Page 30 - BBC Sky at Night Beginners Guide to Astronomy - 2017 UK

So, kilometres or miles are confi ned to the (186,282 miles per second), light can cover
Solar System. We could use something better, 9,460,000,000,000 (9.46 trillion) km, or
but we’re safe and happy with them and there’s 5,860,000,000,000 (5.86 trillion) miles in a year.
some continuity with how we do things on Earth With this unit at hand we can say that Proxima
without getting too space-ifi ed too early on. Centauri is 4.26 lightyears away.
The other ‘something better’ is simply a unit
based on Earth’s mean distance from the Sun – TRAVELS WITH LIGHT MIRRORS ON
instead of it being 150 million km we can say it is Looking into the night sky, most of the stars THE MOON
one astronomical unit or 1 AU. Using this method, we see are tens to hundreds of lightyears away,
all the planets, and anything else fl ying around although there are a few that reach into the
the Sun (or Earth) can be defi ned in terms of an thousands. When we think about how light travels
astronomical unit. For example, Jupiter is about from a star that is, say, 80 lightyears away, we
5 AU from the Sun; that’s fi ve times farther realise that we are seeing that star as
from the Sun than Earth. Travelling out farther, it appeared 80 years ago.
the nearest star, Proxima Centauri, is around The light has been travelling through space
268,000 AU away. You’ll have gathered that the for all that time before it ends up in our eyes;
numbers are growing again, which means we therefore, when we gaze into space what we are
need to switch to another scale – the one that is actually doing is looking back in time.
used by all but the hardiest of scientists. Enter, Don’t think this means any of the stars are The Apollo astronauts left
stage right, the lightyear. not there any longer. Stars live for millions, if laser retrorefl ectors on the
This wonderful length is simply the distance not billions, of years, so even a star that is 5,000 lunar surface
that light travels in one year. Speeding along lightyears away in the night sky is still really Nowadays the distance
at almost 300,000 kilometres per second shining away quite happily. to the Moon is easy to
measure, thanks to the
Apollo astronauts, who
THE SOLAR SYSTEM left some experiments
there back in the early
AND BEYOND 1970s. These were the
marvellously named
‘lunar laser ranging
Local distances are measured by the time it takes light to get there retrorefl ector arrays’
– mirrors, to you and
me. These mirrors have
been used constantly ever
since, and are now the
PAUL WOOTTON; NASA/JPL, SOHO/ESA/NASA, NASA/CXC/SAO, NASA, NASA/ESA/S.BECKWITH (STSCI)/HUDF, NASA/GSFC
longest working Apollo
lunar experiments.
A laser beam from
Earth is fi red at one of
the mirrors, which refl ects
it. The time it takes to
get there and back gives
you the distance to the
Moon, if you know the
MOON MARS speed of light. From these
1.5 lightseconds 5.4 lightminutes measurements we know
the distance to the Moon:
405,696km (252,088
miles) when it’s furthest
away, and 363,104km
(225,622 miles) when it’s
closest. We also know
that the Moon is moving
away from us at about
4cm (1.5 inches) per year.

SUN PLUTO PROXIMA CENTAURI
8.3 lightminutes 5.5 lighthours 4.26 lightyears

30 skyatnightmagazine.com 2012