How is the sky magnitude measured?

[Syntarsus]

Sorry if this is posted elsewhere. Did some searches but couldn't find anything.

So I'm wondering how one measures the magnitude of the sky? I see people saying things like 'under a mag 5.3 sky' etc.

If it's just the faintest naked eye star you can see that's surely highly subjective and says as much about one's eyes as it does the sky.

Or do people work it out from The Night Sky Simulator or somewhere else?

[pete_l]

Sorry if this is posted elsewhere. Did some searches but couldn't find anything.

So I'm wondering how one measures the magnitude of the sky? I see people saying things like 'under a mag 5.3 sky' etc.

If it's just the faintest naked eye star you can see that's surely highly subjective and says as much about one's eyes as it does the sky.

Or do people work it out from The Night Sky Simulator or somewhere else?

Takes deep breath

Yes, the idea behind Visual Limiting Magnitude is that it should give some idea of roughly what you (yourself, not anyone else) would expect to see while looking straight upwards on a clear night - all other factors being equal.

Sadly, it's become a bit like "top speed" values among boy-racers: and just as unreliable.

It turns out that no two individuals will have the same acuity of eyesight. They probably won't want to spend all their time looking at the zenith (traditionally the darkest point, but not particularly useful and with everyone's light pollution situation being different: even from one street to another, not that helpful for general viewing guidance either). The value will change on a night-by-night basis, and definitely varies with the 11 year sunspot cycle, as well as with the seasons, and the moon, and ... well, you get the point.

Personally, I feel that if people quote VLMs with decimal points in the figure, they're just showing their sense of humour!

[ollypenrice]

The magnitude scale was placed on a mathematical footing by Pogson. Five magnitudes equates to a difference of 100x in brightess. He assigned a difference in brightness of 2.512 times between one magnitude and the next. A good sky allows someone with keen eyesight to see down to around mag 6. An excellent site and great eyesight pushes that to around Mag 7. In mainland Europe the difference between zenith and low elevation is large and even in truly remote places it is always considerable.

People involved in variable star observation are very good at estimating fractions of a magnitude based on known nearby stars but for most amateurs an estimation of about a magnitude for a given night is more like it, as Pete says above.

Olly

[Macavity]

There's also this other stuff...

Sky Darkness at Cuevas del Negro

I could NEVER afford a telescope that penetrated to the "sky brightness".

But perhaps it becomes interesting, as I mess with VIDEO astronomy?

Moreover now there's these: http://www.firstlightoptics.com/unihedron-sky-quality-meters.html

[pete_l]

There's also this other stuff...

Sky Darkness at Cuevas del Negro

Wow, I've never seen one of my pages quoted anywhere I didn't think anybody ever read that stuff.

[Macavity]

Wow, I've never seen one of my pages quoted anywhere I didn't think anybody ever read that stuff.

Well, there y'go, Sunshine. Worldwide "Nerd Solidarity" and all that?

[Syntarsus]

Wow, I've never seen one of my pages quoted anywhere I didn't think anybody ever read that stuff.

Well it was useful to me, and I loved the bottom line.

[cantab]

I have about magnitude 3 skies with my shortsighted eyes

[dph1nm]

There's also this other stuff...

Yes, professionally, sky brightness is measured in magnitudes per square arcsecond. However this is wavelength dependent, hence the sometimes conflicting values which are bandied about ( although 24.3 is nuts in anyone's language). Generally speaking, in mag/sq.arc. the sky is fainter (larger numerical values) at bluer wavelengths. So a good dark site will have a b-filter sky brightness ~22.5 mag/sq.arc.. Through an r-filter, the same sky will measure ~21.5, and in the i-band maybe ~20.

You should be able to measure this value from CCD/DSLR data, if you have stars of known magnitude in the image.

NigelM