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The Jansky is the unit for spectral flux density. It is defined as

$$1 {\rm \ Jy} = 10^{-26} {\rm W \ m^{-2} \ Hz^{-1}}$$

in terms of Watts per square meter per Hertz.

I've never quite understood why this unit was chosen or why it is so useful. What are the reasons, historical and otherwise? For the record, this unit is used primarily in Radio Astronomy.

I suspect it is like other non-SI units in physics/astronomy, like the Ångström.


EDIT: To be clear, I am asking the reason for why radio astronomers (i.e. physicists) chose this unit. There are PHYSICAL reasons for this. This is explicitly NOT an off-topic conversation about nomenclature, which is distraction from physics.

To make my point clearer, there were PHYSICAL reasons why radio astronomers derived this unit. Therefore, in my estimation, this question is not at all off-topic. This is a physics forum, and we are discussing physics.

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    $\begingroup$ I'm voting to close this question as off-topic because questions asking for the reasons for notation or terminology are off-topic. $\endgroup$ – ACuriousMind May 29 '15 at 13:23
  • $\begingroup$ In this case, however, the notation has an interesting physical justification. $\endgroup$ – rob May 31 '15 at 21:37
  • $\begingroup$ @ACuriousMind Surely you can see that this is a discussion about physics. We are not arguing about how a certain symbol should be written---we are rather discussing why radio astronomers thought this was an appropriate unit. $\endgroup$ – ShanZhengYang Jun 1 '15 at 14:34
  • $\begingroup$ The link ACuriousMind provided states pretty clearly that asking Why do we use this notation?-like questions is off topic. $\endgroup$ – Kyle Kanos Jun 1 '15 at 16:08
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    $\begingroup$ Voting for a re-open - the existing answer to the question demonstrates pretty convincingly that there is physics to be had here. I agree that it is dancing around the on/off-topic line, though. $\endgroup$ – Kyle Oman Jun 1 '15 at 16:36
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You use units to make it convenient to talk about numbers. It's easy to do mental arithmetic with small numbers and hard to do mental arithmetic with scientific notation. So we tend to choose units where values of interest have numerical values between one-tenth and ten thousand (with a lot of give and take).

For instance, we use the non-SI "astronomical unit" to talk about distances in the solar system. Most professional astronomers could not tell you off the top of their head that the distance between the sun and Saturn is about one billion four hundred million kilometers, but they could tell you that it's a little less than ten AU.

A radiant energy of $1\,\rm W\,m^{-2}\,Hz^{-1}$ would be a huge radio signal. The big radio telescope at Green Bank has a diameter of $100\rm\,m$; a signal that bright would be enough energy to make the receiving antenna catch fire, like trying to observe the sun through an optical telescope. If a typical "bright" radio signal is a few jansky, there's not even a name for the appropriate SI prefix (at least, not a name that ordinary working scientists use in any other context).

Another similar unit is the "barn," a unit of area corresponding to the likelihood of an interaction in nuclear physics. One barn is $100\rm\,fm^2 = 10^{-28}\,m^2$. It got its name because it's much bigger than the physical cross-sectional area of a nucleus ("the cross section for this reaction is as big as a barn!"). Modern colliders like the LHC are sensitive to reactions with cross sections of picobarns and femtobarns; if some standards organization demanded that cross sections be expressed in square meters, the literature would become much more confusing for no good reason.

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  • $\begingroup$ @Thanks for the response! It is true I was asking for the physics behind why radio astronomers use this symbol (much like why particle physicists developed the barn). It may be difficult for some students or theorists to see why this is actual physics. However, this is NOT a question about notation. There are physical reasons for the choice of the Jansky. $\endgroup$ – ShanZhengYang Jun 1 '15 at 14:47

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