one for the maths gurus

[proflight2000]

if the speed of light is 180,000 miles per second, how do I do a maths equation for how long it takes light from the sun to reach the earth from 93,000,000 miles. Also if something is 25,000ly from earth, how long does it actually take the light to reach us. cheers

[Akyra]

A light-year is a unit of distance. It is the distance that light can travel in one year. Light moves at a velocity of about 300,000 kilometers (km) each second. So in one year, it can travel about 10 trillion km. More p recisely, one light-year is equal to 9,500,000,000,000 kilometers.

Why would you want such a big unit of distance? Well, on Earth, a kilometer may be just fine. It is a few hundred kilometers from New York City to Washington, DC; it is a few thousand kilometers from California to Maine. In the Universe, the kilometer is just too small to be useful. For example, the distance to the next nearest big galaxy, the Andromeda Galaxy, is 21 quintillion km. That's 21,000,000,000,000,000,000 km. This is a number so large that it becomes hard to write and hard to interpret. So astronomers use other units of distance.

[proflight2000]

gawd

[Astronut]

Divide distance (93,000,000 miles) by speed of light (approx. 186,000 miles per second) gives number of seconds for light to travel that distance.

It's roughly 8.5 minutes so you could say the sun is at a distance of 8.5 light minutes.

[Cloudgazer]

Using the numbers provided it takes approximately 516.67 seconds for light from the sun to reach the Earth.

Presumeably if something is 25,000ly from Earth then the light would take 25,000 years to reach the Earth (what with light traveling at the speed of light).

Scott

[proflight2000]

thanks guys, its the ly thing thats confusing me, if something is 200ly away then to my small brain that does not seem a lot, its the maths of knowing in distance what something like 200ly is in miles. As the speed of light being done as 180,000MPS does that mean then x60 then x60 then x24 then x365 to get a distance.

[Cloudgazer]

Sounds about right to me..

Scott

[Jamie]

As the speed of light being done as 180,000MPS does that mean then x60 then x60 then x24 then x365 to get a distance.

And thats precisely why they measure some astronomical distances in light years.

[Astronut]

.....As the speed of light being done as 180,000MPS does that mean then x60 then x60 then x24 then x365 to get a distance.

Yes. 1 LY = approx 5,879,000,000,000 miles. It's a big number to get your head around and is not a useful way of measuring astronomical distances.

By definition of the speed of light as 186,000 miles per second (it's actually a little higher than that but not much), a 'light second' is 186,000 miles. Multiply by 60 to give the distance in miles of a 'light minute'. Multiply that by 60 again to give the distance in miles of a 'light hour'. Multiply that by 24 to give the distance in miles of a 'light day'. Finally multiply that by 365 (or is it 365.25?) to give the distance in miles of a 'light year'.

I suppose you could go a bit further and use 'light centuries' or 'light milleniums' for the unimaginably immense distances to, say, those quasars at the edge of the observable universe

13 billion light years? Why not say 13 million light milleniums? It's still a blumming big number to get your head around

[GazOC]

As the speed of light being done as 180,000MPS does that mean then x60 then x60 then x24 then x365 to get a distance.

And thats precisely why they measure some astronomical distances in light years.

yep, its just best to totally forget about miles and kilometers where astronomy is concerned. The numbers are so big they are meaningless to us.

[Whippy]

Also, the distance between the Sun and the Earth is measured as 1 Astronomical Unit (AU). That comes up in a few texts too.

[tenbyfifty]

thanks guys, its the ly thing thats confusing me, if something is 200ly away then to my small brain that does not seem a lot, its the maths of knowing in distance what something like 200ly is in miles. As the speed of light being done as 180,000MPS does that mean then x60 then x60 then x24 then x365 to get a distance.

I worked out recently that a light year is roughly the equivalent of going back and for to Saturn

3000 times!!!

[GordonCopestake]

Dont forget parsecs too A much more sensible way of looking at things!

[Cloudwatcher]

The mind boggles! Several times!!

If the speed of light is an universal constant what is the effect of gravitational lensing?

If light is 'bent' by gravitational forces rather than travelling in the usual straight line

it must take longer to reach us,right? So,does this mean we are seeing further back

when the light has been 'bent'? How would you know which is bent and which is straight?

I wish I hadn't started this!

[tenbyfifty]

yep, its just best to totally forget about miles and kilometers where astronomy is concerned. The numbers are so big they are meaningless to us.

Unless you are J K Rowling in which case the numbers are strangely similar to the money

you've earnt from Harry Pothead or whatever his name is

[Whippy]

The mind boggles! Several times!!

If the speed of light is an universal constant what is the effect of gravitational lensing?

If light is 'bent' by gravitational forces rather than travelling in the usual straight line

it must take longer to reach us,right? So,does this mean we are seeing further back

when the light has been 'bent'? How would you know which is bent and which is straight?

I wish I hadn't started this!

Oh come on now! Go and look it up at Wikipedia or something....

[Kaptain Klevtsov]

The mind boggles! Several times!!

If the speed of light is an universal constant what is the effect of gravitational lensing?

If light is 'bent' by gravitational forces rather than travelling in the usual straight line

it must take longer to reach us,right? So,does this mean we are seeing further back

when the light has been 'bent'? How would you know which is bent and which is straight?

I wish I hadn't started this!

The light travels in a straight line, but space is warped by gravity to accomodate this.

Captain Chaos

[GordonCopestake]

Dont forget also that the universe is still expanding (a la hubble constant) at 77 km/s/megaparsec so the time taken for each photon is extended from the estimated travel time (the distance increases during the transit)

[Cloudwatcher]

Aha. I had forgotten about Red Shift.

Lister,Rimmer,Kryten and Cat et.al.

[themos]

The most mind-boggling fact about light I learned during 7 years of physics training in the 80s is this: A photon experiences no time. You may have heard of time-dilation, a relativistic effect. Light takes this to its ultimate conclusion, time is dilated by a factor of infinity. If every photon could carry a clock with it, they'd all be completely frozen, showing the time of creation of the photon which is the same as the time of annihilation.

[legion48]

As the speed of light being done as 180,000MPS does that mean then x60 then x60 then x24 then x365 to get a distance.

And thats precisely why they measure some astronomical distances in light years.

yep, its just best to totally forget about miles and kilometers where astronomy is concerned. The numbers are so big they are meaningless to us.

So setting up on my shed roof doesn't mean I can drop a barlow size? Now you tell me!

:D

[GazOC]

:lol:Standing on tip toes whilst using binoculars is also of limited value.....

[Kaptain Klevtsov]

Dont forget also that the universe is still expanding (a la hubble constant) at 77 km/s/megaparsec so the time taken for each photon is extended from the estimated travel time (the distance increases during the transit)

Surely once its set off, any movement of whats behind it is irrelevant? Or if not, how does it know? Also it sets off towards us (or in another direction, but we wouldn't see it) at the speed of light irrespective of how fast whatever it sets off from is going and in which direction.

Granted it looks a different colour when it gets here, but that doesn't affect the speed.

Captain Chaos

[GazOC]

Gordon only mentioned the distance and the time taken, not the speed.

[GordonCopestake]

The speed of light in a vacuum is a constant, however the time it takes isn't.... mucho wierd einstein stuff confirms that