Changes

=== Meter ===

=== Meter ===

The '''meter''' (or '''metre''') (symbol m) is the unit of [[length]]. Originally (see [[Metric system]]) it had a definition depending on the dimensions of the earth. It was defined so that the circumference of the Earth would be exactly 40 million meters. However, the circumference of the Earth depends on where you measure it, so the French creators of the metric system chose the circumference which travels through both poles, along the [[meridian]] that passes through Paris. (This was a snub to the British, who had defined longitude by reference to the meridian passing through [[Greenwich]]).

The '''meter''' (or '''metre''') (symbol m) is the unit of [[length]]. Originally (see [[Metric system]]) it had a definition depending on the dimensions of the earth. It was defined so that the circumference of the Earth would be exactly 40 million meters. However, the circumference of the Earth depends on where you measure it, so the French creators of the metric system chose the circumference which travels through both poles, along the [[meridian]] that passes through [[Paris]]. (This was a snub to the British, who had defined longitude by reference to the meridian passing through [[Greenwich]]).

It later had a definition dependent on a particular type of radiation. Today its definition depends upon the second and on the [[speed of light]] in a vacuum--which the CGPM now assumes to be constant. Thus a meter is 1⁄299,792,458th of a light-second. (Therefore the speed of light is exactly 299,792,458 meters per second.)

It later had a definition dependent on a particular type of radiation. Today its definition depends upon the second and on the [[speed of light]] in a vacuum--which the CGPM now assumes to be constant. Thus a meter is 1⁄299,792,458th of a light-second. (Therefore the speed of light is exactly 299,792,458 meters per second.)