Astronomical seeing: to be clear or not so clear


Weather apps may provide accurate information, but perhaps not the information amateur astronomers are looking for.
Credit: iStock


When the weather forecaster makes the statement "tonight will be clear" or "will have clear spells" I have no doubt that astronomers all over the country get excited.

Yet what constitutes a clear sky to the weather forecaster may not really be what we astronomers would prefer, which is an actual cloud and haze-free night sky.

You see a thin layer of haze can still look clear to most people, but seasoned veterans may look up and see even stars high up in the sky twinkling.

They will shake their heads sadly and remain indoors.

Such occasions are a good time to clean or service equipment, as astronomers know the view through a moderate-sized telescope will show the stars slightly blurry whilst detail on the planets may well be lost.

Faint deep-sky targets become impossible to view and as such it’s even possible that newcomers to visual astronomy may well be disappointed and put off by the poor view. 

This is why we describe the ‘seeing’ conditions.

If it is a clear night and the stars are crisp and pin-sharp to the naked eye, then the seeing could well be good.

But even so there may well be another factor to spoil the view if you are into high resolution planetary, lunar and solar viewing or imaging: the jet stream.

Some weather forecasters do mention where it may lie over the land and so this helps in deciding whether it is worth getting that cherished telescope and mount out.

It’s not their fault the weather is sometimes not what we would like or even expect from the forecast.

Even astro weather apps can get it wrong: after all they are usually using the same official source.

Light pollution can exacerbate the situation too: the slightest haze will scatter that unwanted light all over the sky and spoil what could have been a good night.  

So, the next time you hear the weather forecaster mention a clear sky, take a look outside at the real sky first before getting too excited!


The March issue of BBC Sky at Night Magazine is on sale 21 February.



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