<p>I would expect the authors to be talking about the signal in terms of <a href="https://en.wikipedia.org/wiki/Jansky" rel="noreferrer"><em>janskys</em></a>, the now-commonly-used units of flux density. The typical definition is
<span class="math-container">$$1\text{ Jansky}=10^{-26}\text{ Watts meters}^{-2}\text{ Hertz}^{-1}$$</span>
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 <a href="https://en.wikipedia.org/wiki/Hertz#History" rel="noreferrer">the hertz was only adopted on a large scale in 1964</a>, the older term <a href="https://en.wikipedia.org/wiki/Cycle_per_second" rel="noreferrer">"cycles per second"</a> is more fitting, given the time period.</p>

CC BY-SA 4.0
2019-03-04T22:38:51.877Z
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!
Jim421616
CC BY-SA 4.0
2019-03-05T03:35:47.887Z
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!)
Michael MacAskill
CC BY-SA 4.0
2019-03-05T03:50:20.790Z
@MichaelMacAskill Yeah, considering when the Hertz was adopted makes the non-use of Hertz here make a lot more sense.
HDE 226868
CC BY-SA 4.0
2019-03-05T20:16:07.657Z
The context of the second paper makes it clear that "Mc" is "megacycles [per second]".
Mark
CC BY-SA 4.0
2019-03-04T22:37:13.490Z
2019-03-05T12:06:19.027Z
2019-03-05T12:06:19.027Z
HDE 226868
<p>I'm reading up on radio astronomy, and I came across <a href="http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1964Obs....84..191L&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf" rel="noreferrer">this paper</a> 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:</p>

<blockquote>
  <p>Now the outbursts on the Sun give an intensity on Earth of <span class="math-container">$10^{19}$</span> to <span class="math-container">$10^{20}$</span> <span class="math-container">$wm^{-2}(c/s)^{-1}$</span></p>
</blockquote>

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

<p>A similar unit appears in <a href="http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1955ApJ...122..361H&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf" rel="noreferrer">this paper</a> on the first line on page 364:</p>

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

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

<p>On page 362 of the second paper, the unit appears as <span class="math-container">$(Watts/M^2
)/(C/S)$</span> as a unit of flux. There, the <span class="math-container">$C$</span> looks like coulombs, but that makes the <span class="math-container">$3.25 Mc$</span> in the second quote seem weird.</p>

<radio-astronomy><units>
A "strange" unit radio astronomy
Jim421616
# A "strange" unit radio astronomy

(Asked by: username_0 on 2019-03-04)

I'm reading up on radio astronomy, and I came across [this paper](http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1964Obs....84..191L&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf) 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](http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1955ApJ...122..361H&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf) 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.

**Tags:** radio-astronomy, units


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## 1 answers

### Answer by username_1 on 2019-03-04. Score: 23 | Accepted answer

I would expect the authors to be talking about the signal in terms of [*janskys*](https://en.wikipedia.org/wiki/Jansky), 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](https://en.wikipedia.org/wiki/Hertz#History), the older term ["cycles per second"](https://en.wikipedia.org/wiki/Cycle_per_second) is more fitting, given the time period.

> **Comments:**
> **username_0**: 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!  
> **username_2**: 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!)  
> **username_1**: @username_2 Yeah, considering when the Hertz was adopted makes the non-use of Hertz here make a lot more sense.  
> **username_3**: The context of the second paper makes it clear that "Mc" is "megacycles [per second]".  

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