Speed of light and distance...noobie question

[Mr Dobby]

Today I thought I'd just do a bit of self education, I'm aware that the speed of light is 186,000 miles per second, and a light year is the distance light travels in a year (earth year?, which is a tad faster than the missus in her fiesta going to the sales)

I googled it today and the first response was from good old Wikipedia, I started off quite well and then got seriously lost, sadly its all above me, and I'm struggling. So the question I was looking for an answer too is how can it be determined that a star or other celestial body is so many light years away, i.e. according to stellarium Spica is 262.18 light years away, how do they know that?, any replies in easy laymens terms please if possible.

Phil

[dark knight]

Hope this helps

HowStuffWorks "What is a light year?"

[fizzer]

Try looking up Stellar parallax on wiki this should explain the basic but very confusing principle of how it is done.

Good luck

Richard

[Glider]

Nothing to do with your question but just remember, the speed of light in a vacuum is defined as a constant, in anything else it's not, otherwise prisms and lenses don't work, that's why I bought a reflector!!!!

[Astro_Baby]

This may help Finding Stellar Distances

[Mr Dobby]

Many thanks to you all for helping me with this one, I've had a good read so far, but will keep going over it too see how much the little grey cells will take in. I think I'm qualified enough now just to say it's big stuff, but know doubt probably very basic for someone who really knows?.

So yesterday I was poleaxed today I can at least say I'm about a light year ahead of where I was, and now pleasantly parrallaxed:icon_eek:.

The other thing that prompted me to ask the question was due to recently watching the Carl Sagan "Cosmos" on DVD. I doubt if I will quote him exactly so please forgive, I found it fascinating when he states that if I was a passenger on a craft travelling at the speed of light or close to it, time slows down, and in 56 years of my life on that craft I would likely visit much or all of the near universe, but by the time i returned to earth, some 1-2000 years of earth time will have passed, just hope i didn't leave the toaster on it could be expensive.

cheers

Phil

[totalamateur]

Many thanks to you all for helping me with this one, I've had a good read so far, but will keep going over it too see how much the little grey cells will take in. I think I'm qualified enough now just to say it's big stuff, but know doubt probably very basic for someone who really knows?.

So yesterday I was poleaxed today I can at least say I'm about a light year ahead of where I was, and now pleasantly parrallaxed:icon_eek:.

The other thing that prompted me to ask the question was due to recently watching the Carl Sagan "Cosmos" on DVD. I doubt if I will quote him exactly so please forgive, I found it fascinating when he states that if I was a passenger on a craft travelling at the speed of light or close to it, time slows down, and in 56 years of my life on that craft I would likely visit much or all of the near universe, but by the time i returned to earth, some 1-2000 years of earth time will have passed, just hope i didn't leave the toaster on it could be expensive.

cheers

Phil

I know very little about this subject but I do know how the distance from the earth to the moon was calculated. They bounced a laser beam off it and because they knew how fast light travelled, it was easy to calculate the distance. Obviously this doesen't apply to stars because it would take 4.4 years before we got the answer from the nearest star outside our solar system (proxima cenuri), and tens of millions of years to get an answer from more distant stars, so the human race would be extinct before getting an answer.

For distant objects, I believe that they use something called the dopler effect. objects are observed from two different locations, preferably as far apart as possible, which allows them to calculate the distance. They probably do it from satalites nowdays.

You couldn't get more laymans terms as that.

[brantuk]

Hmmm.... I thought our nearest star was the sun - And the light from it takes 8.5 mins to travel 93 million miles to reach us

[Earl]

If I have this right, you can measure the distance a Star is from you by measureing the Red shift in its spectrum of colour, as Red travels slower the further the star is from you the greater this Red Shift.

Now i imagine that is a very basic version of the situation

[themos]

Cosmic distance ladder - Wikipedia, the free encyclopedia

[FraserClarke]

If I have this right, you can measure the distance a Star is from you by measureing the Red shift in its spectrum of colour, as Red travels slower the further the star is from you the greater this Red Shift.

Not really unfortunately. The redshift == distance approximation is valid on cosmological distances (distant galaxies) because the Universe is expanding. It's not that red light travels slower, it's that all light is 'stretched' due to the expansion and therefore looks redder (red light has a longer wavelength). The further away you are, the more stretching goes on, the redder you look...

On a galactic scale though, there is no relationship between distance and redshift.

The fundamental basis of the distance ladder is stellar parallax. A simple geometric measure of how far away something is. Parallax is very easy to demonstrate to yourself; Stick your index finger up at arms length in front of you. Now alternately close your right and left eyes. See how your right index finger appears to move relative to the background? That's parallax.

In astronomical terms, our "left" and "right" eyes are observations taken six months apart, when the Earth is at opposite sides of it's orbit. So that is a baseline of 2 astronomical units (~3x10^8 km). The nearest star (excluding the sun ) is ~4 light years away (~4x10^13 km). So, you can do a little bit of trigonometry and work out what the parallax will be....

[acey]

The redshift tells you the speed at which something is moving away from you (like the Doppler shift of a siren). At cosmological scales there is a relationship between recession speed and distance, but for nearby objects it just says whether they're moving away from you or towards you (the Andromeda Galaxy has a blue-shift: it's getting closer).

[totalamateur]

Hmmm.... I thought our nearest star was the sun - And the light from it takes 8.5 mins to travel 93 million miles to reach us

Thanks for pointing it out, although i think you know what i meant.

[kniclander]

I read quite a good book which i think was called, "measuring the Universe" - it comes from a historical perspective so you learn how we did it when we had very little in the way of gadgets (when it was quite simple) to how we do it now. The problem is (for me cos I am slow...) when you jump straight in to the modern stuff, it is hard to grasp but when you can put it in context and understand how we got to our present stae of knowledge, it's a bit easier.

[pvaz]

I found it fascinating when he states that if I was a passenger on a craft travelling at the speed of light or close to it, time slows down, and in 56 years of my life on that craft I would likely visit much or all of the near universe, but by the time i returned to earth, some 1-2000 years of earth time will have passed, just hope i didn't leave the toaster on it could be expensive.

Phil

That's based on Einstein relativity. The basic idea is that time and space are one thing alone and you move through it. If you're standing still you only move in time, if you are moving you move in both space and time. Now the trick part is that the Theory says the subtraction of the squares (for time and space) is caped at a max value, so when you move you do it in X miles and those miles eat up on the time (remember the subtraction must result in the same value as when you ware standing still and you moved zero miles). Basically time passes slower cause you're also moving, the more you move the more it slows down. Though the kind of difference he talks about demand speeds very close to the light speed.

The GPS is the classical example, the system relies on this as the satellites are going at great speed in orbit, the internal clocks there tick slower (just a bit) so they had to make them run faster (with precise maths) now they seam to be ticking at the same speed as clocks on Earth. If you brought one of them back it would be running faster.

[George Jones]

The GPS is the classical example, the system relies on this as the satellites are going at great speed in orbit, the internal clocks there tick slower (just a bit) so they had to make them run faster (with precise maths) now they seam to be ticking at the same speed as clocks on Earth. If you brought one of them back it would be running faster.

Actually, for GPS, there are two effects that cause clocks on the satellites "to run at different rates" than clocks on Earth:

1) the relative speed between the satellites and the Earth (special relativity);

2) the Earth's gravitational field (general relativity).

Effects 1) and 2) have opposite signs, with the magnitude of 2) being about six times larger than the magnitude of 1). Effects 1) and 2) can be calculated separately, or all in ine go.

[brantuk]

Lol Ray - I was just being cheeky

[pvaz]

Actually, for GPS, there are two effects that cause clocks on the satellites "to run at different rates" than clocks on Earth:

1) the relative speed between the satellites and the Earth (special relativity);

2) the Earth's gravitational field (general relativity).

Effects 1) and 2) have opposite signs, with the magnitude of 2) being about six times larger than the magnitude of 1). Effects 1) and 2) can be calculated separately, or all in ine go.

I'm only through half of a book on relativity... now in the special relativity part.

I«ll be getting to the General Relativity later on, so you should have marked your comment with a spoiler alert .

[George Jones]

I hope you are enjoying the book. I have an ongoing affair with relativity, at least according to my wife. I'm sure the book will talk about gravitational redshift, which is the effect that affects clocks. I hope I haven't given too much of the plot away.

[pvaz]

No problem, I was joking.

it's very interesting, I'm loving it though some parts demand a 2nd read and I'll probably get to the end, digest it, and read it again.

[George Jones]

No problem, I was joking.

it's very interesting, I'm loving it though some parts demand a 2nd read and I'll probably get to the end, digest it, and read it again.

Careful, it can be addictive. More than thirty years after I first started reading about relativity, I'm still digesting.

[totalamateur]

Hmmm.... I thought our nearest star was the sun - And the light from it takes 8.5 mins to travel 93 million miles to reach us

I did state that proxima centuri was the closest star outside our solar system.

[totalamateur]

Lol Ray - I was just being cheeky

OK, No Problem, I really must start to get used to this banter, as I often take it as someone having a snipe. Remember, i'm a newby.