How to work out how good or bad your sky is?

[leo82]

Hi all,

I read somewhere to determine what objects you will be able to see in the night sky you have to find the faintist magnitude star with the naked eye. To do this you find ursa minor and see how many stars you can see. Is this correct? Only last night was the clearest night I've had for observing since I started this hobby and I can see ursa major, Cassiopeia, auriga, perseus, etc even the Pleiades but I can only pick out the polaris star in ursa minor. I guess this would mean I have bad skies to look at? Does this mean I can pick out only the very brightest of things or will I be able to see more of the dimmer things too? I know you would have to take into account what telescope your using etc but I'm enjoying my time observing using binoculars at the moment and just learning my way around the sky. I'm using 10x50 binoculars.

Many thanks.

[Stu]

Hi

I'd be really surprised if you can only see Polaris in Ursa Minor. If that's the case then your skies are quite bad I'm afraid.

Have a look at this link which will help you judge by checking the different stars. Basically if you can see all seven then your NELM is around mag 5 which is a reasonable suburban level I guess.

https://darkskydiary.wordpress.com/2012/01/20/naked-eye-limiting-magnitude-assessing-sky-brightness/

Get this checked correctly and then you can go from there.

[estwing]

funny when you get to really dark skies you have to learn the night sky again!

[Roy Challen]

Do you mean that Polaris is the only star you can see? Or that it's the only star in Ursa Minor that you know? Have a look at a star chart showing UMi when you next observe, then see how many stars you can see. As Stu says, I would be surprised if you could only see the one, even from my town centre LP-riddled location I can see 3 or 4 on a good night. I can see M81/M82 with bins from my garden - but I know just where to look!

[leo82]

I know that ursa minor looks like a miniature version of ursa major and starts with polaris. I have tried a few times to observe this and draco a few times but can hardly make out many stars at all in that part of the sky. I suppose it could be light pollution from the city as it's in that direction. However, next time I go out I will take a copy of the star map from the link Stu posted.

many thanks.

[leo82]

Just come in from a couple of hours observing. I can actually see polaris, kocab and phercab so the faintest magnitude is 3.00 or bortle class 9.

[Stu]

Just come in from a couple of hours observing. I can actually see polaris, kocab and phercab so the faintest magnitude is 3.00 or bortle class 9.

That's pretty bad I would say [emoji20]. Are you in the middle of a city?

It is well worth travelling a small distance to better skies to see deep sky objects. Planets and the moon will be ok from home, but other stuff will be tough. Try to get somewhere at least mag 4.5 or 5.

[leo82]

To be honest I'm just outside the city but looking that way is like looking over the city.

I think I better get a good moon atlas now!

On a plus note though I'm pretty sure I saw m81 and m82 by following an online star map. Would this even be possible with how bad my nelm is? they were 2 hazy patches horizontal to each other and fit in the same field of view.

[Stu]

Well if they are in a bad direction for you they your NELM should be better looking elsewhere, so it is quite possible you saw M81 and 82, the description sounds about right. One is more of an oval glow, the other is a elongated thinner line (it is also called the Cigar Galaxy)

This is the star hop I use:

This shows the kind of orientation you would see relative to each other although I'm sure a lot more washed out than this:

[leo82]

I'm 99% sure I have spotted them and when the cloud clears I will double check. Thanks.

[Stu]

Great stuff.

This website might be interesting for you......

http://washedoutastronomy.com/washedoutastronomy.com/content/urban-galaxies/index.html

[Owmuchonomy]

My sky can vary quite widely and I think it's often moisture dependent. On nights I class as clear I can usually get all 7 major stars in UMi but not always. Other nights I can get more than 7. I am no expert but I judge a good night here as mag 5.5. Are you allowing enough time for dark adaptation of your eyes?

[leo82]

As soon as I stepped outside I glanced over it but I was out for a couple of hours so I made it my last target so I could have maximum dark adaption.

[leo82]

Just spotted your post Stu, thanks for the link i'm sure that will be most suited for me.

[rockystar]

I find ursa minor tends to point down towards the horizon when I go out at 10/11 (don't know if changes throughout the year), I've seen an image were you can also use Cassiopeia to check, which is directly overhead at the moment.

I can see 6 of the 7 in urea minor, but can nearly get to mag 5 in Cassiopeia.

There are a couple of books on amazon about stargazing under washed out skies:

http://www.amazon.co.uk/Urban-Astronomers-Guide-Practical-Astronomy/dp/1846282160/

http://www.amazon.co.uk/Philips-Urban-Astronomy-Guide-Stargazing/dp/1849072752/

[Stu]

This looks like a useful resource, showing different groupings of stars which can be used:

http://dvaa.org/AData/LM/

This looks interesting as well, worth playing around with:

http://www.namnmeteors.org/lm_calc.html

I find Cassiopiea less useful as the main stars don't vary in magnitude as much

[leo82]

Thanks for the links Stu, it's all making good reading. With regards to using Cassiopeia I have no problem seeing the main stars although segin in noticeably fainter than the rest.

[Stu]

Ok, so that takes you to mag 3.37.

There are a few others which you could try to spot which go gradually fainter.

Archird is mag 3.5

Zeta is mag 3.7

Kappa is mag 4.2

Check these out to see where you end up

[leo82]

Getting adventurous now going for mag 4.2. I will definitely try this tonight weather permitting, I'm confident enough that I will be able to see maybe some of those stars because I look at Cassiopeia often and know there is more stars other than the main stars that make the 'w' shape.

[Stu]

Maybe try sketching the positions of the stars you can see then you'll be able to check later

[leo82]

Thanks, that's a good idea, I will do that.

[rockystar]

Yeah, you do need to go away from the main stars in Cass. I'll see if I can find the thread where I saw it.

[Stu]

Yeah, you do need to go away from the main stars in Cass. I'll see if I can find the thread where I saw it.

I keep trying to find a star map with the magnitudes of the fainter stars in Cass marked but can't find one.

[Stu]

Best I can find

[rockystar]

I'm struggling to find it too. There 3 around the middle star that are 4.6 & 4.8. so 1 just above gamma, and 2 just below. I use these 3 at home, on a good night I can get all 3.