2. Units of Length, Mass, and Time

- SI units of length (L), mass (M), time (T):
- Length: the meter
- Was: one ten-millionth of the distance from the North Pole to the equator
- Now: the distance traveled by light in a vacuum in 1/299,792,458 of a second

- Mass: the kilogram
- One kilogram is the mass of a particular platinum-iridium cylinder kept at the International Bureau of Weights and Standards, Sèvres, France.

- Time: the second
- One second is the time for radiation from a cesium-133 atom to complete 9,192,631,770 oscillation cycles.