Light year questions

How, precisely speaking, is a light year measured? I know that a light year is defined as the distance light travels in a vacuum in one year. My question is, how is the year defined? Is it strictly 365 24-hour earth days, or is it the more scientific definition of an earth year, which is approximately 365.25 days?

Also, do astronomers ever use smaller light-based distances such as a light day or a light week?

Not sure about the first question, although I’d suspect it’s the scientific definition.

I have seen “light seconds” used, as in the sun is “x” light seconds from the earth (I think it’s something like 7 or 8). You can do the math.

The Sun is eight light-minutes from Earth, not seconds. If it was eight light-seconds away, it sure as Hell wouldn’t be snowing outside right now.

I dunno about the first question.

I think it’s defined first as a distance, i.e. 9.461 X 10[sup]15[/sup] m. The term “year” is not part of the primary defintion.

Look here.

If it’s important, you would probably use the mean tropical year (the cycle of the seasons, or 365.242191 days). But there are actually quite a few different “years”: You’ve also got the sidereal year (one full rotation with respect to the fixed stars, or 365.256363 days) and the anomalistic year (time from perihelion to perihelion, 365.249635 days), all of which are slightly different, as well as things like the legal year (365 days exactly), the Julian year (365.25 days exactly) and the lunar year (exactly 12 synodic lunar months, or 354.367068 days).

But for purposes of lightyears, it really doesn’t matter. The longest of the principle years (the sidereal) is less than a tenth of a percent greater than 365 days. And there’s absolutely nothing measured in lightyears which is known to anywhere near that precision.

Well that clearly can’t be the case as an original matter, because why would such a distance be arbitrarily chosen? It wasn’t – it’s the distance light travels in a year. Once the measure was originally calculated, you’re right that it is now a specific unit just like a foot or a meter, and therefore the time element is no longer a primary term in the definition, but the concept of a light-year necessarily existed before anyone ever calculated just how long such a distance might be.


For comparison the moon is about 1-1/4 light seconds away. Easily measured with a laser using the reflector planted by the Apollo 11 crew.

FWIW, my encyclopedia mentions the tropical year as the one that matters.

Understood, of course. My point is that one can simply now take the given distance and divide by the speed of light in a vacuum and the result will be a “year” (whether that corresponds to a sidereal year, solar year, or whatever, can/will then be determined).

Professional astronomers, astrophysicists, and cosmologists don’t use light-year as a unit anyways (they use parsecs). Light year is just a bit, how shall we say, “ambiguous” from the point of view of relativity, with Lorentz Contraction and all…

For example, to us it seems like a particular photon took a year to travel X distance, but from the photon’s point of view, it was but a few seconds…

courtesy of

The speed of light in vacuum is exactly 299,792,458 m/s (metres per second)

In 1983 the SI (Systeme International) defined a metre as:

The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
When people refer to the speed of light, they refer to the definition above - the speed of light in a vacuum.

The speed of light is normally rounded to 300 000 kilometers per second or 186 000 miles per second.

Light from the Travel time
Earth to the moon 1.28 seconds
Sun to Earth 8.5 minutes
Sun to Mercury 3 minutes
Sun to Venus 6 minutes
Sun to Mars 12.5 minutes
Sun to Jupiter 43 minutes
Sun to Saturn 1 hour
Sun to Uranus 2.6 hours
Sun to Neptune 4 hours
Sun to Pluto 5.4 hours
Sun to the nearest star 4.3 years
Sun to the furtherest stars 18 billion years]

BTW- The speed of light can be determined through several methods. Many of them are discussed on the cite provided above.

It’s speed is also variable, depending on different sets of circumstances and has been contained (trapped) in a “box” and released minutes later in a Harvard experiment recently.

“food for thought”…:slight_smile:

Professional astronomers, astrophysicists, and cosmologists don’t use light-year as a unit anyways (they use parsecs). Light year is just a bit, how shall we say, “ambiguous” from the point of view of relativity, with Lorentz Contraction and all…

For example, to us here on Earth it might seem that a particular photon took a year to travel X distance, but from the point of view of someone chasing the photon at 0.99c, it was but a few (milli?)seconds…

Well, the speed of light doesn’t change, neither does the distance of a light year measured by any person in a reference frame. What may change is the X distance itself.

A lightyear is a pefectly reasonable unit provided you know how long a year is. And a year is the same length no matter where you are or how fast your going (one year per year). Astronomy employs the parsec instead which is roughly three times the light year. It’s useful because all units it’s derived from are easily defined (semimajor axis of the Earth’s orbit and the number of arcseconds in a radian being the necessary quantities of interest).

It depends on what you’re measuring, and how you’re measuring it. For distances measured by trigonometric parallax, the parsec is the obvious unit to use. Unless, of course, the distances are relatively small. I’ve never heard anyone refer to [symbol]a[/symbol] Cen being 1.something parsecs away; it’s always 4 lightyears. Likewise Sirius, Barnard’s Star, Wolf 359, or [symbol]t[/symbol] Ceti, all of which are close by. But a star in the vicinity of 100 pc away will usually have its distance expressed in parsecs.

On the other hand, you’d never express the distance to the Surface of Last Scatter or the Lookback Horizon in any unit other than lightyears, since those distances are derived from a time in the first place. Likewise, folks refer to quasars as being smaller than a few light-weeks, since they show variability on those time scales (and therefore, the different parts of the quasar must be in communication with each other with less than a few weeks of lag time).

The nearest that you’ll get to an official definition is actually slightly odd at first sight. These days, it’s pretty much the NIST who get to define the SI fundamental constants. Now, even by the obvious possible definitions, the lightyear isn’t a fundamental constant. But they give a conversion factor for it from the SI units, which has the following footnote:

So they’re actually defining the lightcentury and then dividing by 100. Now a Julian century is clearly the average length of a century in the Julian calendar (most years have 365 days, but every fourth has 366). It’s presumably being used because it’s an integer number of days. By defining the day as an integer number of seconds, this means that the only non-integer in the definition is the length of the second, which is an SI fundamental unit. The definition thus technically has nothing to do with the Earth at all.
Given the typical uncertainties in astronomical distance measurements, using any other definition for the year here would make very little difference and be considerably more complicated.

Thinking a bit more after posting, I realise it’s neater than that. The seconds cancel and so its definition can’t matter in converting from the metre to the lightyear.