Seeing and how to record it
Using scales to assess the atmospheric seeing will enable you to predict how much it will affect your observing
The two main seeing scales are the Antoniadi scale (above) and the Pickering scale (below). Others exist but these are the two most commonly used by amateur astronomers. The Antoniadi scale has five values, indicated by Roman numerals, I being the steadiest, V being very unstable.
From the UK, Antoniadi seeing types III and IV are most common, but periods of II do happen, typically at times when high pressure dominates the weather. Antoniadi I conditions are rare, occurring perhaps only on a couple of nights throughout the year. It is important to understand that seeing may vary considerably over the course of a single night. Initially, poor seeing conditions
may give way to much steadier conditions over the course of a night or vice versa.
The Pickering scale was devised by William H Pickering of Harvard College Observatory. It’s based on the views through a 5-inch (120mm) refractor and is a 10-point scale. Just to make things complicated, the Pickering scale denotes the worst seeing at the lower end of the scale and best at the upper end, which is the opposite of the Antoniadi scale. As a rough equivalence to the Antoniadi scale, Pickering 1-2 is considered very bad seeing, 3-4 is poor seeing, 5-6 moderate, 7-8 good and 9-10 excellent seeing.
Seeing is something that gets easier to assess over time as you gain experience,
simply because you need to actually see the effects of each level of stability before you can confidently identify them. Perfect seeing is really something to behold and allows a view which is hard to forget.
As mentioned in the main text, seeing generally occurs in three discrete layers in the atmosphere. The lower and mid layers are greatly affected by wind and it’s important to keep a note of the wind direction when you get good seeing. High level seeing can be predicted to a degree, by the location of the jet stream. It is well worth using a forecast service such as NetWeather (www.netweather.tv/chartsand-data/jetstream), which is very useful for this purpose.