It's hard to tell, if you don't already know the star from a prior evening's viewing. I recognize stars by their relationships to each other, and if it's the first star I see, there aren't any other stars. That leaves me with guessing on the basis of how high it is in the sky, the time of year, and so on.
Stars change their positions in the sky, which is why I use the relationships between them to identify them. Tonight Vega will appear up there, in a month's time it'll be over there. If I don't get a chance to look at it in between, I might think I'm seeing Arcturus when it first appears.
Location Matters
On top of that, when I leave my most frequented observing locations, the sky appears to change. The parts of the sky I can see, and the parts that are blocked by local scenery change. I might not know which way is north as well, or as accurately. This happens most often when I'm at public star parties in new locations. There I am, supposedly one of the expert astronomers. Someone walks up to me as the first stars are appearing and I'm still trying to get my bearings.
"What star is that?" they'll ask.
And I won't know. One of the very brightest stars in the sky, and I can't tell which it is.
What kind of an astronomer am I anyway?
Get It Wrong
Fortunately I can usually make a guess. And I'm honest about it. I'll say, "I don't know, but from where it is and how bright it is I think it's..." whichever. I guess, and then in the next half hour I'll find out if I'm right or wrong.
When I'm by myself, I guess, too. I try to figure out what star I'm seeing. I try to put together a few facts, then make a guess and check back on myself later when I can see the star's neighbors. The only way for me to get better is to be willing to get it wrong, and make a guess. Later, when I find out whether I was right or wrong I can re-assess the reasons I used for guessing as I did. Then I learn something.
Like, yes, Arcturus really can appear that low in the sky, during this time of year. Or, yes, we're far enough along in the year for Altair to start to show. Sometimes it sticks, sometimes it doesn't. But over time, it adds up and I get better at it.
Start at Dark
At the outset, I had to learn the stars when I could see them among the other stars. I didn't learn the Winter Triangle and Summer Triangle at twilight, I learned them when it was fully dark. I also had to learn to remember which was which, and which constellation went with each. The same goes for the other bright stars.
As I often tell people when they're impressed by what I know, I wasn't born knowing this stuff. I had to learn it piece by piece, just like anyone else. The first pieces were the basics. The next step was challenging myself to do more. Like pick out the first stars and know what they are--maybe.
The next step in expanding on the ability to tell one star from another comes with trying to pick out the stars earlier when you're already familiar with the sky. You've been out under the dark sky within the past few days. You pretty well remember what you saw in the early part of the night. Now you see what you can see earlier. The stars will be in slightly different positions, shifted to the east from where you remember them being. But not due east, since the stars travel in circles about the pole.
Then as you see stars, guess to yourself. "I think that's Spica, because that other star over it is probably Arcturus." Then check up on yourself later, when more stars are out.
Optical Illusions
As it is, I now press myself to try to find stars when the sky is still bright. When doing this, I remind myself that the definition of "detectable" is that you can find it 50% of the time. And that's about how it works out. I'll scan the sky, and it's hard not to have my eyes either glaze over or focus on some near object while looking at the apparently empty sky. Then I'll see a little pinprick of light standing out, barely. Aha! A star!
Then I'll move my head. And it'll disappear. It can take me several minutes of looking to find it again. If I've got a scope, I'll try to keep it in view by staring at it as I bend toward the viewfinder. And it'll disappear as I start to crouch. It's funny, but it often seems it's harder to see things that are just detectable if my eyes aren't level with each other.
But sometimes, I can manage it. What helps the most in re-finding a star at this stage is locating the star with respect to something on the horizon. It's one hand span above that tree top. If I put my hand just like so it'll be just over the tip of my pinky. And so on.
It really does feel like an optical illusion. You can practically feel it slipping in and out of view sometimes. It's like one of those pictures where you have to really concentrate to see the picture hidden within the picture.
I know it doesn't sound relaxing. Bit it is, if you don't get wound up over it and just sort of let it happen. After all, in a few minutes the sky will be a little darker. So things will be a little easier to see. Can't catch it now? Wait a few minutes. Check some other part of the sky in the meanwhile. Carry on a conversation while you scan.
Most of all, don't be afraid of being wrong. Make guesses. As they say, you miss every shot you don't take. Take a shot, and see what happens. If nothing else, you'll have to dredge up some star names from memory, even if you aren't sure which stars they apply to, yet.
Relax to See More
The most important reason to stay relaxed? Your eyes are far more sensitive to small differences in contrast when you're relaxed. If you take it easy, and enjoy the view, you'll see more.
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