It’s amazing what a difference observing conditions make on the things you can see when you look into the sky. Until you experience different conditions, it may never cross your mind that wind, moisture in the air, artificial light, or moonlight can completely disrupt your viewing experience. So I wanted to take some time to hit on some of the conditions that are impactful for amateur astronomers and explain them. Then I wanted to bring attention to how important and how amazing it is to visit a dark site as quantified via the Bortle scale. I will explain the Bortle scale briefly in this post, but it’s simply a scale that objectively measures the level of artificial light that a location is affected by.
There are four main conditions that affect our ability to view celestial objects. These conditions include cloud cover, transparency, seeing, and darkness. There are also some important tools that I will get into that are helpful for forecasting these conditions when planning an observing session. There is also a Bortle Scale Google Maps overlay to help get an idea about total darkness and how far away from city lights you need to go in order to experience dark skies. It’s quite useful in finding the closest dark site to your location.
Cloud cover is obviously one of the most detrimental things affecting amateur astronomy and stargazing. Clouds can completely ruin an observing session by blotting out everything that’s possible to see. Obviously you want to keep an eye on the weather when planning an observing session and always avoid cloudy nights.
Transparency is a measure of moisture in the atmosphere, it can take into account humidity, or in my location for example, frozen moisture in the air during the winter. This can be highly reflective and make viewing unsatisfying. This is why humid locations are often less conducive to astronomy then other areas like Arizona, New Mexico, or my home state Wyoming.This is a separate measure from cloud cover that measures moisture in the air. Clouds are moisture in the air, but not the kind of moisture that the transparency data point measures.
Seeing conditions refer to how stable the atmosphere is in your location, and is important for determining how much detail you will be able to see on planets in particular. Seeing conditions detrimentally affect the amount of magnification you can use when viewing things like binary star systems and planets alike. When light enters the atmosphere the incoming light rays are bent and blurred and the degree of that disruption depends on air density and air movement. There are different layers of air stacked from the ground to space, and they are often moving at different speeds, in different directions, and have different moisture densities. If there are many different layers with these varying characteristics, the seeing can be quite poor. Planets and stars will look like they are twinkling and exhibit no detail. Under good conditions fine details visible on the brighter planets like Jupiter and Mars, stars are pinpoint images, and double stars are easily differentiable. Under poor seeing conditions you may not be able to make any detail out on Jupiter, Mars might look like a blurry red star, and the stars twinkle. In fact one way to judge the seeing conditions before you even look through your telescope is to look up and judge how much the stars are twinkling on a given night.
Darkness is one of the most important variables that particularly affects your ability to view deep sky objects in a location or on a certain night. The biggest offenders causing sky illumination are the moon, and sky glow from cities’ artificial light. However, even auroras and other natural sky glow can end up having a big affect on what you can see on any given night. The main variables you can control are the nights you view and the location you view from. The moon is particularly disrupting in the week and a half or so leading up to the full moon each month. The full moon is one of the worst nights to view anything besides the moon, so try to plan on observing other objects in the weeks after the moon reaches full illumination. The new moon and days surrounding it are the darkest and best times to observe deep sky objects and other faint targets.
The second variable you can control is your location. The Bortle scale is a very helpful tool that measures the amount of artificial light or lack there of in different locations. It is a nine level scale that measures the brightness of the sky. It objectifies the observability of different celestial objects and helps us to understand and quantify the amount of light pollution affecting various sites. It was published in February 2001 in Sky and Telescope magazine to help amateur astronomers evaluate the darkness of an observing site. I’ve found it indispensable for locating dark skies around my town. I can’t even explain what a difference it makes when you travel from a level 6 area on the scale to a level 2. The views are absolutely astounding! Try your best to visit darker sites, it is absolutely worth the time.
I will cover the Bortle scale in more depth in a separate post, as well as explain how to overlay the Bortle Scale map onto Google Earth. It is an accurate and interactive way to map out your area and decide on a dark site to visit. Again I found this indispensable when trying to find a dark site to visit near my town. It let me optimize my travel time by finding the closest dark site with a good area to set up and view the sky.
One final note, there is a handy website that forecasts these observing conditions that I’ve covered in this post. It’s helpful to use for planning either the upcoming night or the next day’s observing sessions. It helps with deciding how good the nights will be, especially if you are planning to visit a dark site and contemplating if it’s worth the drive. The website is cleardarksky.com. Once you’re there, input your town and it’ll pull up a chart with the forecasts for the various observing conditions over the next 48 hours. I highly recommend you check it out!