How do you measure light year? I know light year (ly) is an unit of time taken by light to travel in one year. I am looking for details like concept or experiment used to measure the light year. Being at a point on earth, how do you say this light has travelled one light year and the light has travelled 3 or so light years?
A light year is defined specifically by the International Astronomical Union based on the speed of light in a vacuum. In terms of what scientists mean when they say a star is X light years away, they are generally getting that number by calculating the redshift of the body in question. Redshift refers to the lengthening of electromagnetic waves as they travel through the universe. Since the universe is expanding at a known rate, scientists can work backwards and see how much the light from a distant star has shifted toward red in order to determine how long that light wavelength was traveling through expanding space. The more shifted the light is, the farther away the object is. An exact calculation can yield a relatively precise result for the distance of the object, which can then be converted into the familiar unit of light years from earth.
One minor correction: a light year is not a unit of time but a unit of distance. It is the DISTANCE traveled by light in one year. Since we know the speed at which light travels, and the length of a year, finding the distance of a light year is a very simple calculation: speed = distance / time distance = speed x time distance = (3.00e5 km/s)(31,536,000 s) = 9.46e12 km. So, a light year is about 9.46 trillion kilometers, or roughly 63,200 times the distance between the Earth and the Sun. We wouldn't use the light year to measure distances on Earth, or even within the solar system (although we may speak of light-minutes or light-hours). Light years are only used to express the distances between stars. The nearest star to our Sun is just over 4 light years away, for example. I hope that helps. Good luck!
I know light year (ly) is an unit of time taken by light to travel in one year. Wrong. A light year is a unit of distance, representing the distance light travels in one terrestrial year. IT is equal to a little under ten trillion kilometers, which is to say about 70000 times the distance from the Earth to the Sun.
You don't really measure a light year, any more than you measure a mile. You measure a distance and then say what the distance is using miles or light years. People have measured (and theoretically calculated) how fast light moves. It is about 186,000 miles a second. The measurements is done over a short distance of a few miles or less, by measuring the fraction of a second the light takes to go that far. This is usually done by sending a short flash of light to a distant mirror and timing how long it takes the flash to be reflected back. Then if you calculate the distance to some star to be 58 trillion miles, you just say 10 light years instead, because 58 trillion is just too big a number because miles are just too short for measuring such distances. Using miles to measure the distance to another star is like using inches to measure the distance from New York to Paris.
