Anyone with a large telescope who also likes to look at the Earth?

Question

Say seeing is 1 second of arc one night, near the zenith. Is it possible to get close enough to a building to see better than 1 second of arc on it?

Because there's less total atmosphere between you and the building than between the star and space

How close do you have to be?

Assuming your scope is large enough to be able to see sharper than seeing.

Typo! Sorry! I meant earth to space

Answer 1

You're confused about what arc seconds are. Arc seconds are little slices out of a circle. Any circle, no matter how big or small. Sixty arc seconds make an arc minute, 60 arc minutes make a degree, 360 degrees make a circle. There are 60X60 or 3,600 arc seconds in a degree and 216,000 arc seconds to make a full 360 degree circle.

Out in space we can't see *how far* things are (we have to measure that differently), but we can see how *how far apart they appear*, measured as parts of a circle. You could have two GALAXIES that are a few arc seconds apart, you could also have TWO STARS. The STARS would be MUCH CLOSER to us and to each other than two GALAXIES that are also separated by two arc seconds.

Your telescope magnifies things by "zooming in" on them. So you can see Epsilon Lyrae as one star to your eyeball but in your telescope it is the famous double double. The pair of stars is separated by 3 arc minutes and 26 seconds written 3' 26". Each side of the pair splits into another double star, those are separated by about 2 arc seconds written 2". The small components of these stars are separated by about 1/10th of a light year according to Internet sources--farther away than their ANGULAR DISTANCE would indicated.

Now if you focus across the street, your telescope will still have the same RESOLVING POWER which is measured usually as a fraction of one second of arc. An eight inch telescope has a resolution of 0.58". But if you're looking at the neighbor's garage door you'll be seeing two flecks of paint separated by a very small distance. If the building is 100 meters away the distance, in units of arc, between two specks of paint 1 mm apart is 2.07 arc seconds--about the same as the separation between the tight pairs of the double double. So your telescope's ANGULAR RESOLUTION (sometimes just called resolution), measured as how much of piece of a complete circle it can see, is the same, regardless of whether you look 100 meters at two specks 1mm apart, or whether you are looking up at two stars 160 light years away, and about one tenth of a light year apart.

As you are aware, pointing straight up or "at zenith" gets you the best atmospheric conditions, and usually your telescope's real performance is, because of the air above, much worse than it is in theory. This does not change if you point the telescope across a lake or down the street. In fact the thermal mess very close to the Earth is much worse than it is looking straight up in the cool of the night. So the odds are that you will find it as difficult to see two specks 1 mm apart (2 arc seconds) on a garage door at a distance of 100 meters as you will two stars that are 2 arc seconds apart 160 light years away.

Or look at it th is way. Drop a 10mm eyepiece into an eight inch telescope. Look at two specs separate by 1mm at a distance of 100 meters. The distance between those two specks in your eyepiece will be *the same* as it is between the stars of the Lyra double double. That's because the angular distance is a function of the telescope and how much of the sky it seems out of a full potential 360 degrees.

Arc seconds stay arc seconds because the "circle of view" goes from the end of our noses out to as far as we can see. If you sit cross-legged on the ground, mid-way between the ends of a Toyota Corolla, at a distance of one meter, the Corolla fills up just under 180 DEGREES of space in front of your eyes. You see the car and nothing but the car. On the moon, it would be a tiny speck that you can't see--about 0.0018 arc seconds, to be precise. Your eyes resolve to 20 arc seconds and that isn't enough to see something that is 0.0018 arc seconds. Even the biggest telescope currently on the drawing board won't be able to resolve that finely.

Hope that helps. GN

Answer 2

You words say nothing. What does, "less total atmosphere between you and the building than between the star and space" even mean? Of course there is less atmosphere between you and a nearby building on Earth than there is between you and a star IN space, but there is NO air between a star and the space around the star.

Anyway, seeing is usually worse near buildings, not better. The best seeing is from a large open grassy area or from a small island surrounded by water,

Answer 3

Well, the image of earth which God would reveal to aliens living thousands of light-years away would really depend on what the aliens expected to see relative to their own planet... Bloody hell, God must be really knackered working all this stuff out! It would probably have been a hell of a lot easier to let all that primordial hydrogen float around for a bit and see what sort of universe it eventually formed. God is apparently ageless, so a few billion years wouldn't have been such a great wait...

Answer 4

Telescopes designed for astronomy aren't designed to focus on something that close up.