How are light-years measured?

Jacob

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Velocities tend to hold the key yo understanding the universe far more than distance.
That a velocity exists at all speaks to an object moving relative to another object (or to the space "the moving object" is in); an object that is not stationary has a velocity.
 

gcthomas

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If you have no length between the sun and the earth, do you have the diameter of either (not to mention that of Venus and the other planets) to begin with? Do you know, prior to the measurement mentioned in this article, that the orbit of the earth around the sun is a circle rather than an oval or something in-between? Or, would you need to know?

Only the angle is measured, if you know the size of the earth. The distance to the sun is then calculated from the angle.

The orbit must be approx circular because the seasons are about the same length.
 
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CabinetMaker

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That a velocity exists at all speaks to an object moving relative to another object (or to the space "the moving object" is in); an object that is not stationary has a velocity.

Yes. But remember that velocity is speed and a direction. How fast something is moving and in what direction gives a lot of information about what is going on in the universe.
 

Jacob

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Only the angle is measured, if you know the size of the earth. The distance to the sun is then calculated from the angle.
What was the task? Finding the length of time for the traversal of Venus across the surface of the sun somehow helps us to determine an angle. I do not know the size of the earth or how to determine it. But if the earth is a sphere (it basically is, though maybe not exactly) then there may be a task of determining the angle from more than one point on its surface. So here if we have two locations on the earth from which the observation is made, we have two different angle measurements at least.
The orbit must be approx circular because the seasons are about the same length.
Of course this is an assumption.
 

Jacob

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Yes. But remember that velocity is speed and a direction. How fast something is moving and in what direction gives a lot of information about what is going on in the universe.
You are talking about a velocity vector. Is this how you determine the difference between a velocity and a speed?

An orbit (I have heard there is an escape velocity but can't remember how it works) may have a direction or velocity, but in what sense is it a vector? Is it only a vector perpendicular relative to the distance from the earth assuming this remains the same? Or would we say the vector is constantly changing, perhaps in an ideal situation at an even rate relative to some point. I think of the object's orbit path.
 

PureX

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Hmmm, … how did we measure the speed of light? I mean, we measured it against what, exactly?
 

Jedidiah

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Hmmm, … how did we measure the speed of light? I mean, we measured it against what, exactly?
That was the exact question that led Einstein to Relativity because the only thing resembling a static or neutral frame of reference from which to make measurements is the speed of light itself, which is always measured to be the same magnitude in every direction.

So, good question. :thumb:
 

OCTOBER23

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Untillectual,

Did you get that long long stick that I told you to get

to measure the distance to the Light above your head ?
 

OCTOBER23

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It is because you use words like assumption and yet you ask a simple question
that you should have Googled long ago.

Would you like to talk to Einstein about the Speed of light or how it bends around a sphere ?

Google it.

Da 12:4 But thou, O Daniel, shut up the words, and seal the book,

even to the time of the end:

many shall run to and fro, and knowledge shall be increased.
 

Jacob

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It is because you use words like assumption and yet you ask a simple question
that you should have Googled long ago.

Would you like to talk to Einstein about the Speed of light or how it bends around a sphere ?

Google it.

Da 12:4 But thou, O Daniel, shut up the words, and seal the book,

even to the time of the end:

many shall run to and fro, and knowledge shall be increased.
It's not about a lot of knowledge. It's about following the train of thought and understanding the history of how we have the values (for example, for constants like the speed of light) and how we have chosen to measure distance in terms of light years if that is even possible.

But though I don't see the connection to Daniel, I do believe you put forth an interesting inquiry in line with the types of things we have discussed in this thread so far.
 

Stripe

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I am interested to know the answer to this question. It seems to me that something that is for example a light-year away, if that could be measured, would only appear to us as it did a year ago and not as it is now. If we cannot observe it as it is now, how do we know how far away it is?

I think your question boils down to: How is the speed of light determined?

There are some old-fashioned methods that could work quite well to look at how fast light travels, but all of them have been set aside in favor of the assumption that lightspeed is a constant (called "c").

This led to the calculation of a light year, which is a useful tool for working within the field of astronomy under the assumption of c.
 

Stripe

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There are some old-fashioned methods that could work quite well to look at how fast light travels.
The first successful entirely earthbound measurement of the speed of light was carried out by Hippolyte Fizeau in 1849. Fizeau's experiment was conceptually similar to those proposed by Beeckman and Galileo. A beam of light was directed at a mirror 8 km away. On the way from the source to the mirror, the beam passed through a rotating cog wheel. At a certain rate of rotation, the beam could pass through one gap on the way out and another on the way back. But at slightly higher or lower rates, the beam would strike a tooth and not pass through the wheel. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, the speed of light could be calculated. Fizeau reported the speed of light as 313,000 kilometres per second. Leon Foucault improved on Fizeau's method by replacing the cogwheel with a rotating mirror. Foucault's estimate, published in 1862, was 298,000 kilometres per second.
 
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