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Jupiter’s clock
Galileo (1564–1642) was the first person to see some of the moons of Jupiter, in January 1610. He
saw the four biggest moons, Io (pronounced ‘eye-O’), Europa, Ganymede and Callisto. Even today,
these four moons are sometimes called the ‘Galilean’ moons.
Galileo also tried to measure the speed of light. His plan was to have two people with lanterns.
One person would uncover their lantern. The second person would uncover theirs as soon as
they saw the light from the first lantern, and the first person would watch for the light from this
lantern. They would do this close together at first, so they could get used to their reaction times.
They would then try the experiment again, standing two or three kilometres apart. They would use
telescopes so they could see each other clearly. Galileo thought that the light should take longer to
travel the greater distance, but he could not detect a time lag.
Later that century, in 1676, a Danish astronomer called Olaus Roemer (1644–1710) was working
at the Paris Observatory. He was observing Io, and recording the times when it went behind
Jupiter. Io orbits Jupiter at a steady rate, so the times when it was eclipsed by Jupiter could be
predicted for several months ahead. But Roemer found that the predicted times were not always
correct. Eventually he noticed that at one time of the year the eclipses were up to 8 minutes early,
and about six months later they were 8 minutes late. This cycle repeated itself. Roemer realised
that this was connected with the orbit of the Earth.
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Roemer realised that the effect must be caused by the changing distance between the Earth and
Jupiter. Light took longer to travel from Io to the Earth when the Earth was further from Jupiter. He
used his measurements to calculate that the speed of light was 241 000 km/s.
orbit of Earth
J
E1
S
E2
not to scale
The eclipses were 8 minutes early when the Earth was at the closest position to
Jupiter (E1), and 8 minutes late when it was furthest from Jupiter (E2).
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Jupiter’s clock (continued)
You may find the following information helpful when answering the questions.
•
•
•
•
•
•
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a
radius of Earth’s orbit = 149 600 000 km
radius of Jupiter’s orbit = 778 300 000 km
length of Jupiter’s year = 11.86 Earth years
speed of light = 300 000 km/s
speed = distance ÷ time
time = distance ÷ speed
1 We know today that Jupiter has more than 60 moons. Why could Galileo only see four of them?
2 a If the two people in Galileo’s experiment stood 2 kilometres apart, how long would light take
to travel that distance and back?
b How long would the light take if they stood 4 kilometres apart?
c Explain why Galileo could not detect a time lag when he tried the experiment.
3 a Why were the eclipses of Io 8 minutes early when the Earth was at position E1, and 8 minutes
late when it was at position E2?
b Make a copy of the diagram on the previous page. Draw on it two positions for the Earth
when the eclipses of Io would have been ‘on time’.
4 Why wasn’t Roemer’s value for the speed of light the same as the value in the
information box?
5 a According to Roemer’s measurements, how long does light take to cross the distance between
E1 and E2?
b Use this time, and the distance for the radius of Earth’s orbit in the box, to calculate the speed
of light in metres per second. (Hint: There are two sets of units to convert.)
c Why isn’t your answer exactly the same as the answer in the information box?
6 The diagram you have copied into your book is not strictly correct. If Jupiter was at the position
shown when the Earth was at position E1, where would it be a year later when the Earth was at
E1 again? Mark its position on your diagram.
I CAN...
• explain how Galileo and Roemer attempted to calculate the speed
of light • use calculations to discuss accuracy.
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