Hertz The dimension of the unit hertz is 1/time (1/T). Expressed in base SI units it is 1/second (1/s). In English, "hertz" is also used as the plural form. As an SI unit, Hz can be prefixed; commonly used multiples are kHz (kilohertz, 10 Hz), MHz (megahertz, 10 Hz), GHz (gigahertz, 10 Hz) and THz (terahertz, 10 Hz). One hertz simply means "one cycle per second" (typically that which is being counted is a complete cycle); 100 Hz means "one hundred cycles per second", and so on. The unit may be applied to any periodic event—for example, a clock might be
Astronomical unit expanding outward from the Sun. This has led to calls to abandon the astronomical unit as a unit of measurement. As the speed of light has an exact defined value in SI units and the Gaussian gravitational constant "k" is fixed in the astronomical system of units, measuring the light time per unit distance is exactly equivalent to measuring the product "G" in SI units. Hence, it is possible to construct ephemerides entirely in SI units, which is increasingly becoming the norm. A 2004 analysis of radiometric measurements in the inner Solar System suggested that the secular increase in the