Seeing Forecast for Lincoln Astronomical Society

Seeing Forecast

The word 'seeing' in astronomical terms has nothing to do with a star's brightness, but refers to definition. The apparent brightness of a star is a measure of transparency, and is entirely different from seeing, though often confused with it. If you want to take a detailed look at the planets or our Moon you need to pick a night with good ‘seeing’. Leaving aside telescopic faults, it may be said that the majority of poor definition is due to atmospheric disturbances.

The best time to get detailed views of the planets is when the atmosphere is steady from ground level up through the jet stream. Low or high level winds cause turbulence and the mixing of cold and warm air leads to blurring of the image.

Following are three charts:

The first chart shows the wind at 5000 feet and the temperature over Europe. It covers the next 3 days weather in 12 hourly increments. Ideally low wind speed is what you are looking for. This usually occurs under high pressure. Look for days when the UK is free of isobars.

The next chart for the high level winds (jet stream). It covers the wind in 12 hourly intervals for the next three days. Look for periods when the wind isobars are clear of the UK.
This indicates a time when there is less turbulence in the upper atmosphere. Looks for days when the UK is covered by mauve, avoid days when covered by green or yellow.

Both charts will help you to plan for steady observing conditions over the short term.

The next chart covers the period from two to ten days in the future.

To avoid low level turbulence, either only observe in the early morning when the heat of the day has radiated away from buildings,
or pick a night when the daytime temperature and that in the evening are very similar.

Having said all this, is there an easier way to find out whether the atmospheric ‘seeing’ is good on a particular night?
The easiest method is simply to go out and look at the stars. If those overhead twinkle –forget about the planets that night.
If the stars about 20 degrees above the horizon are steady, there is a chance of good ‘seeing’.

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