A Beginner's Map of the Night Sky: What You Are Actually Looking At

Most of us can name the Moon, and after that it gets vague. The sky is on show every clear night, free of charge, and it is quite possible to walk under it for decades without knowing what any of it is.

This article is a first map. It needs no telescope and no app. It covers what the sky is doing, why it does it, and what the points of light actually are. Much of it comes from the opening chapters of my book Astronomy, which begins in the back garden for the same reason.

Start by giving your eyes half an hour

The best instrument for the night sky is the one you already own, provided you give it time.

Step outside from a lit room and your pupils widen within seconds, from about 2 millimetres to as much as 7 in young eyes. That helps, but the real change is slower and chemical. The rod cells of the retina rebuild their light-sensitive pigment, rhodopsin, over 20 to 30 minutes, until the eye is around a million times more sensitive than in daylight.

So the first rule of looking up is patience. The second is to leave the phone in your pocket. A single glance at a bright screen undoes most of the work. This is why observers carry a dim red torch: rods barely respond to deep red light.

Where you stand matters as well. A 2016 sky atlas found that almost 80 per cent of North Americans and about 60 per cent of Europeans can no longer see the Milky Way from home. If your first attempt from a town garden is disappointing, the fault is probably the street lamps and not your eyesight.

The sky is a dome with borders drawn on it

Stand in a field at night and the sky looks like the inside of a vast dome. Greek thinkers such as Eudoxus, around 370 BC, took that impression literally and pictured the stars fixed to a crystal shell. We now know the stars lie at wildly different distances, but the idea survives as the celestial sphere: an imaginary globe of infinite size centred on Earth, onto which every object is projected. It is a fiction, and an extremely useful one, because it gives the sky poles, an equator and a grid.

The constellations sit on that grid. To an astronomer a constellation is not a stick figure but a piece of territory. In 1922 the International Astronomical Union adopted an official list of 88, and the Belgian astronomer Eugène Delporte then drew their borders. Every point of the sky now belongs to exactly one constellation, so a newly found comet or galaxy arrives with a ready-made address.

It helps to remember that the patterns are flat only from where we stand. Orion looks like a figure pinned to a wall, but its stars lie at very different depths. Learning the constellations is learning the map, and the map is not the terrain.

Why everything rises, sets and changes with the seasons

Every star rises in the east and sets in the west, yet nothing in the night sky is doing the moving. Earth spins from west to east, and we ride along.

There is a small detail here that explains a great deal. Measured against the distant stars, one turn of the Earth takes 23 hours 56 minutes and 4 seconds. The familiar 24-hour day is about four minutes longer, because Earth also moves roughly one degree along its orbit each day and must turn a little extra to bring the Sun back to the same place. The result is that each star rises about four minutes earlier every night, which adds up to two hours a month. That is why the stars you see on a winter evening are not the ones you see in summer.

The seasons themselves come from tilt, not distance. Earth's spin axis leans 23.4° from upright relative to the plane of its orbit, and it keeps pointing the same way in space, towards Polaris, all year round. In June the northern hemisphere leans towards the Sun; six months later the southern hemisphere does. The tilt changes two things at once: how high the Sun climbs, and how long it stays up.

So the nightly turning is the Earth's spin, the slow change of constellations through the year is the Earth's orbit, and the seasons are the Earth's lean. Three motions, all ours.

The Moon: half lit, always

The Moon makes no light of its own. It reflects about 12 per cent of the sunlight that falls on it, which makes it about as dark as worn tarmac. It only looks bright against the black sky.

The phases confuse people because they look like something is covering the Moon. Nothing is.

Half the Moon is always sunlit; the phase is how much of that half faces us

At every moment the Sun lights exactly half of the Moon. What changes through the month is how much of that lit half is turned towards Earth.

Eclipses are a separate matter, and they need a straight line. In a solar eclipse the Moon passes in front of the Sun, which can only happen at new Moon. In a lunar eclipse Earth's shadow falls across the Moon, which can only happen at full Moon. They do not happen every month because the Moon's orbit is tilted by about 5° to Earth's, so the three bodies line up only now and then. These eclipse seasons come round about every 173 days, giving between four and seven eclipses of both kinds each year.

One more thing about the Moon, for perspective. Laser measurements show it drifting away from Earth by about 3.8 centimetres a year, roughly the rate at which your fingernails grow.

How bright, how far, and what the pale band is

Astronomers grade brightness with a scale invented more than two thousand years ago. Around 130 BC, tradition says, the Greek astronomer Hipparchus sorted the stars he could see into six classes, with the brightest of "first magnitude" and the faintest of "sixth". The scale is still in use and it runs backwards: the smaller the number, the brighter the object. Every five steps down the ladder is a hundred times fainter. The handful of stars that outshine the old reference star Vega, led by Sirius, Canopus and Arcturus, carry negative magnitudes.

Distance needs its own unit. A light-year is a distance, not a length of time, whatever science fiction suggests. Light in a vacuum travels at 299,792.458 kilometres per second, and in one year it covers about 9.46 trillion kilometres. Light from the Moon takes about 1.3 seconds to arrive.

As for what the stars are doing: look at any star in the night sky and the odds are that it is quietly doing what the Sun does, fusing hydrogen into helium in its core. Stars spend about 90 per cent of their lives in this long, stable phase. The Sun is 4.6 billion years into a stay of about ten billion.

On a dark, moonless night a pale river of light arches across the sky. That river is the Milky Way, our own galaxy, seen from inside its disc.

Every star you can see with the naked eye belongs to it, and so do between 100 and 400 billion more.

That sentence sets the scale. Everything described so far, every constellation and every named star, is local.

There is one easy exception. The Andromeda Galaxy is the most distant thing most people can see with the naked eye. From a dark site it is a faint oval smudge in the constellation Andromeda, and its light has travelled about 2.5 million years to reach you. It left before our own species existed. The Persian astronomer al-Sufi described it as a "small cloud" around 964. Long-exposure photographs show that it stretches about three degrees across the sky, six times the width of the full Moon. We simply cannot see most of it without help.

What to take from this

• Give your eyes 20 to 30 minutes in the dark, and keep bright screens away. Use a dim red torch if you need light.
• The sky's turning is the Earth's spin. Each star rises about four minutes earlier every night, so the evening sky changes through the year.
• Seasons come from the 23.4° tilt of Earth's axis, not from its distance from the Sun.
• The Moon is always half lit. The phase is how much of the lit half faces us.
• On the magnitude scale, smaller numbers mean brighter objects.
• A light-year is a distance: about 9.46 trillion kilometres.
• Every naked-eye star is in the Milky Way. The Andromeda Galaxy is the far smudge beyond it, about 2.5 million light-years away.

If you would like to carry on from here, Astronomy: The whole sky, explained goes from the back garden to the edge of the observable universe in 100 chapters, each with a detailed plate, and says how we know each thing. It is a 309-page PDF and EPUB, and it costs £9 on Gumroad: matsiems.gumroad.com/l/astronomy.