Perfect skies are rarer than you think! When you’re deciding on a good sky, you need to keep several things in mind. The first is how well your seeing conditions are. The second is how well the sky contrasts. The third is of course how much light pollution is in the sky without the moon! When you understand these aspects, then finding the right time and the right sky for you is easy!
You will also about apps and websites that help you decide on making the trips!
What do you mean by Seeing Conditions?
Seeing refers to Astronomical Seeing – the amount of blurring and twinkling you see due to turbulence in our atmosphere combined with temperature differences.
The larger the telescope, the more it is affected by the seeing conditions. The better the seeing, the more magnification you can use before objects get too blurry.
Under good seeing, objects at high magnification will have crisp clear views; but under bad seeing, the same objects will appear to be rippling under water, and will not show much detail. Sometimes, the conditions flip back and forth on a whim, creating frustrated observers.
- Most major observatories are located on mountaintops, as they often represent the best conditions for good seeing. For example, Mt. Wilson’s location elevated above Los Angeles was chosen due to it naturally having steady air.
- If you are below any inversion layers, then the local seeing conditions can get pretty bad, especially when trying to view objects low above the horizon through thick haze.
- Besides high winds, radiative cooling can also create air currents that cause bad seeing conditions.
- Average seeing conditions typically limit your magnification to 100-200x at best, depending on local currents. This makes those nights when you can go to 300x and above a rare treat.
What is Transparency?
It means exactly what the word implies – the total transparency of the atmosphere from Earth to Space. This is factored by how much water vapor is in the air.
If the night is clear, but there is a lot of humidity or water vapor, then the sky can still be hazy. You also need to watch out for dew, as the last thing you want is dew to build up on your optical equipment.
Good transparency is the difference maker in your views on low contrast deep sky objects like galaxies, nebulae, and globular clusters.
- While windy conditions create poor seeing conditions, they do create excellent transparency as they push away all the moist air. The best nights that have both good transparency AND good seeing are after immediately the nights after the wind has settled down.
- The higher the object is, the less atmosphere you have to look through to view it. This is especially true when trying to view or photograph dim objects in the sky.
- Smoke from nearby wildfires or volcanic ash puts a lot of dust in the atmosphere, and unfortunately affects the transparency of the sky. If you observe on these nights, keep watch to make sure no ash is falling on your equipment.
Websites such as cleardarksky.com currently specialize in giving you a forecast on seeing conditions and transparency levels. It also tells you how much wind, humidity, and cloud cover is expected for a certain hour. Most of the time these forecasts prove to be true.
HOWEVER, even the website will tell you that it does not account for low clouds or afternoon thunderstorms. I’ve had my share of nights with clear skies in the forecast that didn’t account for fast rolling fog or monsoon flows quickly making the sky overcast. Always check the local weather as well.
This app, also called Dark Sky, is a very useful tool on cloud cover and rain forecasts, and often catches things that other weather forecasts don’t.
So How Do I Get Away From Light Pollution?
It depends on how far you are willing to go. Some locations you don’t have to go that far to get to an excellent dark sky site, while others require at least a couple hours minimum of driving time.
if you’re an iPhone user, the best app to use to find a dark sky is Dark Sky Finder by Skidmore Properties, LLC . If you’re not an iPhone user, you can find similar apps!
The colors represent the Bortle Dark Sky Scale. To understand the colors, look a the following chart (You can click on it to view bigger):
White (Class 8-9) means “bad,” red and orange (Classes 6–7) are “below average,” yellow (Class 5) is “average,” green (Class 4) is “above average,” blue (Class 3) is “good,” while grey and black (Classes 1–2) are “excellent.”
I live in a red zone, which is Class 7 on the scale, and Griffith Observatory is easily in a Class 9 sky. From my home, I can see the huge differences between a 7 and 9, and I can observe most things just fine. However, I need to drive at least a couple hours to get to a Class 3 sky. Obviously the lower the number the better, but I personally would not go any higher than Class 3 for deep sky viewing.
Do NOT Rule Out The Moon!
Depending on how prominent it is, The Moon washes out the Milky Way and makes the deep sky a shell of its true self. Crescent moons aren’t much to worry about, as they have little interference and are not in the night sky for very long. On the other hand, a half moon (first/last quarter) will brighten any dark sky site to a class 5! A full moon will brighten the sky up to a class 7, as bright as my light polluted Woodcrest, CA sky is!
The moon’s prominence during the hours you want to observe makes it completely pointless to drive out to far away dark sky locations, as you’ll get the same bright sky from nearby suburban locations. The only difference is the color of the sky will be blue rather than orange-brown.