Planning a clear night around the moon, the forecast and the boat

The moon decides the month

A lunation runs about twenty-nine and a half days, and of those perhaps ten give a properly dark sky for faint objects. I plan the month around that before looking at a single weather map. The week on either side of new moon belongs to galaxies and nebulae. The days around first quarter belong to the moon itself, which is far better with the terminator across it than when it is full and flat. The nights after full, when the moon rises late, are still perfectly good for double stars and clusters.

Altitude matters as much as phase up here. A full moon in December rides high because it sits opposite a low sun, so it floods the entire sky and there is nowhere to hide from it. The same full moon in June scrapes along the southern treeline and barely troubles anything overhead. That reversal takes a while to internalise if you have read observing guides written for lower latitudes, where the difference between a summer and a winter full moon is much less dramatic than it is here.

What the forecast can and cannot tell you

Cloud forecasts for this coast are honest about the next six hours and largely speculative beyond that. The pattern that actually produces clear nights is cold high pressure with a northerly component, and the pattern that ruins them is anything bringing moist air over water that is still warmer than the land. In late autumn the sea holds its summer heat for weeks, which is why September and October evenings so often begin perfectly clear and end inside a fog bank that arrived from nowhere in about twenty minutes.

I have stopped believing published forecasts of transparency and started judging it by eye instead. If I can see the fainter stars of the Little Dipper from the jetty, the night is good enough for faint objects. If only the two bright ones show, it is a night for the moon, doubles and bright clusters, and I adjust the list rather than going back inside. That single test has saved me more disappointment than any amount of studying satellite images on a telephone screen.

The boat, the bus and the cold

Getting out to the island in winter is its own problem. The boat timetable thins from October, the bus that meets it does not always wait, and an evening that ends at two in the morning means staying over rather than travelling home. I plan the journey first and the observing second, which sounds backwards and is not. An excellent forecast on a night I cannot reach, or cannot return from, is simply a night I read about afterwards in somebody else's account of it.

Cold is the other constraint, and it removes more observing time than cloud does. Minus eight with a breeze off the water is a very different proposition from minus eight in a sheltered garden inland, and the wind on an exposed jetty finds every gap in your clothing. I dress for standing still rather than for the walk down, and I keep sessions to about ninety minutes with a warm break in between rather than trying to be heroic. Nobody sees anything useful once they have started shivering.

• Winter boat timetable checked the same week, never from memory
• Layers chosen for standing still, plus something dry to sit on
• Red torch, and spare batteries kept warm in an inside pocket
• Eyepieces in a closed box until needed, to slow the dewing
• Ninety minutes out, twenty minutes in; repeat rather than endure

Writing it down

Every session gets an entry, including the failures, and the failures are the more useful half of the notebook. Date, time, temperature, wind, moon phase and altitude, the faintest star I could see near the zenith, and then the objects with a sentence each. That limiting-magnitude line is what makes the whole record comparable from year to year, because an impression of a faint galaxy means nothing at all unless you also know what the sky was doing behind it at the time.

After a few years the record starts answering questions I did not know I had asked. It told me that my best nights cluster in the fortnight after the first hard frost, that fog has ended more sessions than cloud ever has, and that I have never once regretted going out on a marginal forecast. It also stops me repeating the same fruitless hunt for the same object at the same hopeless altitude, which I managed three winters running before I started writing things down.