Magnitude of the sky - how can you tell?

[Planetesimal]

Reading some of the threads here I've noticed people occasionally estimating the sky's magnitude.

Now I'm not sure what this means but I'm interested. My guess is that it must relate to the dimmest magnitude of stars that can be seen with the naked eye. So for example, if the dimmest star I can see is Sheliak, which is mag 3.5, then I would say the sky was mag 3.5. Have I got it right?

[Spec-Chum]

Yep, spot on.

I can see down to about mag 5 from my back garden on a good night, which is ok but not great

[Bizibilder]

Details and definitions are here: http://en.wikipedia.org/wiki/Bortle_Dark-Sky_Scale

[Mike73]

You can use the stars in Ursa Minor to work out your estimated limited magnitude.

Have a look at this post on my blog, you can download and print off all the info you need.

http://darkskysketches.blogspot.co.uk/2012/05/naked-eye-limited-magnitude.html

[cantharis]

Yep, spot on.

I can see down to about mag 5 from my back garden on a good night, which is ok but not great

A sight better than I can do at present!!

[Qualia]

Clearly an aproximation and will vary significantly from person to person but from my experience under clear, dark conditions, with fully dark adapted vision, most people can see stars at the zenith of magnitude around 5, 5.5. Those with better eyes or better skies observing under ideal conditions can often see down to magnitude 6.0 to 6.5 at the zenith. Under very clear conditions and with a good set of eyes, I guess some may be able to see stars as dim as 7.0 to 7.5. I have read reports of folk claiming sightings of 8.0 magnitude stars and dimmer with the naked eye, but frankly I'm inclinded not to believe them.

Ursa Minor is a good testing ground for your evening's naked eye star magnitude. Wait a good half hour or so without any white light or the flame from a ciggy lighter and then check out what stars you can make out. In an LP city on an exceptionally good night, you should be able to see with the naked eye Pherkad; out in the sticks, you should be able to do a lot better. In the arid-lands I go to on a good night of transparency and seeing I can make out all the stars of the 'plough shaped thingy' of Ursa Minor and often a few more like Theta, 19 and 20 Ursae Minoris. Needless to say, if you can see this plough-like asterism of Ursa Minor, you're in for a good night.

[Planetesimal]

Thanks all! Very useful... there's a lot more to this astronomy lark than buying a telescope, as it turns out!

It'll have to wait until the next new moon / clear sky combo to check, but I suspect the view from my garden will be around class 4 on the Bortle scale (i.e. mag 6.5'ish) on the best nights. I can certainly see Ursa Minor without any difficulty, although I've yet to manage M33 without the 'scope...

[knightware2]

There is another way to look at the darkness of the sky besides the magnitude of the dimmest star that you can see visually, and that is an intgrated magnitude value of a small area of sky. This is typically recorded in magnitudes per square arcsecond (mpsas), and sometimes magnitudes per square arcminute. Professional observatories often do studies of a location to determine its mpsas, and they also collect readings continuously to callibrate images.

If you have heard of the Sky Quality Meter, it acquires light from a cone of sky and integrates the data into a reading in mpsas. When you use one of these, you find that different areas of sky on the same night give differnt results because of light domes, the Milky Way, etc.

The star magnitude measurement, also called naked eye limiting magnitude (NELM), is helpful when comparing relative sky darkness and naked eye observing. The mpsas measurement is more appropriately compared to an extended object's surface brightness. I've found that I can expect to see objects in my telescope with 1.00 or higher surface brightness values than the Sky Quality Meter reading on the same night. I believe that the eye is a smarter integrator of light (and detector of low contrast differences) than the device.

- Phyllis