You often here that light from other galaxies travelled 100 light years or a million light years.How do they figure out how old light is that reaches earth?
it is not possible to tell how old light is, because light doesnt get old it just comes and goes but the way they know how far the light has gone is by the wave length they produced cause over long distances light looses some of its value and the more wavelengths the closer it is viceversa.. :o)
find its source measure the distance divide by the speed of light
At distances over 100 million light years or so, you can measure the age by how much the expansion of the universe has stretched it's wavelength. But mostly you just measure how far away the object is that emitted it.
Good question! They can tell the distance to nearby stars by watching for parallax; that is the change in the star's apparent position in two observations six months apart. Since the Earth is on opposite sides of its orbit, the closest stars shift slightly in position. There is even a unit of distance called a parsec, which is the range for an object to be to have one arc-second (1/3600 of a degree) of parallax; 'parsec' is short for 'parallax second'. A parsec is 3.26163626 light years according to Google. (The nearest known planet outside our solar system is about 10.5 light years away, or roughly 3 parsecs.) Of course, with really distant objects that won't work. In those cases astronomers look carefully at the spectrum of the light. It includes bright or dark bands at known positions, from the emission or absorption spectra of various elements. It is also known that, since space itself is expanding, that the further an object is away from us the more it will appear to be moving away (although it isn't really; space is actually getting bigger). This has the effect of stretching light waves from distant stars, which shifts the colour spectrum toward the red end of the spectrum. This red-shift can be measured, and used to calculate the apparent speed of the star, and from there determine how far away the light source is, and therefore how old the light is. Of course, the distance scale needs to be calibrated. Conveniently, there is a kind of exploding star called a Type 1A supernova which, because of the way it occurs, always has about the same luminosity. So by looking across many, many galaxies for the light signature of Type 1A supernovas, and measuring their apparent brightness, it's possible to determine the distance to them directly. Then, by looking at their redshift, you can figure out how much the universe is expanding and use that as a guideline for determing distance with red shift. And there you have it.
it's all science dude! science is a mystery!
