dalia asked:
“A light year is the distance that light will travel in a year. Light travels at 186,000 miles per second.”
the light year distance when comes from far in another galaxie. How to know that is 200 light years or other?
How much time needs the sun light from the crown to reach earth? Thank you.
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I presume your question is about determining distances to stars, and galaxies.
For relatively near stars, the parallax method is used, where the orbit of the Earth around the Sun, with observations done 6 months apart, will create a very small, but still measurable, apparent motion of nearby stars against a much more distant background.
For more distant stars, the spectral type to luminosity relationship is used. Since the stellar theory provides a relation between stellar type (the color of the star) and how bright it is in absolute term, how bright it appears as seen from here is thus a function of how far away the star is.
For galaxies, yet another couple of techniques are used. One uses the variables stars, the brightness of which is a function of the absolute luminosity; and another one uses supernova, again, theory shows how bright they should be. Of course, this requires a supernova to be observed.
Finally, there is the Doppler effect, as galaxies are usually moving away from one another in the expansion of the universe, the speed a galaxy is moving away from us correlates with how far it is; see about the Hubble constant for more details.
The Earth is 8.3 light minutes from the Sun, these 8.3 minutes is how much time it takes for the light from the Sun to reach us.
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They measure the redshift of the light. It’s similiar to the Doppler effect of a moving sound source. If you know the frequency of the sound being emitted, then you can calculate how fast the source is moving. Redshift is similiar with light. As a light source moves faster away, the light it emits appears to go redder, due to a longer apparant wavelength. Conversely, if it moves towards you, it appears more blue, due to a shorter apparant wavelength. This, combined with Hubble’s Law, can tell you how far away it is.
Another technique is to measure the brightness of “standard candles”; an object is used that has a known, constant luminosity. An example would be Cepheids. If you know the brightness of one of these objects, you use a simple inverse square law to compute the distance.
We’re 8 light-minutes and 19 light seconds away from the sun, which means light takes 8 minutes and 19 seconds to reach us, on average (our orbit isn’t perfectly circular, so it’ll oscillate around that number)
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Here’s 26 ways to find interstellar and intergalactic distances -