Planning a Night

Before you pick a site, pick a date, and start with the Moon. This is the next full Moon: up all night, it turns a Bortle 3 site into a Bortle 6, and faint galaxies and nebulae are gone.

The next new Moon, the same site: no Moon all night, and the Milky Way is back. For faint things pick the week either side of it. Then, the day before, check the clouds: the Tonight page has a forecast for your site.

On the night, face north first, for the stars that never set. The Big Dipper and its neighbours round the pole are up every clear night of the year: the place to practise. And read the altitude on Polaris: north of the equator, that number is your latitude.

Face south. Everything that rises crosses the meridian here, at its highest and through the least air. At 30° up you look through twice the air overhead, at 20° three times, each extra layer dimming a star by a third of a magnitude.

Face east to plan the rest of the night. Everything here is on its way up as the sky turns 15° an hour: a star rising due east now crosses the south six hours later. So work west to east — western targets while they last, eastern ones once they have climbed into good air.

Now navigate. Never start from a faint target: start from a shape you are certain of and hop outward. From Vega and Deneb, go south to the third corner of the Summer Triangle.

The whole night as a graph: the three stars and the Moon, height against the clock. The top of each curve is its transit, the meridian crossing from three steps ago: the time to look. The Tonight page draws this for your own targets.

Pack: a dim red torch, more layers than feels sensible, a chair, binoculars, and this map in night mode. On site, give your eyes 30 minutes before you judge the sky, and start from a shape you know.