What is light year: Definition and 21 Discussions

A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (9.46×1012 km), or 5.88 trillion miles (5.88×1012 mi). As defined by the International Astronomical Union (IAU), a light-year is the distance that light travels in a vacuum in one Julian year (365.25 days). Because it includes the time-measurement word "year", the term light-year is sometimes misinterpreted as a unit of time.The light-year is most often used when expressing distances to stars and other distances on a galactic scale, especially in non-specialist contexts and popular science publications. The unit most commonly used in professional astronomy is the parsec (symbol: pc, about 3.26 light-years) which derives from astrometry; it is the distance at which one astronomical unit subtends an angle of one second of arc.

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  1. K

    B Exploring the Standardization of One Light Year Distance | PF Forum

    Dear PF Forum, It's been a while since I logged in here. And I really do appreciate all the answers that I've been getting here. Now, I wonder. Is there any standardization for 1 light year distance? Is it 10 trillion kilometers, or 299,792,458 * 60 * 60 * 24 * 365.256 = ...
  2. Simon Peach

    Space and time -- Is the term "light year" really correct?

    When something is described as say 7 billion light years away would't it be more accurate to 7 billoin years ago?
  3. S

    I How to calculate the wavelength of light coming from 1 light year away

    How to calculate the wavelength of light coming from 1 light year away. Is there any formula for it. Please help.
  4. H

    B Debate about how a light year can be used

    If I say that a star will die after 50 million light years, can I also say that that after 50 million EARTH years, the star will die BECAUSE DISTANCE AND TIME ARE INTER-CHANGABLE (if a airplane travels from point A to point B and the distance is 200 miles, I can also say that the distance is...
  5. B

    1 light year of mirror reflections?

    Just a thought...I know it is impossible, but... What would you see in a mirror if you set up millions of mirrors, each reflected to the next mirror (each not too far apart), maybe in a spiral format, so that the total distance of reflections was one light year. The last mirror was angled to...
  6. B

    How Can We Measure the Speed of Light Over a Long Distance?

    The main question stems from the answer for this one: If I have a light year long rod and push it on one end, how long would it take for the other end to move? Still Unsure of this answer. Main Question: If I had 2, light-year long conductive rods with a voltmeter attached at the ends then...
  7. I

    Light year long stick question

    If I had a light year long stick that was incredibly strong, made of diamond for example and held it out at arms length, when I let go of the stick, the end closest to me would start to fall to the ground but 2million+ miles down the stick would still be completely level right? Because those...
  8. rrw4rusty

    All stars in Milky Way equal one square light year?

    Hello, Can anyone confirm or, refute and correct the following statements? The volume of our galaxy, is roughly 8 trillion cubic light years. The combined volume of all the stars in our galaxy only equals one cubic light year. Thanks for any help you can provide! Rusty
  9. M

    Question between friends - How long to travel 1 light year?

    Question between friends! -- How long to travel 1 light year? Homework Statement Me and a few friends were trying to work out how long it would take a human to travel one light year. We were using the these variables. Distance = 9.4605284 × 1015 m Fastest speed for manned space...
  10. K

    Why I doubt the generality of Gauss' law: A Gaussian sphere 1 light year across

    Let's say I have a Gaussian sphere 1 light year across with synchronized clocks and sensors all over its surface. All clocks are co-moving, not accelerating, and the spatial curvature is negligible. If I have only one charge inside the Gaussian sphere, 1 centimeter from its surface for an entire...
  11. S

    The speed of light, the term light year , and reference frames.

    The speed of light, the term "light year", and reference frames. Hi everyone. This is my first post, and I post out of desperation. A friend of mine and I were casually discussing Time Dilation, interstellar travel, etc. when we came to a point we fundamentally disagreed upon. Neither of us...
  12. O

    Light year, explanation required?

    I am a lay man but having lot of interest in understanding about out space & universe. In connection to that, I need a explanation for my confusion regarding light years, which is denoted for mentioning the distance of an object located far away from the earth. Let say a star located a light...
  13. S

    Anomalous Quasar Observations 200k Light Year Jet? And So On.

    If Astronomers stated that they had using the Hubble telescope found a remote island on the Earth with 1000 meter tall Homo sapiens we would based on scientific knowledge and logic concerning the biological limits of animal structure (any land based animal that moves on the surface of the earth)...
  14. M

    Alpha centuri .4 light year relative to who?

    Alpha centuri...4 light year relative to who? Hi, Here it goes. Suppose it takes 4 light years to get to alpha centuri. So if I could travel at 99.99999% the speed of light and the people on Earth remains stationary(as in moving very slowly which is << C). So does my clock shows 4 years...
  15. H

    Where is a light year inside the earth?

    Raymond explains in his radically modern physics text on page http://physics.nmt.edu/~raymond/classes/ph13xbook/node61.html that a constantly accelerated observer (acceleration a) never increases his distance from the point x_0-c^2/a where x_0 is the point where he started his acceleration...
  16. R

    Light Year Question: When Will We See It?

    Homework Statement If a star explodes 30 light years from the Earth when will we be able to see it? Homework Equations one light year = 9.47 x 1012km speed of light = 3.00 x 108m/s The Attempt at a Solution the answer in my workbook says 30a but idk what the value of a is
  17. P

    Mystery of the "Cosmic Blank Spot": A Billion Light Year Void

    Recently, a rather large nothing was discovered in the Universe (last August by the University of Minnesota team). This cosmic blank spot has no stray stars, no black holes, not even dark matter, and is a billion light years across. Is there a possibility that this could be the result of a...
  18. D

    Circuit with superconducting wire 1 light year in length

    Question regarding the propogation of the e-field.. would it propogate at nearly the speed of light? If a lightbulb were put on a circuit, with half a light year of cable coming from each terminal on the bulb and connected to a sufficient battery/power supply, 1/2 a year until the bulb...
  19. B

    How long would it take to move a pole one light year long one foot?

    A pole one light year long... laying in bed last night I had this question pop into my pointy little head. If I had a pole one light year long sitting between points A and B and I pushed it forward one foot from point A toward point B how long would it take to move at point B? I've...
  20. W

    The light year long wall

    Ok I have had a course is general relativity, but not on special. I was wondering something though... If you have a brick wall a light year long (when at rest) and then have it thrusted at you at a speed such that it is only 1ft long, i.e. .99999c, when would you be able to stand in front of...
  21. L

    What Is a Light Year? Understanding the Value of 1 Light Year

    what is the value of 1 light year