Changes

Historically the second was without definition, until 1820, when it was defined as 1/86,400 of the mean solar day. In 1956 the CGPM adopted the ''ephemeris second'' definition of 1/31 556 925.9747 of the tropical year 1900. The present definition dates from 1963.

Historically the second was without definition, until 1820, when it was defined as 1/86,400 of the mean solar day. In 1956 the CGPM adopted the ''ephemeris second'' definition of 1/31 556 925.9747 of the tropical year 1900. The present definition dates from 1963.

=== Metre ===

=== Meter ===

The '''metre''' (or '''meter''' in the US) (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 metres. 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''' outside the US) (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 metre 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.)

* The '''centimetre''' (or '''centimeter''') (symbol cm) is one-hundreth of a meter.  It is not a base unit but is included here because it is a commonly used unit of length.

* The '''centimeter''' (or '''centimetre''' outside the US) (symbol cm) is one-hundreth of a meter.  It is not a base unit but is included here because it is a commonly used unit of length.

=== Kilogram ===

=== Kilogram ===