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How do I judge sky conditions from my site?
Home » Astronomy » Choosing an Observing Site » How do I judge sky conditions from my site?While some objects, such as the Moon, planets, double stars and bright clusters are less affected by sky conditions, others are far more sensitive—such as faint nebulas and galaxies. It is for that reason that we try to gain a sense of limiting magnitude. While this sounds like a complicated procedure, it’s nothing more than gauging the dimmest star that you can see unaided after dark adaptation.
For the most part, a reasonably dark sky means the magnitude—or brightness—of the dimmest stars you can see unaided is probably near 4 or 5. For example, if you can see the “bow” stars in the constellation of Orion, you are seeing at least magnitude 4. The dimmer the star, the higher the number. If you are seeing little sparkles of light west of the “belt,” you are seeing stars of magnitude 5, and so increasing your limiting magnitude to 5.
Does the image waiver when viewed? If so, the sky might be unstable. This can either be atmospheric conditions, or nothing more than rising heat. It’s best to observe objects when they are as high as possible to escape the thicker atmosphere along the horizon, but it’s not always practical.
While it's not entirely important you know limiting magnitude numbers right from the start, it will help you learn through experience what you can see. It would be next to impossible on a moonlit night to see a faint galaxy—even with a very large telescope! The limiting magnitude is the magnitude of the dimmest star you can see unaided. For example, if you can spot a magnitude 6 star, this means you stand a good chance seeing much fainter objects with smaller optics.
The next factor is stability. These numbers are on a scale of 1 to 10—with 10 being the steadiest. On a very clear night, regardless of light, you might have poor stability—causing difficulty splitting a double star or seeing a planetary feature well. It's like looking at something under clear running water. For example, a turbulent night would be 2/10, making magnification useless —while an 8/10 means very little waiver and crisp, clean images.
Related Choosing an Observing Site Articles
- Introduction
- Why is it important to get away from the light?
- I don't have dark skies. Can I still observe?
- What should I look for in an observing site?
- How do I judge sky conditions from my site?
- Where can I go to find a safe, dark place to observe?