The definition uses such an odd number because it was designed to refine an already existing definition. The original definition used a nice, round number - the distance from the equator to one of the poles divided by 10,000,000. However, that distance was difficult to measure accurately, and as better measurements were taken, it would have had the effect of changing the length of a standard meter.
To avoid that problem, the meter was redefined as the distance between two scribed marks on a special platinum-iridium bar kept in France, near Paris, at the Bureau International des Poids et Mesures. (In English that translates to International Bureau of Weights and Measures.)
Since then, the ability to measure distances using such things as lasers has gotten more accurate than the ability to duplicate the marks on a bar. So, in order to make sure that everyone is using the same definition of a meter, the new standard meter is defined in terms of the speed of light.
To avoid that problem, the meter was redefined as the distance between two scribed marks on a special platinum-iridium bar kept in France, near Paris, at the Bureau International des Poids et Mesures. (In English that translates to International Bureau of Weights and Measures.)
Since then, the ability to measure distances using such things as lasers has gotten more accurate than the ability to duplicate the marks on a bar. So, in order to make sure that everyone is using the same definition of a meter, the new standard meter is defined in terms of the speed of light.
