Why did Uuq become Fl? Why is the sky blue? Why is the sky black? What is spaghettification?There's a problem with the typical quiz. It always features far too much sport, 1980s pop and celebrity gossip â and not nearly enough science.How Many Moons Does the Earth Have? is the ultimate solution. Test your knowledge to the limit with a sizzling collection of brain-stretching, science-based questions in two eight-round quizzes.Turn the page to get the answer immediately â and as each answer page explores the subject in more depth, this the only quiz that's just as entertaining to read from beginning to end as it is to play competitively.Where was the Big Bang? What links the elephant Tusko and Timothy Leary? What is the significance of 6EQUJ5? Science explainer extraordinaire Brian Clegg tells allâŚ
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The largest moon in the solar system is Jupiterâs moon Ganymede, which has a radius of around 2,600 kilometres, more than one third the size of the Earth.
There is evidence already of moons around planets in other solar systems.
The Earth has one moon
This may seem an obvious answer to a ridiculously easy question, but viewers of TV show QI have been told that it isnât true. While the show has been on air, the number they have provided has varied from 0 to 18,000 â but in reality, the obvious answer, 1, is the best.
The reason given for a large number is that lots of little lumps of rock get captured by Earthâs gravitational field for a few days and while captured are natural satellites, making them moons. The zero figure suggests that the Moon is a planet, not a moon, because it is unusually large compared with the Earth â but this decision is arbitrary and is not accepted by the astronomical community. (And as âthe Moonâ it is just a moon.)
There is not as definitive a definition of âmoonâ as there is of âplanetâ, but there are still clearly intended consequences from using the word âmoonâ. These are that the body in question should be:
Long-lasting â I suggest staying in orbit for at least 1,000 years
Sizeable â say at least 5 kilometres across
This would still allow moon status for the pretty dubious companions of Mars, Phobos and Deimos, which are about 20 kilometres and 10 kilometres across.
Clearly such rules are implied when we talk about moons. If the time rule didnât exist, then every meteor that spent a few seconds passing through our atmosphere would be a moon, while without the size rule, we would have to count every tiny piece of debris in Saturnâs rings as a moon â each is, after all, a natural satellite.
Further reading: Near-Earth Objects
QUESTION 2
Space Station blues
Weâve all seen astronauts floating around pretty much weightless on the International Space Station. What percentage of Earth normal is the gravity at the altitude of the ISS?
Answer overleaf
While youâre thinking âŚ
The first part of the International Space Station was launched in 1998.
The orbit of the ISS varies between 330km and 435km above the Earth â call it 350km for this exercise.
One of the favourite sections of the ISS for astronaut photographs is the Cupola, an observatory module that has been likened to looking out of the Millennium Falcon in Star Wars.
At the ISS, gravity is around 90 per cent Earth normal
Allow yourself a mark for anything between 88 and 92 per cent. Newton gives us a value for the gravitational attraction (F) between two bodies as: F=Gm1m2/r2.
We can use this to work out the difference between the ground and the ISS. Luckily, practically everything cancels out. G (gravitational constant) is the same, m1 (the mass of the Earth) is the same and m2 (the mass of a person) is the same. So the ratio of the gravitational forces ForceISS/ForceEarth is just r2Earth/r2ISS, where rEarth is the distance from the Earthâs centre to its surface and rISS the distance from the Earthâs centre to the ISS.
Weâre saying the ISS is 350 kilometres up. And the radius of the Earth is around 6,370 kilometres. That makes rISS equal to rEarth+350, or 6,720 kilometres. Not very different. So the ratio of the forces is (6,370 Ă 6,370)/(6,720 Ă 6,720) â which works out around 0.9. To be more precise, the force of gravity at 350km is 89.85 per cent of that on the Earthâs surface.
So how come the astronauts float around, pretty much weightless? Because the ISS is free-falling under the force of gravity â which means it cancels out the gravitational pull. It might seem something of a headline news event that the Space Station is falling towards the Earth, but thereâs another part to the story. The ISS is also travelling sideways. So it keeps missing.
Thatâs what an orbit is. The object falls towards Earth under the pull of gravity. But at the same time it is moving sideways at just the right speed to keep missing the Earth and stay at the same height. As a result every orbit has a specific velocity that a satellite needs to travel at to remain stable.
Further reading: Gravity
QUESTION 3
A question of dropping
Who dropped a hammer and a feather on the Moon to demonstrate that without air they fall at the same rate? (For a bonus â which mission was it?)
Answer overleaf
While youâre thinking âŚ
It is very unlikely that Galileo dropped balls of different weights off the Leaning Tower of Pisa to show they fall at the same rate. The story came from his assistant, shortly before Galileoâs death. Galileo was a great self-publicist and would surely have mentioned it had it been true.
What Galileo did do, though, was compare the rate of fall of pendulum bobs and balls of different weights rolling down an inclined plane â much easier than getting the timing right with the Leaning Tower.
The Ancient Greeks thought that heavier objects fall faster because they have more matter in them, and matter has a natural tendency to want to be in the centre of the universe. So with more matter, a heavy object should have more urgency in its attempt to reach its preferred place.
David R. Scott dropped a hammer and feather on the Moon
I will let you off the middle initial â and have a bonus point if you knew that the mission was Apollo 15. Scott beautifully demonstrated that the only reason a feather falls more slowly on the Earth is because of the resistance of the atmosphere. (You can see him in action here: http://youtu.be/KDp1tiUsZw8)
The Ancient Greeks were perfectly capable of trying this out (not the hammer and the feather on the Moon, but dropping similar sized balls of different weights), but it didnât fit with their approach to science, which was all about logical argument rather than observation and experiment.
Although Galileo did plenty of experiments, which mostly confirmed that different weights fall at the same speed, he also found a logical argument that would have worked for the Greeks if they had thought of it, and that would have enabled a much earlier development of an understanding of gravity.
Galileo imagined you had two balls of different weights, and the heavier did fall faster than the lighter one. You would equally expect a third ball of the combined weight of the two to fall faster still. But letâs make that third ball from two separate parts, one for each of the two original weights, joined by a piece of string. The heavier of the two should fall a bit more slowly than it would otherwise, because the lighter weight would slow it down. Similarly the lighter weight should fall a bit faster than it otherwise would. So, the connected weights should fall at an intermediate speed.
But that means the same weight, depending on whether or not it is split, has two totally different speeds â showing that the idea doesnât make sense.
