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# What is a Light Year?

OCCOM BILL

1
Thu 27 Aug, 2009 09:46 pm
@ebrown p,
ebrown p wrote:
The sound travels a little more than 10 miles per second... it gets their much more slowly.
Do you mean about 12 miles per minute? I think you do. About 5 seconds per mile is about right.

I think the standard answer is about 761 miles per hour... but it is far from constant. On a hot day it's faster, and on a cold day, it's slower. It slows down quite a bit with altitude, too, but mach 1 remains the speed limit.

For all practical purposes, the speed of light may as well be considered a constant... but the things that effect the speed of sound are calculated constantly by people in the business of flying... and especially those in the business of flying close to the speed of sound.
0 Replies

Setanta

1
Fri 28 Aug, 2009 02:32 am
The speed of sound in the atmosphere at sea level is roughly 1100 feet per second, so Bill is correct that this is about one mile in five seconds. For the distance measurement challenged, that's about one kilometer in three seconds.
gungasnake

1
Thu 8 Jul, 2010 12:10 pm
@Dorothy Parker,
Missed this one when it was posted...

The idea of the distance light travels in a year doesn't give you much of a feeling for the distance in question. A bit better to my thinking is this:

A light year is a reasonable measure for the distances between stars and galaxies, whereas miles or kilometers aren't.

If you scale our own system to about a yard in diameter i.e. use a scale for which Pluto's orbit is about a yard in diameter, then our sun is about the diameter of the width of a human hair, Earth is an inch or two out from the sun, and the nearest star, the Alpha Centauri triple system at about four light years distance in real life, is about four miles off.

BillRM

1
Thu 8 Jul, 2010 02:47 pm
@gungasnake,
Gungassnake you can get a feeling for a light year by the following the moon is roughly 1.3 sec light distance , the sun is 8 light minutes from the earth and the outer planets are 4 to 9 light hours out from the sun.
0 Replies

High Seas

1
Mon 20 Sep, 2010 09:11 am
@gungasnake,
gungasnake wrote:
.....
A light year is a reasonable measure for the distances between stars and galaxies....

That sounds reasonable for the immediate vicinity calculations, but doesn't compute exactly for the outer edges of the universe. I don't know if that's because photons near the edge are in some lower energy state or because of some other reason, but alpha seems to have changed since the big bang:
Quote:
.....the fine-structure constant is actually a compound of several other physical constants....
..start with the square of an electron’s charge, divide it by the speed of light and Planck’s constant, then multiply the whole lot by two pi. The point of this convoluted procedure is that this combination of multiplication and division produces a pure, dimensionless number. ...the result is 1/137.036

For alpha to be getting larger near the edge - assuming the calculations hold up, of course - it must be that the speed of light has increased since the beginning; pi can't change, neither can Planck's constant, and the electron's charge isn't likely to change as long as we're still in the same universe.

http://www.economist.com/node/16941123?story_id=16941123&CFID=143595100&CFTOKEN=62911885
0 Replies

High Seas

1
Mon 20 Sep, 2010 01:54 pm
@Setanta,
This is the most poetic explanation of alpha I could locate; the writer is the late Prof. Richard Feynman in his book QED:
Quote:
There is a most profound and beautiful question associated with the observed coupling constant, e the amplitude for a real electron to emit or absorb a real photon. It is a simple number that has been experimentally determined to be close to 0.08542455. (My physicist friends won't recognize this number, because they like to remember it as the inverse of its square: about 137.03597 with about an uncertainty of about 2 in the last decimal place. It has been a mystery ever since it was discovered more than fifty years ago, and all good theoretical physicists put this number up on their wall and worry about it.) Immediately you would like to know where this number for a coupling comes from: is it related to pi or perhaps to the base of natural logarithms? Nobody knows. It's one of the greatest damn mysteries of physics: a magic number that comes to us with no understanding by man. You might say the "hand of God" wrote that number, and "we don't know how He pushed his pencil." We know what kind of a dance to do experimentally to measure this number very accurately, but we don't know what kind of dance to do on the computer to make this number come out, without putting it in secretly!

0 Replies

HexHammer

1
Sat 2 Oct, 2010 01:13 am
@Dorothy Parker,
exactly 9,460,730,472,580.8 km (about 9.5 Pm)

http://en.wikipedia.org/wiki/Light-year

Dorothy Parker

1
Wed 6 Oct, 2010 07:26 am
@HexHammer,
thanks HexHammer.

I find that sometimes googling stuff throws up too much information and I don't know where to start with it, whereas if I ask questions on A2K, another user will answer in a simple way that I understand. And sometimes it's someone I am familiar with and who's opinion I respect and know I can trust.
blueveinedthrobber

1
Wed 6 Oct, 2010 09:38 am
about 7 to 36 pounds or 10.00 same as in town
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HexHammer

1
Wed 6 Oct, 2010 11:20 am
@Dorothy Parker,
Dorothy Parker wrote:

thanks HexHammer.

I find that sometimes googling stuff throws up too much information and I don't know where to start with it, whereas if I ask questions on A2K, another user will answer in a simple way that I understand. And sometimes it's someone I am familiar with and who's opinion I respect and know I can trust.
Very well.
0 Replies

snj

1
Sat 29 Jun, 2013 09:58 pm
@Dorothy Parker,
The distance travelled by light in a year is called a light year. 1 light year =9.4605284 × 1015 meters . https://en.wikipedia.org/wiki/Light-year
McTag

1
Sun 30 Jun, 2013 12:19 am
@snj,

Well how informative. Except for the spelling.
0 Replies

stefen

1
Sun 30 Jun, 2013 10:15 am
@Dorothy Parker,
Light year is an astronomical unit of distance traveled by light in 1 year.
9.46 * 10^15 meters

Dorothy Parker

1
Fri 30 Aug, 2013 03:53 pm
@stefen,
Alright science!
0 Replies

Banana Breath

2
Tue 30 Dec, 2014 02:14 pm
A Lightyear? It's a Buzz.
0 Replies

carloslebaron

2
Mon 5 Jan, 2015 12:22 am
A Light Year is a hypothetical measurement.

It assumes a "constant" speed of light in vacuum at 300,000 km per second multiplied by 60, and by 60 and by 24 and by 365 in order to obtain a super huge distance.

This Light Year measurement is good for playing numbers in physics, but, as I have stated before, the traveling of light in space is not "constant" because the outer space doesn't enjoy of "perfect vacuum" (you can consult about this fact with any expert of NASA), and this measurement becomes hypothetical.

0 Replies

33export

1
Sun 10 Jan, 2016 07:46 pm
A downturn in end -of-year net profit. Seasonally adjusted, of course.
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Glennn

1
Sun 10 Jan, 2016 07:53 pm
A light-year is the distance light will travel in a year at the rate of 186,000 miles per second highway; 150,000 miles per second city.
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raygmurphy

0
Tue 22 Mar, 2016 03:53 am
@Dorothy Parker,
Defination=>
A light-year, also light year or lightyear (symbol: ly) is a unit of length, equal to just under 10 trillion kilometres (10×1015 metres, 10 petametres or about 6 trillion miles). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year.

USE=>
The light-year is often used to measure distances to stars and other distances on a galactic scale, especially in non-specialist and popular science publications. The preferred unit in astrometry is the parsec, because it can be more easily derived from, and compared with, observational data. The parsec is defined as the distance at which an object will appear to move one arcsecond of parallax when the observer moves one astronomical unit perpendicular to the line of sight to the observer, and is equal to approximately 3.26 light-years.
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