How are light-years measured?

[gcthomas]

Would you differentiate between the light of a lazer and the light of a distant star here?

No, I wouldn't.

Laser light spreads out by the same principle (inverse square law) - it is just that the light that comes from the business end of as laser has already bounced back and forth many times in the device, so it doesn't appear to spread out as if from a point source if you only look at the beam spread for a few metres. When the distances are large compared to the internal path lengths than the inverse square law becomes apparent.

If you shone a 1mm diameter laser beam at the Moon, for example, it will have diverged to approx 200 km across when it arrives. The inverse square law applies even to lasers, then, for large distances.

 

[Eeset]

If you shone a 1mm diameter laser beam at the Moon, for example, it will have diverged to approx 200 km across when it arrives. The inverse square law applies even to lasers, then, for large distances.

Your spread is far too wide. This from Wikipedia ....

At the Moon's surface, the beam is about 6.5 kilometers (four miles) widehttp://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment#cite_note-ApolloLaser-6. The reflected light is too weak to be seen with the human eye: out of 1017 photons aimed at the reflector, only one will be received back on Earth every few seconds, even under good conditions. They can be identified as originating from the laser because the laser is highly monochromatic. This is one of the most precise distance measurements ever made, and is equivalent in accuracy to determining the distance between Los Angeles and New York to 0.25 mm (0.01 in).

 

[CabinetMaker]

Okay. I still can't tell how many light years away a distant galaxy is by looking at it.

Technically, that is incorrect. The only way we can tell how far away something is is by looking at it. By observing the light that arrives here, and looking for specific indicators and doing some math, we can estimate the distances.

 

[CabinetMaker]

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....

The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some suggested defining the meter as the length of a pendulum having a half-period of one second; others suggested defining the meter as one ten-millionth of the length of the earth's meridian along a quadrant (one fourth the circumference of the earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the earth, affecting the period of the pendulum.

The metal meter rod was a standard based on the above definition. And a metal rod will change length based on temperature.

 

[Stripe]

If you shone a 1mm diameter laser beam at the Moon, for example, it will have diverged to approx 200 km across when it arrives. The inverse square law applies even to lasers, then, for large distances.

I did a presentation at NTNU on the setup they have on the moon and how they detect returning photons -- like about one every other second compared with how many were sent out. Brilliant stuff, and intriguing how they can tell one photon apart from all the noise.

 

[serpentdove]

...What about the speed of light, how fast it travels?

It's God's speed limit.

Light is fascinating (Gen. 1:3–5).

See:

Let there be Light (right click, open) by Daryl Ferguson

 

[Jacob]

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.

If this is true at all I have heard it is only true for a near star or should I say a star that is near to us.

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.

This is at least different than different sides of the earth/globe.

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.

Of course I have not seen this done. But I do know that the furthest distance as you put it hear would be from one side of the earth's orbit to the other, though I don't know what the angles to a given star would be.

 

[Jacob]

The length of a meter per wikipedia: "the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second."

Google is our friend.

I wonder how this was determined.

 

[Jacob]

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.

I'm not sure of these figures. First off I don't know if we need a reflector or not, per this thread we are posting on. Then I don't know how to determine if the moon is spiraling away. Is our orbit around the sun a perfect circle?

 

[Jacob]

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.

A unit of measure and a unit of measurement are two different things.

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.

I'm not sure of your point, but I have nothing to challenge it with/by at this time. I don't know if I will revisit this post or not. Thanks for posting.

What is a light minute? Similar to a light year I presume, in the grammar of it.

 

[Jacob]

No, I wouldn't.

Laser light spreads out by the same principle (inverse square law) - it is just that the light that comes from the business end of as laser has already bounced back and forth many times in the device, so it doesn't appear to spread out as if from a point source if you only look at the beam spread for a few metres. When the distances are large compared to the internal path lengths than the inverse square law becomes apparent.

If you shone a 1mm diameter laser beam at the Moon, for example, it will have diverged to approx 200 km across when it arrives. The inverse square law applies even to lasers, then, for large distances.

The light of a laser might be "more concentrated"? But the inverse square law you are talking about would be unaffected?

 

[Jacob]

Your spread is far too wide. This from Wikipedia ....

At the Moon's surface, the beam is about 6.5 kilometers (four miles) widehttp://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment#cite_note-ApolloLaser-6. The reflected light is too weak to be seen with the human eye: out of 1017 photons aimed at the reflector, only one will be received back on Earth every few seconds, even under good conditions. They can be identified as originating from the laser because the laser is highly monochromatic. This is one of the most precise distance measurements ever made, and is equivalent in accuracy to determining the distance between Los Angeles and New York to 0.25 mm (0.01 in).

Interesting claim.

I don't have a very good grasp of what a photon is. I don't really have an understanding of what a photon is at all.

 

[Jacob]

Technically, that is incorrect. The only way we can tell how far away something is is by looking at it. By observing the light that arrives here, and looking for specific indicators and doing some math, we can estimate the distances.

We use different words, such as brightness or luminosity, or perhaps faintness, to describe what we see.

 

[Jacob]

The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some suggested defining the meter as the length of a pendulum having a half-period of one second; others suggested defining the meter as one ten-millionth of the length of the earth's meridian along a quadrant (one fourth the circumference of the earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the earth, affecting the period of the pendulum.

The metal meter rod was a standard based on the above definition. And a metal rod will change length based on temperature.

Good discussion.

 

[Jacob]

I did a presentation at NTNU on the setup they have on the moon and how they detect returning photons -- like about one every other second compared with how many were sent out. Brilliant stuff, and intriguing how they can tell one photon apart from all the noise.

Difficult. Was this having anything to do with the size of the reflector vs. how much area the laser covered? I don't know for sure that this has been done.

 

[Jacob]

It's God's speed limit.

Light is fascinating (Gen. 1:3–5).

See:

Let there be Light (right click, open) by Daryl Ferguson

There is feedback in this sound after clicking on the link.

 

[Stripe]

Difficult. Was this having anything to do with the size of the reflector vs. how much area the laser covered? I don't know for sure that this has been done.

Like GC said, if you wanted to reflect all of the photons, you'd need a 200km-wide mirror on the moon. The ones they have are about the size of a shoebox.

 

[Jacob]

Like GC said, if you wanted to reflect all of the photons, you'd need a 200km-wide mirror on the moon. The ones they have are about the size of a shoebox.

Again, I don't even know what a photon is.

 

[Stripe]

Again, I don't even know what a photon is.

:thumb:

 

[Jedidiah]

The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some suggested defining the meter as the length of a pendulum having a half-period of one second; others suggested defining the meter as one ten-millionth of the length of the earth's meridian along a quadrant (one fourth the circumference of the earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the earth, affecting the period of the pendulum.

The metal meter rod was a standard based on the above definition. And a metal rod will change length based on temperature.

An AU happens to be 500 light-seconds (at parts-per-thousand precision -- i.e., 0.10-0.90%). The average diameter of the earth's orbit is 1 thousand light-seconds long. This doesn't seem contrived to anybody else ?

:crackup: