I'm reading up on radio astronomy, and I came across this paper from 1964. At the bottom of page 193, the author uses a unit that I've not seen before in discussing radio power emission from stars:

Now the outbursts on the Sun give an intensity on Earth of $10^{19}$ to $10^{20}$ $wm^{-2}(c/s)^{-1}$

I'm guessing it's "Watts per square meter per [something] per second", but I'm not sure what the [something] is.

A similar unit appears in this paper on the first line on page 364:

The comparison band in the radiometer, being separated approximately 3.25 Mc from the signal band, never encounters the hydrogen range of frequencies.

Again, this looks to me like mega[something]. Can anyone shed some light on this?

On page 362 of the second paper, the unit appears as $(Watts/M^2 )/(C/S)$ as a unit of flux. There, the $C$ looks like coulombs, but that makes the $3.25 Mc$ in the second quote seem weird.


I would expect the authors to be talking about the signal in terms of janskys, the now-commonly-used units of flux density. The typical definition is $$1\text{ Jansky}=10^{-26}\text{ Watts meters}^{-2}\text{ Hertz}^{-1}$$ One hertz is one cycle per second, which makes me suspect that the "c" stands for cycle. It might seem curious that the authors choose to use cycles/second instead of hertz, but as the papers were published in 1964 and 1955, and the hertz was only adopted on a large scale in 1964, the older term "cycles per second" is more fitting, given the time period.

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    $\begingroup$ The fact that they are from older papers makes me agree with you, that it's an old convention. Jansky is consistent with flux. Thanks! $\endgroup$ – Jim421616 Mar 4 at 22:38
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    $\begingroup$ I hadn't seen c/s before, but cps (cycles per second) was certainly a common abbreviation back in the olden days (and people would commonly refer to radio frequencies in units of kilocycles and megacycles, dropping the "seconds" entirely). When the SI was introduced in 1960, everyone standardised on Hz (even in the US!) $\endgroup$ – Michael MacAskill Mar 5 at 3:35
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    $\begingroup$ @MichaelMacAskill Yeah, considering when the Hertz was adopted makes the non-use of Hertz here make a lot more sense. $\endgroup$ – HDE 226868 Mar 5 at 3:50
  • $\begingroup$ The context of the second paper makes it clear that "Mc" is "megacycles [per second]". $\endgroup$ – Mark Mar 5 at 20:16

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