A light year is the distance that light travels in one Julian year in a vacuum. By using the fact that the speed of light () = 299,792,458 m/s, a light year can be calculated by dimensional analysis (a fancy term for "solve for x") to equal (299,792,458 m/s x 3600 s/h x 24 h/d x 365.25 d/y) = 9,460 billion kilometers (9.46 * 1015 meters). These fixed units have been officially defined since 1984, based on SI units (seconds for time and for the speed of light) derived from physical constants - general and special relativity show the need for that. Prior to this definition by the International Astronomical Union (IAU), the solar year and a measured speed of light was used, producing a slightly different value.
The light year is commonly used by laypersons, amateur astronomers, and sci-fi authors (who may also refer to a "light minute" or "light second" for comparably shorter distances) while professional astronomers tend to prefer to use the parsec (~3.26 light years) as it is based on movement of 1 arcsecond of parallax motion, and therefore most easily convertible to observational data.
A nanosecond is occasionally referred to informally as a "light foot", because the distance that light travels in a billionth of a second is approximately 11.8 inches (299,792,458 meters divided by 1,000,000,000 is very nearly 30 centimeters).
 Speed of light
In an ad hoc attempt to counter overwhelming evidence that the universe is billions of years old, young Earth cretinists have resorted to the claim that the speed of light is not a fixed constant but instead a variable parameter (meaning that a "light year" will change in size over time). The only evidence which has been brought forward for this is highly dependent upon old measurements which were much more inaccurate than the more recent ones. Curiously, as the measurements became more accurate, the speed of light appears to have approached its present value and stopped changing. Even taking the old measurements at face value, they would only support a small change in the speed of light, far too small to be compatible with the universe being younger by a factor of a million (thousands of years, rather than billions). Although some ideas regarding a variable speed of light were worked on in the 1990s, it has been in the confines of very hypothetical cosmology and even if true, it wouldn't be great enough to reduce the age of the universe by the required amount. Moreover, these studies have shown that over the last billion years or more, if there has been any change in the speed of light it has been a matter of a small fraction of a percent. A fraction of a percent in a few billion years, rather than a multiplier of a million in a few thousand years. It is also interesting to note that such a varying speed of light would be grossly incompatible with the Anthropic principle in the form that God had to fix the values of the constants of physics (such as the speed of light) near to their present values in order to make a universe fit for life.
Another option which some Young Earth creationists have chosen is that light was created in transit from distant stars. So that when we see a supernova from a hundred thousand light years away, we are not really seeing the event which took place a hundred thousand years ago, but a manufactured (that is, "designed") image which merely looks like a supernova exploding. When Adam looked into the sky in the first year of his life in Eden, he didn't see stars, but light which was made to look like it was from stars which were more than a light year away. (There being no stars to be seen, other than the sun, which are less than one light year away.)
 See also
- ↑ Defined as 365.25 days, where 1 day is defined 86,400 SI seconds long.