greatest duration - Greatest duration (for total eclipses) is defined as the instant when the duration of totality reaches a maximum along the path of a total eclipse. The calculation of greatest duration requires an accurate lunar limb profile to account for the effects of mountains and valleys around the circumference of the Moon on the duration of totality. The length of totality calculated at greatest duration may differ by 1-2 seconds compared with greatest eclipse, and the geographic location may differ by a hundred kilometers or more.
greatest eclipse - Greatest eclipse is defined as the instant when the axis of the Moon's shadow cone passes closest to Earth's center. The computation of the duration of totality at this point is typically done using a smooth edge for the Moon that ignores the effects of mountains and valleys along the lunar limb. For total eclipses, the instant of greatest eclipse offers a good approximation (1-2 seconds) to the maximum duration of totality along the entire eclipse path. Far more rigorous calculations using the Moon's limb profile are required to predict the instant of greatest duration (of totality) and a more accurate value for the maximum length of totality. For annular eclipses, the instant of greatest duration may occur either near the time of greatest eclipse or near the sunrise and sunset points of the eclipse path.