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Science of Maps Weekend Reads Maps

Mapping the stars may be a challenge beyond the final frontier

How do we map the stars? Representing the round surface of the Earth on a flat piece of paper has posed a major challenge to cartographers over the centuries, but it pales beside the demands of representing an ever-expanding three-dimensional universe.

First light

For much of recorded history, people simply recorded what they saw, in two dimensions, grouping stars into constellations that resembled human or animal shapes.


constellations The traditional Western constellations of the northern hemisphere, from Harmonia Macrocosmica by Andreas Cellarius, 1661


Throughout antiquity and the Middle Ages, people believed that everything in the universe revolved around the Earth – the Sun, the Moon, the planets and, outermost of these concentric circles, a sphere to which the stars were fixed, as if they were painted on the inside of a dome. All the stars were therefore the same distance from the Earth, and their different levels of brightness was because of their size, not their distance.

From a point of view on Earth, the stars all appear to move together, and not in relation to one another, so this theory worked well enough to predict their positions, and was of great assistance to sailors navigating the high seas.

What it failed to explain was the movement of the planets, which appeared to overtake one another in their journeys across the sky, and sometimes even to move backwards. In fact the word ‘planet’ derives from the ancient Greek for ‘a wanderer’, and many outlandish theories were devised to explain their movements.


Ptolemaic-geocentric-model The geocentric model of the universe, from Harmonia Macrocosmica by Andreas Cellarius, 1661


The Age of Discovery

The theory that the stars all moved together, and were all the same distance from the Earth, began to break up in the Renaissance. Nicolaus Copernicus (1473-1542) first proposed that the Earth and the other planets revolved around the Sun, an idea so radical it took the best part of a century to become accepted and attracted fierce opposition from the Church.


711px-Cellarius_Harmonia_Macrocosmica_-_Scenographia_Systematis_Copernicani The Copernican model of the universe, from Harmonia Macrocosmica by Andreas Cellarius, 1661


In 1610, Galileo pointed his newly invented telescope at the Milky Way and saw that it was made up of individual stars – a cloud of stars, one on top of another, receding into the depths of space.

Around the same time, Johannes Kepler worked out that the orbits of the planets were not circular, as previously thought, but elliptical. His discovery – arrived at by mathematical calculation based on the detailed observations kept by his mentor Tycho Brahe – explained the strange movements of the planets and did away with the need for fanciful theories to account for them.


ptolemaic-orbits Diagram of the apparent motion of the planets around the Earth, known as epicycles


Expeditions to the southern hemisphere led to the addition of many new constellations to celestial charts and globes. In 1676, Edmond Halley visited Saint Helena to study the stars of the southern hemisphere, publishing the results in his Catalogus Stellarum Australium (1679); and between 1750 and 1754 the French astronomer Nicolas de Lacaille observed more than 10,000 stars from the Cape of Good Hope, naming 15 new constellations.

In 1718 Halley announced his discovery that the stars Sirius, Arcturus and Aldebaran had drifted more than half a degree away from the positions charted by the ancient Greek astronomer Hipparchus 1850 years earlier. The fixed stars were no longer fixed: they moved in relation to one another.

By now, many scientists accepted that the stars were different distances from the earth, but this could not be proved until 1838, when the German astronomer Friedrich Wilhelm Bessel used parallax to measure the distance of the star 61 Cygni, and determined that it was 10.3 light years (64 trillion miles) away.

Two months later, the Scottish astronomer Thomas Henderson calculated the distance to our nearest star, Alpha Centauri, as 3.25 light years. The limitations of the available technology meant that both measurements were slightly out, but they were close enough to prove the relative distance of stars.

The Big Bang

In 1927, Georges Lemaître proposed that the universe was expanding from a single point of origin – the big Bang Theory. Two years later, Edwin Hubble discovered that the Milky Way was just one of many galaxies, and that they were all drifting apart at high speeds. The accidental discovery of cosmic microwave background radiation in 1964 lent support to the theory, which predicted its existence as an after-effect of the Big Bang.


heic0406a_small The Hubble Deep Field View covers a speck of sky 1/30th the diameter of the full Moon. A few foreground stars in our Milky Way are vastly outnumbered by more distant galaxies. Credit: Robert Williams and the Hubble Deep Field Team (STScI) and NASA


The use of space telescopes such as Hubble is constantly revealing new stars in the depths of space. Exoplanets – planets revolving around other stars than the Sun – were once the realm of science fiction, but more than 1800 have now been discovered.

The mind-expanding universe

Now that we can begin to grasp the awe-inspiring scale and complexity of the universe, how do we even begin to map it? As we now know, the familiar constellations – the Great Bear, Orion, Cassiopeia and the rest – are made up of unrelated stars massively distant from one another; they only appear connected from the perspective of the earth. Seen from another planet orbiting another star, they would fall into completely different patterns.

Not only that, but the stars orbit the centre of their galaxy; our Sun, for example, circles the Milky Way at a speed of 140 miles per second, taking an estimated 230 million years to complete one circuit.


What is needed is a three-dimensional map, and this is where computer technology can help us. In 2012, scientists from the Sloan Digital Sky Survey released the most detailed three-dimensional map of the universe ever made, containing details of 200 million galaxies, while online maps like this one allow viewers to fly through distant galaxies on their home computers.

But we may need new computers entirely before we can properly wrap our minds around it. Computer screens are 2D, even if they’re showing 3D worlds like in computer games, and the most common control systems like mice or touchpads are also 2D. If you want a good excuse for finding it hard to visualize 3D space, then blaming our tools is quite a fair one.

image creditNASAESA/Hubble and the Hubble Heritage Team

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