Academic Reading — Test 153

Reading passage

For most of human history, the night sky appeared as a flat ceiling on which countless points of light were fixed. Determining how far away those points lay seemed an impossible task, since the stars offered no obvious clue to their remoteness. The breakthrough came not from building larger instruments but from a deceptively simple idea: that the position of a nearby star, when viewed against the background of more distant stars, would appear to shift very slightly as the Earth moved around the Sun. This apparent shift is known as stellar parallax, and it remains the most direct method astronomers have for gauging the distances to nearby stars. The principle behind parallax can be demonstrated with a everyday gesture. If you hold a finger at arm's length and look at it first with one eye closed and then the other, the finger seems to jump against whatever lies behind it. Bring the finger closer, and the jump becomes larger; push it further away, and the jump shrinks. The two eyes provide two slightly different viewpoints, and the size of the shift depends on the distance to the object. Astronomers exploit exactly the same geometry, but instead of two eyes they use two positions of the Earth in its orbit, separated by six months. Because the Earth travels to opposite sides of the Sun in that time, the two observing points are enormous distances apart, which is essential for detecting the tiny shifts involved. The angle by which a star appears to move is extraordinarily small, and this explains why parallax went undetected for so long. Even the nearest stars produce a shift of less than one second of arc, a unit equal to one three-thousand-six-hundredth of a single degree. To picture such a quantity, imagine the apparent width of a small coin viewed from several kilometres away. Early astronomers searched in vain for this shift, and some concluded, wrongly, that the Earth could not be moving at all. It was only in 1838 that the German astronomer Friedrich Bessel succeeded in measuring the parallax of a star called 61 Cygni, becoming the first person to establish the distance to a star other than the Sun by this technique. From the measured angle, calculating a distance is a matter of straightforward geometry. The radius of the Earth's orbit around the Sun is known with great precision, and this serves as the baseline of a very long, thin triangle whose apex sits at the target star. Knowing the length of the baseline and the angle at the apex, astronomers can work out the distance to the star. The relationship gives rise to a convenient unit called the parsec, defined as the distance at which a star would show a parallax of exactly one second of arc. A parsec is roughly three and a quarter light years, and the closer a star lies, the larger its parallax angle becomes. In other words, distance and parallax are inversely related: a doubling of the distance halves the measured angle. For all its elegance, the parallax method has firm limits. The shifts grow smaller and smaller as distances increase, until they become too faint to separate from the unavoidable errors of measurement. Observations made from the surface of the Earth are blurred by the constant churning of the atmosphere, which restricts ground-based parallax to relatively nearby stars. The most dramatic advances have therefore come from space. The European Space Agency launched the Hipparcos satellite in 1989, and it catalogued the positions of more than a hundred thousand stars with a precision unattainable from the ground. Its successor, the Gaia mission, launched in 2013, has measured the parallaxes of well over a billion stars, extending reliable distance measurements across a large part of our galaxy. The importance of parallax reaches far beyond the immediate neighbourhood of the Sun. It supplies the foundation for an entire sequence of distance-measuring techniques, often described as the cosmic distance ladder. Each higher rung relies on objects whose distances were first established, directly or indirectly, by parallax. If the lowest rung were faulty, every measurement built upon it would be thrown into doubt, including estimates of the size and age of the universe itself. For this reason, the patient refinement of parallax measurements continues to occupy astronomers, who recognise that the grandest conclusions about the cosmos ultimately rest on the careful observation of small angles.