What is a light year?

Wednesday, 12 September 2018

– Technology

by Contributed ItemContributed Item

A few weeks ago, a contestant on a popular TV quiz programme was asked this question and she was given three captioned options from which to choose her answer:

Is it a measurement of time, distance or temperature? As I recall, the lady quickly discarded option three (temperature) but pondered for some little time over the other two before finally opting for time. Her answer was locked in and, after the usual short wait to build suspense, it was shown to be wrong. Distance was highlighted as the correct answer.

So why is a word relating to time (year) used in a measurement of distance, causing understandable confusion to quiz contestants?

Well, the stars are so far away from us that if we simply express their distances in miles and kilometres, we end up with impossibly huge numbers, so instead, astronomers speak of their distances in light-years.

Light is the fastest-moving stuff in the universe. In old money, it travels at an incredible 186,000 miles per second. That’s very fast. If you could travel at the speed of light, you would be able to circle the Earth’s equator nearly eight times in just one second.

The Earth’s equator is almost 25,000 miles so eight times that is 200,000 miles. A light-second is defined as the distance light travels in one second, i.e. approx. 200,000 miles. The Moon is 230,000 miles away from Earth, so that’s a distance of just over one light second.

Following on from this, a light-year is the distance light travels in one year. How far is that? Multiply the number of seconds in one year by the number of miles that light travels in one second, and there you have it: one light-year is very nearly six trillion miles.

The 20th century astronomer Robert Burnham devised an ingenious way to portray the distance of one light-year and of expressing the distance scale of the universe in simple, understandable terms. He did this by relating the light-year to the distance from the Sun to the Earth. That’s just about 93 million miles – a distance of eight light-minutes (if the Sun were ever to blow up or go out, we wouldn’t actually know about it until eight minutes after the event).

Robert Burnham noticed that, quite by coincidence, the number of Earth-Sun distances in one light-year and the number of inches in one mile are virtually the same. There are 63,000 Earth-Sun distances in one light-year, and 63,000 inches in one mile. This wonderful coincidence enabled him to bring the light-year down to Earth.

If we think of the Earth-Sun distance as just one inch then, on this scale, a light year represents one mile. The closest star to Earth, other than the Sun, is Alpha Centauri at some 4.4 light-years away. Thinking of the Earth-Sun distance as one inch places this star at a distance of 4.4 miles. Using this same scale, here are the distances to some other rather well known heavenly objects.

Sirius: 9 miles; Vega: 25 miles; Betelgeuse, the orange star in the constellation Orion, 427 miles; the black hole at the centre of our galaxy, 27,000 miles; and Andromeda, our nearest galactic neighbour comprising hundreds of billions of stars, 2.3 million miles.

Andromeda, incidentally, is visible to the naked eye in the northern sky as a faint smudge and now is a good time to look for it. You’ll need a truly dark sky and a keen eye, but you can find it by star hopping from the constellation Cassiopeia, the Queen. Cassiopeia is very easily identified in the northeast sky at nightfall as a very distinct W shape. You’ll note that one half of the W is more deeply notched than the other half. This deeper V is like an arrow in the sky, pointing directly towards Andromeda at a distance of just over twice the length of one side of the arrowhead (notch).

If, like mine, your eyes are not what they once were, then binoculars will certainly help. If you can spot it, remember that the actual light you are seeing began its journey 2.3 million years ago.

Norman Lloyd

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