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CALCULATING THE SPEED
OF LIGHT
In the 17th century, the Italian
astronomer Galileo tried to work
out how fast light travels. He asked
two people with lanterns to stand
on top of neighbouring hills. The
first person flashed their lantern
and when the second person saw
this, they uncovered their lantern
too. Using the time between the
two flashes, and knowing the
distance between the hills, Galileo
hoped to calculate the speed of
light. Unfortunately, the two hills
were less than a mile apart, making
the time gap too short to measure.
WHAT IS A LIGHT YEAR?
USING LIGHT FOR ‘TIME
TRAVEL’
In the 1670s, the Danish astronomer
Rømer used eclipses of Jupiter’s
moon, Io, to calculate how long it
took light to travel from the Sun to
the Earth. With this much greater
distance, he estimated it took
somewhere between 10 and 11
seconds. We now know it takes just
over 8 seconds, making Rømer’s
350-year-old estimate impressively
accurate. At such a speed, light can
travel all the way around the Earth’s
equator 7.5 times in a single second.
Phew!
Since light takes time to travel,
looking at things in the distance is a
bit like time travel. When you look
at the Moon, you see an image
which was true just over one second
ago because it has taken that time
for the light to travel from the Moon
to the Earth. If you use a special
filter to look at the Sun, you see a
snapshot of how it was 8 seconds
ago (never look directly at the
Sun, it can damage your eyes).
Use a telescope to look further
away, and you are looking even
further back in time.
Let’s imagine that we are standing
on a planet that is 67 million
light years from Earth. Let us also
imagine that we have an enormous
and very powerful telescope that is
perfectly focused on Earth. The
telescope would show us light
which left the Earth 67 million years
ago … that’s when Tyrannosaurus
and Triceratops roamed the land. We
would see dinosaurs! It is
theoretically possible, but first we’d
have to get 67 million light years
from Earth, then build a gigantic
telescope… (better get started,
ed) .
NEW DISCOVERIES
We are learning more about light all
the time! In the summer of 2018, the
Canadian Hydrogen Intensity
Mapping Experiment (CHIME)
discovered some interesting bursts
of photons coming from distant
galaxies. The team of scientists were
very excited by their discovery, and
by the time you read this they might
have announced why!
Strangely it is not an amount of
time, but a measure of distance. It is
the distance that light can travel in a
year, in certain conditions. It is equal
to about 9.5 trillion kilometres
(9 500 000 000 000 km) or 5.9 trillion
miles (5 900 000 000 000 mi).
Non-specialists use it to describe
astronomical distances because it
makes some sort of sense.
Astronomers generally describe
distances in parsecs (one parsec is
equal to about 3.26 light years).