# The Light Years- when devised? **URL:** \ **Category:** Factual Questions\ **Created:** [May 8, 2003, 1:28am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805 "2003-05-08T01:28:18Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![Rhino](https://avatars.discourse-cdn.com/v4/letter/r/a5b964/32.png) [@Rhino](https://boards.straightdope.com/u/Rhino)\ **Post date:** [May 8, 2003, 1:28am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/1 "2003-05-08T01:28:18Z")
What astronomer(s) is credited with devising the light year. And how did he/they decide the distance of what ever source of light they were measuring.? ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [May 8, 2003, 1:56am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/2 "2003-05-08T01:56:23Z")
The speed of light was first measured in 1644, when it was noted that eclipses and transits of the Galilean satellites relative to Jupiter occurred 1000 seconds later when Jupiter was on the other side of the Sun from us than when we and it were on the same side. The astronomer in question, whose name I don’t recall, realized that this was the result of the position of the Earth, and that therefore light moved at 2 A.U. per thousand seconds, or 0.002 AU per second. Some time thereafter, the Earth-Sun distance was measured at 93 million miles on average, meaning that light moved at 186,000 miles per second – a figure remarkably close to the more precise measurement accepted today. At 60 seconds per minute, 60 minutes per hour, 24 hours per day, and 365.25 days per year, it’s easy to calculate from this how far it would travel in a year – which works out to 5,878, 612, 843, 200 miles – and this measurement was therefore called a light year. I’m not sure who came up with the idea of using that as a good distance for measuring stellar distances, but it was obvious once they thought of it. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 8, 2003, 1:57am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/3 "2003-05-08T01:57:32Z")
All you need to do to create a light year is to multiply the speed of light (about 300,000 kilometers/second) by the number of seconds in a year. There’s no need to measure anything. It’s just a convenient way to express large numbers. Much easier to say 1 light year than 10 trillion kilometers (6 trillion miles). ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [May 8, 2003, 2:05am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/4 "2003-05-08T02:05:52Z")
> [@](#): > > \*Originally posted by Exapno Mapcase \* > \*\*All you need to do to create a light year is to multiply the speed of light (about 300,000 kilometers/second) by the number of seconds in a year. > > There’s no need to measure anything. Ummm…what? So, the value of _c_ just magically appeared out of thin air? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 8, 2003, 2:17am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/5 "2003-05-08T02:17:52Z")
> [@](#): > > \*Originally posted by Q.E.D. \* > \*\*Ummm…what? So, the value of _c_ just magically appeared out of thin air? \*\* No more than the number of seconds in a year. But a light year did not come about because an astronomer was “measuring a source of light”. ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [May 8, 2003, 2:51am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/6 "2003-05-08T02:51:13Z")
Merriam-Webster shows it dating from 1888. However, it doesn’t say who coined it. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [May 8, 2003, 3:04am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/7 "2003-05-08T03:04:45Z")
That would be the Michaelson-Morley Experiment, **Earl**. But the earlier figure is documented, and was even reported in science popularizations (I think it was in one of the Gould essays on history of science, but I’m not sure). ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 8, 2003, 3:32am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/8 "2003-05-08T03:32:16Z")
This is from the OED Online: > [@](#): > > 1888 Athenæum 27 Oct. 558/2 The distances in \*light-years of the last two stars. 1890 C. A. YOUNG Elem. Astron. xii. §433 It is better, and now usual, to take as the unit of stellar distance the so-called ‘light year’; i.e. the distance light travels in a year, which is about 63,000 times the distance of the earth from the sun. **Polycarp** , I think you’re mixing up when the speed of light was measured with when the term light year came into general use. Michaelson had been measuring the speed of light for a [decade](http://hum.amu.edu.pl/~zbzw/ph/sci/aam.htm) before 1888 and had a very accurate figure years before the Michaelson-Morley experiment. And it was Ole Roemer who used Jupiter’s orbit to measure the speed of light, but in [1676](http://www.what-is-the-speed-of-light.com/roemer-speed-of-light.html). However, he was born in 1644 and his measurement was way off, not even close to 186,000 mps. Various sites give it as 120,000 or 140,000 mps. ---
**Author:** ![JasonFin](https://avatars.discourse-cdn.com/v4/letter/j/4af34b/32.png) [@JasonFin](https://boards.straightdope.com/u/JasonFin)\ **Post date:** [May 8, 2003, 5:35am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/9 "2003-05-08T05:35:17Z")
I don’t think it’s fair to blame Roemer for being off on the speed of light by 50% or so. Before he did his calculation, no one had the faintest idea how fast light traveled, other than that it must be significantly faster than sound. It wasn’t even known that light didn’t travel instantaneously. The very idea of calculating the speed of light based on deviations in timing of the eclipses of the moons of Jupiter was an extremely creative one for which Roemer deserves credit. ---
**Author:** ![eburacum45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/eburacum45/32/8690_2.png) [@eburacum45](https://boards.straightdope.com/u/eburacum45)\ **Post date:** [May 8, 2003, 8:27am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/10 "2003-05-08T08:27:37Z")
James Bradley in 1728 determined the speed of light much more accurately by watching the star Eltanin in Draco, as a similar effect called stellar aberration caused the star to apparently move slightly in the sky as the Earth orbited the sun. He was looking for parallax, but found instead the distortion caused by the relationship between the earth’s motion and the angle and speed of the lightbeam. ---
**Author:** ![Rhino](https://avatars.discourse-cdn.com/v4/letter/r/a5b964/32.png) [@Rhino](https://boards.straightdope.com/u/Rhino)\ **Post date:** [May 8, 2003, 7:37pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/11 "2003-05-08T19:37:28Z")
[QUOTE] \*Originally posted by Polycarp \* \*\*The speed of light was first measured in 1644, when it was noted that eclipses and transits of the Galilean satellites relative to Jupiter occurred 1000 seconds later when Jupiter was on the other side of the Sun from us than when we and it were on the same side. The astronomer in question, whose name I don’t Polycarp, that is just amazing information. thank you. I did not know that time could be measured in seconds as early at the 17th cent. was that about that time that that guy written about in the book’LONGITUDE" made the chronograph? I wonder how they measured seconds? some instrument with springs and gears? ---
**Author:** ![Rhino](https://avatars.discourse-cdn.com/v4/letter/r/a5b964/32.png) [@Rhino](https://boards.straightdope.com/u/Rhino)\ **Post date:** [May 8, 2003, 7:47pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/12 "2003-05-08T19:47:31Z")
> [@](#): > > \*Originally posted by Exapno Mapcase \* > \*\*No more than the number of seconds in a year. > > But a light year did not come about because an astronomer was “measuring a source of light”. \*\* But Exapno , I was using the words"the source of light" to mean the star or the plant that was emitting light ; how far from earth was that light–that star,that plant. ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 8, 2003, 8:32pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/13 "2003-05-08T20:32:42Z")
Boy, has this gotten confused. Start over. Even before Roemer, astronomers had started to realize that the earth’s movement around the sun would have consequences for viewing the planets and the stars. By checking the position of a nearby star against the more distant background at one point and doing so again six months later, the star would appear to shift position. This is known as parallax. A nice explanation, with diagram, is given [here](http://www.noao.edu/outreach/nop/nophigh/steve9.html). The problem is that the shift is too small to be seen except with telescopes that didn’t come into existence until the 19th century. F. W. Bessel of Prussia was the first to measure parallax in 1838. What he actually measured, though, was the change, in arcseconds, the star appeared to make. By basic trigonometry you can apply this measure against your baseline - which is the diameter of the earth’s orbit around the sun, a known quantity - and come up with a distance. [Here’s](http://www.astronomynotes.com/starprop/s2.htm) a better explanation of how to do that. Now, the question is what measure do you use for distances to the stars? At first, astronomers used the Astronomical Unit (AU), which is the distance from the earth to the sun, or half a diameter. This is very unwieldy, however, as the distance even to the nearest star is well over 250,000 AU. Astronomers came up with the parsec, or the distance at which a star has a **par** allax of one arc **sec**. The nearest star has a parallax of a mere 1.3 parsecs. I’m not sure why the term light year started being used instead. I assume that it was because parsec is such a technical term without any intuitive base. OTOH, a light year could be expressed in miles or kilometers or other familiar units. One parsec is about 3.26 light years. Astronomers use both terms, since they have different uses and applications, and may be better suited for some calculations. The public just knows about light years. And George Lucas doesn’t even know that much, or he wouldn’t have had Han Solo do the Kessel run in 26 light years (a unit of distance, not time) or whatever exact numerical idiocy he committed in Star Wars. Is that the answer you were really looking for? ---
**Author:** ![Exapno\_Mapcase](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/exapno_mapcase/32/1051_2.png) [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)\ **Post date:** [May 8, 2003, 8:35pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/14 "2003-05-08T20:35:42Z")
Goofed up one paragraph. Astronomers came up with the parsec, or the distance at which a star has a **par** allax of one arc **sec** ond. The nearest star has a **distance** of a mere 1.3 parsecs. ---
**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\ **Post date:** [May 9, 2003, 12:43am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/15 "2003-05-09T00:43:47Z")
As usual, the OED is unlikely to have found the _first_ use of the term. The very fact that, only two years after 1888, Young is describing the usage as “now usual” suggests otherwise. There’s the additional complication that the term might as easily have originated in French or German as English. Furthermore, it’s easy to find proto-instances of the basic idea. In his _Darkness at Night_ (Harvard, 1987, 141-7), Ed Harrison cites several examples of 19th century astronomers describing large distances as the time taken by light to cover them, starting with Herschel in 1802. Then you even get passages like this: > [@](#): > > According to the measurements of Maclear, Bessel and Struve, of the parallaxes and distances of three fixed stars of very unequal magnitude ([alpha] Centauri, 16 Cygni, and [alpha] Lyrae), a ray of light requires respectively 3, 9 1/4, and 12 years to reach us from these three bodies. This is Alexander von Humboldt in volume I of _Cosmos_ (1845), as translated by Otte in 1848 (reprinted Johns Hopkins, 1997, p153). By this stage, the likes of Humboldt having realised that these are convenient numbers to discuss, inventing the actual term “light-year” is a mere matter of useful abbreviation. ---
**Author:** ![Polycarp](https://avatars.discourse-cdn.com/v4/letter/p/82dd89/32.png) [@Polycarp](https://boards.straightdope.com/u/Polycarp)\ **Post date:** [May 9, 2003, 3:06am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/16 "2003-05-09T03:06:38Z")
**Exapno Mapcase** is correct about my error. I was attempting to show when the speed of light (and therefore, by definition, the distance light travels in a given time) was first measured. ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [May 9, 2003, 8:24am UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/17 "2003-05-09T08:24:38Z")
Quoth **Rhino** : > [@](#): > > I did not know that time could be measured in seconds > as early at the 17th cent. Measurement of time precise to seconds dates at least to Galileo, who discovered that a pendulum takes a constant amount of time to make a swing. The big challange for measuring longitude was to build an accurate timepiece _which could work on a ship at sea_, since a simple pendulum will get screwed up by the rocking of the boat. But for measuring the eclipses of Jupiter’s moons, you can stay on land with a nice stable clock. And c can, in principle, be determined without any light sources at all. One of the simplest ways to get it (which my physics lab did in college) involves measuring two other constants relating to the strengths of the electric and magnetic forces, from which you can derive the speed of light. ---
**Author:** ![dylan\_73](https://avatars.discourse-cdn.com/v4/letter/d/ecccb3/32.png) [@dylan\_73](https://boards.straightdope.com/u/dylan_73)\ **Post date:** [May 9, 2003, 12:25pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/18 "2003-05-09T12:25:02Z")
> [@](#): > > _Originally posted by Chronos \*\*\* > And c can, in principle, be determined without any light sources at all._\* Indeed. My favourite is measuring the speed of light with [chocolate](http://physics.about.com/library/weekly/aa012703b.htm)! ---
**Author:** ![FriendRob](https://avatars.discourse-cdn.com/v4/letter/f/ecc23a/32.png) [@FriendRob](https://boards.straightdope.com/u/FriendRob)\ **Post date:** [May 9, 2003, 2:30pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/19 "2003-05-09T14:30:13Z")
Galileo also used his pulse to time his experiments, IIRC. BTW, Galileo was possibly the first to attempt to measure the speed of light: he stood on a hilltop with a lantern and had an assistant on a different hilltop with his own lantern. When the assistant saw G’s light, he opened his lantern. G concluded that the speed was too fast for his experiment to measure. ---
**Author:** ![dylan\_73](https://avatars.discourse-cdn.com/v4/letter/d/ecccb3/32.png) [@dylan\_73](https://boards.straightdope.com/u/dylan_73)\ **Post date:** [May 9, 2003, 3:26pm UTC](https://boards.straightdope.com/t/the-light-years-when-devised/173805/20 "2003-05-09T15:26:29Z")
Actually, Galileo thought it was instantaneous. It wasn’t till Roemer that it was discovered that light had a finite speed. A history can be found [here](http://www.njsas.org/projects/speed_of_light/). [Next page](https://boards.straightdope.com/t/the-light-years-when-devised/173805.md?page=2)