How do astronomers measure distances that light travels?

Answer 1

There are several methods. For example, for nearby stars, the parallax method is used - the star will change its apparent position, between the two extreme positions of the Earth in its orbit (e.g., now, and 6 months later - a distance of 300 million km.). The greater the angle, the nearer the star. For remote stars or galaxies, they measure the intensity and spectrum of lightn to tell how far the star is. If you see some cars on a road during the night, you can tell how far they are based on how bright the lights are. Pretty easy, because all cars have allmost same road lights. But the problem is that if you're looking at stars in the night sky, if a star is at a certain distance from you its brightness can't really be used as a measure of how far away it is because a bigger star will be brighter and because light gets dimmer the farther it is from you a big star can be a lot farther away than a small star and yet they'll both appear exactly the same brightness. How do you solve that one? This kept astronomers guessing for a very long time until about the turn of last century. A woman in contact with Hubble, after whom the Hubble Space Telescope is named, solved the problem. Her name was Henrietta Levitt and she was looking at star charts. She noticed that some star appeared to get bigger and brighter and then dimmer and weaker. They did it with a regular period. These have now become known as the stellar yardsticks. They're called Cepheid variables. They're stars that swell up and shrink down. Because the period at which they do that varies with the size of the star you therefore know, if you look at how often a star like that is blinking on and off, you know how big it is. Therefore you know how bright it is. Because light follows an inverse square law you can work backwards to work out how bright that star must be and therefore how far away it is. Scientists now use these Cepheid variables when they look at a distant star structure they can use the period of any Cepheid variables that are there to work out how far away those particular entities are. That's a stellar yardstick and it was solved by a lady at Harvard a hundred years ago Read more: How_astronomer_measure_distances_among_the_stars

Answer 2

AU or Astronomical Units - This is the distance from the Earth to the Sun and is used only within our solar system. Light years are used to measure distances in the remainder of the universe.

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They also use the PARSEC for extreme distances, based on a trigonometrical use of the parallax given by observing the same object from opposite "sides" of the Earth's orbit.

Answer 3

Scientists measure distances within Solar System using parallax measurements and sometimes radar.

The units used are kilometers (or miles) and Astronomical Units (the average distance from Earth to Sun).