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...change the rpm of the crankshaft to turn the wheels at different rates so that the car can move at varying speeds. The downside of this is that you have less driving force (torque), which is why it’s easier to accelerate using lower gears. It’s a bit like riding a bike, where it takes less effort to go up a hill in low gear, but you don’t go as far. The Ice Cold In...

...change their position night by night and can look like an arrow pointing at the ground. They were much loved by medieval artists. Planetary conjunction The planets in our solar system look like bright stars and move in relation to the familiar backdrop of constellations in our night sky. Sometimes two or three planets may appear to come close together looking like they...

...change its chemical characteristics through time, and therefore so will the zircon crystal, growing continuous concentric ‘zones’ of progressively different chemistry, until the molten rock has cooled enough and has solidified. Each of these zones, therefore, preserves a chapter in the life of the rock. [Zircon] This zircon grain is smaller than a grain of sand. This...

...changed over the 60 years of Mars exploration and will continue to adapt to new scientific understanding of the planet and of life itself. However, the central principle is to minimise the amount of biological material (bioburden) carried by a spacecraft that might contaminate the martian surface. Setting standards It was required that the Viking landers that reached Mars...