Because of tidal locking, the Moon only presents one side to the Earth. However, the eccentricity of its orbit gives rise to the lunar libration of longitude. Because the Moon's rotation alternately leads and lags its orbital position, an observer on Earth can see an extra 6 degrees of longitude (approximately 200 km) along the lunar equator. The Earth locator (at a focus of the ellipse) controls the eccentricity of the orbit. The Moon's black and white disk shows the libration of longitude.