The prism works on the principle that each wavelength is refracted a different angle as it enters and exits the prism.
How are light-years measured?
- Thread starter Jacob
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The prism works on the principle that each wavelength is refracted a different angle as it enters and exits the prism.
This is what I was looking for in what you were presenting. Thank you.
If the wavelength is different, is the speed? We must also think of different mediums, such as glass or water.
No you haven't. You've been aging your pet project, not his question.
It is not unable.
We can measure the distance to the moon, sun and numerous other stars in light years.
We can't MEASURE by light years.
Yes we can and we do. A light year is a unit of distance so we can use it to measure distance. You just need to keep in mind that distances are measured by several different methods that all are capable of giving results in the unit of "light years". You must also remember that the calculated distances are approximations. That doesn't mean the method or the unit if length are some how wrong, it just means there are certain variables that cannot be determined with sufficient accuracy for a perfect calculation.
We can speak of distance in terms of light years, if we know first what that distance is by other means. In this sense it can be a measure. But we don't MEASURE in terms of light years because light years describes what was presumably already measured, not what is being measured.
Light years are accurate measurements to where the light was emitted. Yes, it is possible that the light we see is hundreds or thousands of years old and maybe older. That in no way invalidates the measurement. It is also why I said velocities are so important. If we take measurements over a period of years we dan know the velocity. If you take the starting position and add to it the velocity times the time, then you can predict where the object is today. You won't see the light from today's position for some time, but that doesn't mean you don't know where it is.
I believe you are speaking of the unit. But taking a measurement or measuring by a standard are both different from defining something as a measurement in the sense you are doing.
Do we determine the distance by anything in the light itself? No.
Then to say that an object is a certain distance away because we measured it in terms of light years might be wrong. We need to find out how we indeed measured the distance to these objects without light, or we need to find out how we can measure something with light, when light in itself does not have anything that gives us a distance that it has traveled.
A light year is a distance. Exactly like a ruler, a yard stick, a mile or a fathom. You cam measure the distance between stars in miles if you wish. It is just that there are so many miles that a light year becomes a much easier unit to deal with. We have already figured out how to use received light to measure distance.
Please, read the links provided. It will help.
Have you followed in this thread that you can't use light as a ruler?
There are properties of light that we can use to estimate its point of origin. Remember the 1/r^2 law? And a light year is defined as the distance light will travel in one year in a vacuum.
Okay. I still can't tell how many light years away a distant galaxy is by looking at it.
The distance can be known in light years, but you can't use a light year to mark out the separation.
Just like you can "measure" the distance from the US to the UK in miles.
Jedidiah
New member
What is the official definition of a meter ? IUPAC's definition.
Not IUPAC. SI ? Who sets international standards for physical constants ? Whoever they are, how do they define a meter ?
It used to be a metal bar or rod or something like that, it was the official meter. Meter with a capital M. They had it in Europe I think. They almost completely assuredly still do have it, and whoever these people are, changed their definition of a meter from that piece of metal, that would never change in length, to something else, having to do with light, and I think it just might be as simple as saying that a meter is now the distance light travels in a vacuum for 1 three hundred millionth of a second. So the meter is also a measurement of time.
And that means that Science with a capital S authoritatively defines the Meter with a capital M to be something like 1 ten million billionth of a lightyear.
Whatever that means....
Not IUPAC. SI ? Who sets international standards for physical constants ? Whoever they are, how do they define a meter ?
It used to be a metal bar or rod or something like that, it was the official meter. Meter with a capital M. They had it in Europe I think. They almost completely assuredly still do have it, and whoever these people are, changed their definition of a meter from that piece of metal, that would never change in length, to something else, having to do with light, and I think it just might be as simple as saying that a meter is now the distance light travels in a vacuum for 1 three hundred millionth of a second. So the meter is also a measurement of time.
And that means that Science with a capital S authoritatively defines the Meter with a capital M to be something like 1 ten million billionth of a lightyear.
Whatever that means....
The poster is correct the light that reaches our eyes from a star, other than our own sun, is "old" light. The nearest other star is 4 plus light years away, so the light we see it by is 4 plus years old.
The distance to the star is knowable with great exactitude. It is measured by using trigonometry. The same way you would measure the height of a mountain, without climbing the mountain. A sighting is taken of the star and its angle from the horizon is carefully noted. Several months later another sighting is taken and the angle noted. You know how far the Earth has traveled between the first sighting and the second and you know the angle at which the star appeared to be at each sighting. With the rules of trigonometry, if you know the length of one side of a triangle and you know the angles at the of the other two sides, you can calculate the legnths of the other sides. There is nothing "made up" or "guessed at" in this process.
The distance to the star is knowable with great exactitude. It is measured by using trigonometry. The same way you would measure the height of a mountain, without climbing the mountain. A sighting is taken of the star and its angle from the horizon is carefully noted. Several months later another sighting is taken and the angle noted. You know how far the Earth has traveled between the first sighting and the second and you know the angle at which the star appeared to be at each sighting. With the rules of trigonometry, if you know the length of one side of a triangle and you know the angles at the of the other two sides, you can calculate the legnths of the other sides. There is nothing "made up" or "guessed at" in this process.
It is really very simple. We use a laser with a millisecond pulse length bounced off a retroreflector array left on the surface of the moon by the Apollo landers.
The distance to the Moon is calculated approximately using this equation:
Distance = (Speed of light × Time taken for light to reflect) / 2. Since the time as measured is approximately 2.564890 seconds the equation then becomes 186 282.397 (speed of light in miles per second) times 2.564890= 477794/2 = 238,897 miles. By repeated observations over time we now also know that the moon is spiraling away from Earth at the rate of approximately 3.8 centimeters per year. That means that in 42,351 years the moon will be a mile further away from Earth if the rate remains constant.
There is some discussion of wether we can measure in light years or if we must first measure in miles or kilometers and then translate that to light years. This ia a silly thing to argue over. We certainly can use a light year as a unit of measurement in astronomy. The Earth changes position about 16 light minutes between january and june. If you are taking your sightings on the star 6 months appart 16 light minutes is the length of one side of the triangle. The math will work fine either way.