Why did ancient people measure the distance to other planets and to the sun?
They did not measure that.Ancient people had no clue of how the solar system worked. For them, the solar system was either governed by gods or the things they saw were attached to firmaments or spheres in the sky.They had even less any possible method for measuring anything outside their immediate surroundings. A distance such as that to the Moon was far beyond their grasp.Some scholars like Erastothenes were able to figure out the circumference of the Earth, but he was the rare exception.
Why are the distances to other planets measured in light years? What is a light year?
Same reason why we dont measure our weight in milligrams.ConvinienceLight travels at 299,728 kilomoters per secondIn one year it travels 9.5 trillion Km.Light-year - Wikipedia.Now the nearest star (List of nearest bright stars - Wikipedia) Alph centari is approx 4 light years away.. or in other words you have to say 38 trillion km away.How many km it would be for a star which is billion light years away?
How is the distance between the planets and the stars measured?
You can't measure distances such huge in kilometres or miles.So a need to come up with a new unit of measurement was required.What it could be, then a thought struck someone.Such distances could be measured by LIGHTYEARS.No brother it ain't a measure of time.It is the distance travelled by light in one year.We know light travels 3,00,000 meters per second.That's makes around 9 trillion kilometres per year.That's quite a huge distance.Do you know that the nearest star next to sun is 4.2 light years away.See this new measurement of distance has made easy to crunch down big numbers.Or you can take another fact, one of the farthest quasars like ABEL IR 1315 is a whopping 13 billion light years away, you see now if you were to find it out in kilometres it would be cumbersome. It is so far that even measurement units like light-years are looking cumbersome.Another fact, our galaxy contains over 100 billion stars and from end to end it's 1,00,000 light years.The bulge in the middle of our own galaxy is 15,000 lights years thick.Our solar system is 30,000 light years from galactic center point.I hope you got to know what a light year is.EDIT 1: As the question has been changed to distance between planets and stars, I would add up some points regarding it.Distance between the stars and planets can be measured in miles/kilometres or LIGHTMINUTES.LIGHTMINUTES is the distance covered by light in a minute.Like the distance between sun and earth is:149 million kilometres or8 light minutes.
How to measure distance in space?
Your question is really good. But, I'll object your statement "if some thing happens in universe,immideatly telescopes finding it". No!!! Any happening in the universe will be propagated with the speed of light. For e.g. If in our solar system, we know that light takes around 8 min to reach from sun to earth. so imagine a condition If somehow you take off sun from solar system, it will take 8 mins to let you know that sun has disappeared even though you are looking at sun (through a proper filter) continuously. Astronomical objects are so distant that whatever object you see today, light has emerged from those objects 100's of years back... Its not possible to communicate any incident faster than the speed of light. This was highlighted in Special theory of relativity by Einstein. Now if your question is how to measure a distance between object just by looking at a light coming from it, it may done by looking at half-life time of photons. Somebody technical at particle physics may give a detail answer on this.
How to measure the distance to the Moon?
Hi. Yes, the size of the earth was determined with pretty good accuracy in ancient times using nothing more than two sticks. One was in a city where the sun was directly overhead on a particular day and one was in a more southern city where the stick cast a shadow (you can research which ones). They estimated the earth to be about 25,000 miles in diameter based on the angle of the shadow. During a lunar eclipse the earth casts a shadow on the moon with a radius equal to earth's. They determined the relative size of the moon relative to the earth. Two sticks and observing. Clever. Based on the diameters they estimated the distance.
How can NASA measure the distance between Earth and other planets or stars when they are 1000 lightyears away? Light needs 1000 years to get there.
Look around the room, and find two small objects, one near you and one further away, both in easy view without having to move your head.Now, close one eye, and look at the two objects without moving your head. Now switch eyes.Notice how the two objects appeared to move… but the nearby item moved a lot and the far away object moved a little?This is a phenomena called parrallax. It’s the main way humans use to determine how far away things are.Stars are things.So if you use two telescopes, and precisely determine the angle you have to use in order to see the star from two different locations, you can see how far away that star is.Nearby items, you can do this with binoculars, just a couple inches apart like your eyes.Further items, you set the telescopes up on two hills across the valley from each other. Or across the state from each other. Or across the country from each other. Or across the world from each other.Or you do one measurement in spring, and then a second in fall, when you’re 2 AU away from the first measurement.Yep, two (virtual) eyeballs twice the distance apart as the Earth is from the Sun.That’s one big pair of binoculars!
Why are the distances to the planets given in kilometers, while the distances to stars and galaxies are...?
We could measure the distances to stars and galaxies in kilometers, but the numbers get hard to handle. The nearest star is over 40,000,000,000,000 km away, and the Andromeda galaxy is more than 20,000,000,000,000,000,000 km distant. Light years are just more convenient when discussing interstellar and intergalactic distances.
How is the distance between planets determined?
Using ta technique called parallax that utilised the orbit of the earth and the orbit of the other planets.You can test this by holding something such as a pencil or your finger and then alternating between closing one eye and then the other eye while keeping the other eye open. the position of the object will appear to move depending on which eye is open. This is the phenomenon of parallax.Parallax involves observing planets in different parts of the Earth’s orbit, usually six months apart (or different places on the earth depending on what is being measured) of the planet at each of those times.Usually the observing something 6 months apart methods usually works on stars as they don’t move much in the sky in that time frame. But it is difficult to use this method for a planet as it is made a bit complicated by the fact that the planets move during that time and don’t stay fixed like stars do.So instead you could stand on different points on the Earth and observe the different using the stars behind the planet. One observer should be able to see some stars and not others and vice versa for the other observer. This can be used to calculate the distance between the Earth and the planet and, using the distance between the earth and the Sun how far away that planet is from the Earth.