The meter is the unit of length in the International System of Units (SI). It is one of the seven SI base units. The origins of the meter go back to at least the 18th century. At that time, there were two competing approaches to the definition of a standard unit of length. Some suggested defining the meter as the length of a pendulum having a half-period of one second; others suggested defining the meter as one ten-millionth of the length of the Earth’s meridian along a quadrant (one-fourth the circumference of the Earth). In 1791, soon after the French Revolution, the French Academy of Sciences chose the meridian definition over the pendulum definition because the force of gravity varies slightly over the surface of the Earth, affecting the period of the pendulum.