Reading the sky

Step outside on a clear night and look up. Almost every point of light you see is a distant sun, and its light has travelled years to millennia to reach you. To begin you need no telescope, only your eyes, some patience and a few tricks. Those are exactly what you will learn here.

Why the sky turns

Watch the sky for a few hours and you will notice: the stars are moving. They rise in the east and set in the west, just like the sun. But it is not the sky that moves, it is you. The Earth spins once around its own axis in just under 24 hours, so the whole sky appears to circle around us once in that time. Rounding to 24 hours, that is 360° divided by 24 hours, so 15° per hour. This is why the constellations stand somewhere else at midnight than in the evening, and why long exposure photos show the stars as arcs.

Why stars twinkle

Stars do not twinkle on their own. On the last few dozen kilometres their light crosses the Earth's atmosphere, and that is constantly in motion: parcels of warm and cold air swirl around and keep nudging the light ever so slightly. Because even through large telescopes a star is only a tiny point of light, this jitter is enough to make it dance and flicker. In space, without air, the same stars shine perfectly steadily. Planets, by the way, barely twinkle: they are so close that they appear as a small disc, and across that disc the jitter largely averages out. So a bright point that shines strikingly steadily is often a planet.

Stars have colours

Look closely: stars are not all white. Betelgeuse in Orion glows reddish, Rigel diagonally opposite shines bluish white. The colour reveals the surface temperature, and the other way round than everyday life suggests: reddish stars are comparatively cool at around 3000 to 3500 kelvin, our yellowish sun sits at about 5800 kelvin, and bluish stars like Rigel exceed 10000 kelvin. Blue means hot, red means cool. So with the naked eye you can roughly sort stars by temperature.

Finding dark skies

In the city you might see a few dozen stars, under a truly dark sky several thousand, plus the delicate band of the Milky Way. The difference is called light pollution: street lamps and advertising brighten the sky and drown out faint stars. So if you can, drive out of the city, and give your eyes time. They need 20 to 30 minutes of complete darkness to reach their full sensitivity: the pupil widens from around 2 mm to 7 or 8 mm, and the retina additionally switches over to faint light. A single glance at your phone screen undoes that adaptation.