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The jansky (Jy), is sometimes called the flux unit, is the unit used mostly in radio astronomy to measure the strength, or more precisely the flux density[?], of radio signals from space. When comparing flux densities measured in other wavelengths, all fluxes are usually converted to janskies.

In measuring signal strength, it's necessary to take into account both the area of the receiving antenna and the width of the frequency band in which the signal occurs. Accordingly, one jansky equals a flux of 10-26 watts per square meter of receiving area per hertz of frequency band (W/m2Hz).

Although it is not an SI unit, the jansky is approved by the International Astronomical Union and is widely used by astronomers. The brightest radio sources have flux densities of the order 1 jansky, which makes jansky a suitable unit.

It is named after Karl Jansky, the American physicist who discovered radio waves from space in 1930.

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