '19 at 8:05
• @AxelP: A geostationary satellite might eclipse the sun once a day (assuming it had a small amount capability for maneuvering) but most certainly not more than that. – Peter Shor Jan 6 '19 at 18:29

A much cheaper way to create an eclipse like effect is a coronagraph.

A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved. Most coronagraphs are intended to view the corona of the Sun, but a new class of conceptually similar instruments (called stellar coronagraphs to distinguish them from solar coronagraphs) are being used to find extrasolar planets and circumstellar disks around nearby stars.

The coronagraph was introduced in 1931 by the French astronomer Bernard Lyot;

Coronagraphs are used in both observatories on Earth and space observatories such as the Hubble Space telescope.

Of course coronagrphs only work for observations through the telescope they are attached to and thus do not make it possible to see ordinary stars with the naked eye during daytime. However, the brighter stars can be seen in telescopes during daylight if aimed at the calculated position. And daytime air is usually more turbulent than night air making observations less useful.

As far as I know, nobody has yet invented a method of making the stars visible during daytime for people using the naked eye.

You can put your umbrella in the Lagrange point between earth and sun (there are a few satellites positioned there observing the sun), trouble is L1 is 1.5 million kilometers away, so your umbrella needs to be tremendously (5 times our moon) large, and you need to move it forth and back by ~12000 km per day. (Side note: your town has to be somewhat close to the equator, because otherwise you need constant additional propulsion to keep the shadow from moving towards the equator.)

In short: Impossible.

(Also you wouldn't be making friends with your neighbours, because a terribly large area around your town, much larger than with an ordinary solar eclipse, would not see much of the sun either. The way weather works, everybody in the vincinity will get a severe temperature depression from not getting much sun plus a constant downstream of cold and dry air from the upper atmosphere. Freakin' cold, no snow.)

• Great answer Karl, but could one not make the shadow area somewhat smaller than a typical solar eclipse from the moon and still get the advantage of seeing a starry sky permanently in the day time. Also, as the shadow path only has to track a single point on the earth as the earth rotates into and throughout the daytime from such a distance, are you sure about the need to move the satellite 20000 km each day? I know I'm not being practical here it is just a mad thought experiment and a bit of fun. – Cfrancisco Jan 6 '19 at 7:33
• Yes, I'm perfectly sure. Relative to earth center, a place on the surface moves by a significant fraction of the earth diameter. And look at the size of the partial shadow of a regular solar eclipse. It's huge, and here it would be five times as big. No, my other answer with the plane is much more realistic imo. ;-) – Karl Jan 6 '19 at 13:54
• Correction: an earth diameter is not 20000, but ~12000km. – Karl Jan 6 '19 at 13:56
• I understand that the shadow would have to shift by the diameter of the Earth. But wouldn't the satellite, being so far away, only have to move a fraction of that distance? What I mean is that, if you were to consider the shadow like a 3rd order lever (weird analogy I know but it demonstrates what I mean). The shadow on the Earth would represent the load, the satellite the effort, and the Sun the fulcrum. If you want the load (the shadow) to move 12000km, wouldn't you only need to move the effort (the satellite) a fraction of that. – Cfrancisco Jan 7 '19 at 7:36
• Looking from the sun, earth and sunscreen are practically at the same distance. The levering effect would be marginal. – Karl Jan 7 '19 at 8:22

If you have steady enough wind, you might just build an oversized airplane and let if hover over your town. It's huge, lightweight, and flying very slowly, so it won't need much energy. You could probably power it with photovoltaics. Much friendlier than my other proposal above.

Of course you need to ground it overnight. Where do you land with a plane that is several kilometers large?

• @AlexP OP wanted a permanent eclipse over one location! The plane has to fly from horizon to horizon (120km perhaps?) in +-12 hours, depending on the season. 10 km/h, a bit faster in the morning and evening. – Karl Jan 4 '19 at 20:48
• You are right. I was wrong. – AlexP Jan 4 '19 at 21:27