Sonnensekunde: Commonly, 1⁄86400 of a mean solar day (24 * 60 * 60 = 86,400). A mean solar day is an average of the time the earth takes to rotate around its axis.
Ephemeridensekunde: The fraction 1/31 556 925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris time (note: 24 * 60 * 60 * 365 = 31,563,000). This definition is based on earth’s rotation around the sun.
Atomsekunde: The time that elapses during 9,192,631,770 cycles of radiation produced by the transition between two levels of the cesium 133 atom. This is the International System of Units (SI, Système international d’unités) definition of a second.