The best seasons for observing the night sky

East is east and west is west, but Sussex is the best, although George V would disagree.

The question has often been asked: “Which is the best season of the year for astronomical observing – spring, summer, autumn or winter?”

This sounds quite a simple question to answer but in fact it isn’t. There are so many factors to be taken into account.

In some countries, of course, things are so much easier because you can have a very good idea of what the weather is going to do. Even so, there can be disappointments.

I remember that years ago I was at Port Elizabeth, in South Africa, where there is a good observatory. A transit of Mercury was due, and I had never seen one before.

At that time of the year Port Elizabeth always has clear skies. I was there for six weeks, and during that period there was one cloudy day – the day of the transit...

Of course, our weather in the British Isles is quite unpredictable.

Some areas have more cloud and rain than others; during the 1960s I spent some time in Ireland, and half-expected to learn that children were born there with webbed feet and fins.

But nothing can ever be planned weather-wise, and there is very little that observers can do about it.

Moreover, we have to put up with the ever-increasing menace of light pollution. Anyone who lives in a city or large town will never see the Milky Way, and telescopically will probably have to confine himself to studying the Sun – unless, of course, their telescope is portable.

Things here are better than they used to be, because some really good telescopes are small enough and light enough to fit into the boot of a car.

Even so, the sky is always polluted to some extent. This was brought home to me once when there was a power failure, which blacked out the whole of South England, including my home county of Sussex.

For several hours the sky really was inky black, and I could see stars that were normally too faint for me to see without optical aid.

A patient wait for power

There was, of course, a snag. I made haste to my observatory and opened the dome, expecting to enjoy myself – but I had forgotten that the drive of my 15-inch reflector was electric.

Muttering things that I will not write down here, I closed the dome and waited patiently for the power.

My home is in Selsey, near the tip of the Bill; I am only just out of spray range. In fact this area has a sort of microclimate, generally a degree or two warmer than most other places, with less rain.

This includes the Bill, Bognor Regis, and a slice of the coast of the Isle of Wight (which is why King George V went to Bognor to recover, and made that unfortunate remark about it).

My main interest is with the Moon and planets, and my ideal is a very steady atmosphere. I do not in the least mind a bit of mist; it calms the air down.

Over the years I have found that mist can most often give me a spell with seeing Antoniadi 1.

The Antoniadi scale gives five grades of seeing, from 1 (well nigh perfect) down to 5 (so poor that observing would probably not be attempted except for some special purposes, such as an occultation).

A glorious sky, with stars blazing down and twinkling strongly, may be useless from my point of view – but certainly not to deep-sky enthusiasts, such as nova-hunters.

We depend upon our chosen targets being available, and there are dead periods; for instance Mars is virtually out of view for long periods, and we lose the Moon for part of each month – to the joy, I admit, of the nova-hunters! But there are always plenty of stars around.

Stellar observers hate the Moon

Summer has its advantages, but there are fewer hours of darkness, and at full Moon, or a night or two either side, there is no real darkness at all – I am speaking about Selsey, and naturally I realise the situation is different elsewhere: north Scotland, for example.

I am not too bothered, because the planets at least are often well seen with steady air and a lightish sky.

Catch Mars at dusk or dawn and the image can often be really good. Stellar observers cannot operate properly under these conditions, and they positively hate the Moon, which is understandable!

For me, summer nights are very variable, but every now and then we do have a succession of exceptionally clear ‘astrometric’ nights.

There is plenty of darkness in winter, with cold, transparent nights. I used to go out and open my dome, but sometimes I knew from the outset that I was in trouble. Twinkling was the clue.

If Sirius flashed violently in various colours, the image was bound to be unsteady. And if Capella also twinkled strongly from near the zenith, I prepared for seeing Antoniadi 3 or worse.

But a night of this sort would be good for concentrating on dim variable stars.

Spring can be favourable, and if Selsey has cloud, most other places are likely to have more cloud (yes, I know we have had a couple of tornadoes, but I assure you these were atypical).

Over many years, I have found that in this part of Sussex, October is probably the best month.

The only thing we can do is to make the most of our opportunities. And in a way I suppose we are lucky.

I remember talking to an astronomer at what was then Leningrad, now St Petersburg, where there is a very famous observatory, Pulkova.

“We haven’t had good conditions here,” he said mournfully. “We really have to keep to April and September. Near midsummer it never gets dark, and in winter it’s always overcast.”

And then, talking to an astronomer in Singapore, practically on the Equator, where it is fun to look up after dark and see the Great Bear on one side of the sky and the Southern Cross on the other. “Seasons?” he said with a chuckle. “Well, we really don’t have any.”

I’m not sure whether I was envious or not.