How do we measure distance?
Over the centuries, there have been countless means of measuring distance, but today all scientists use metres. Since 1793 when the French first made the effort, there have been many attempts to define a metre. The book “You Are Here” [my review here] explains that, since 1983, the metre has been defined as the distance that light travels in a vacuum in 1/299,792,495 of a second. So, now you know.
January 19th, 2010 at 6:44 pm
Amazingly it has been pointed out to me – by a special anorak of a friend (OK, he was originally a physicist) that there is an error in “You Are Here” and that in fact 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.
For a definitive source, see here.