Tuesday 24 April 2018

How high is the sky?

Dr Justin Donnelly: Imagine shooting straight up in the air at 100 kilometres (km) per hour, which is about the speed that cars travel on a motorway.

After 30 seconds, you pass through a flock of birds and even though you're less than 1km up, you already shoot head first into the lowest clouds. If you've ever seen fog, you already know what it's like being inside a cloud.

After three minutes, you're 5km above the ground. At this height half of the weight of the atmosphere is already below you and the air is becoming thin. Its cold up here and the damp from the cloud starts to freeze into ice in your hair.

Six minutes after taking off and at 10km, you're higher than Everest. You whistle past a jet airliner.

After nine minutes (15km) you've passed through the last few scraps of cloud. Looking up, the Moon doesn't seem any closer, but it does seem brighter. Air scatters blue light around, which is why the sky looks blue. Up here there's less air, so the sky above is slowly turning from blue to black. The stars are starting to come out and you realise that you're approaching the edge of space. The atmosphere gets thinner and thinner until it gradually turns into space. And space is mostly just that -- empty space! The horizon looks curved now and you're beginning to see that the Earth is just a huge round ball with a thin blue layer of air around it.

After an hour you've reached a height of 100km, marking the official end of the atmosphere and the beginning of space. Most of the things you called "the sky" are now below you, but the Moon still doesn't look any closer! In fact, it's so far away that you'd have to continue on at the same speed for over five months to reach it (and over 170 years to reach the Sun)

You decide to move at the fastest speed possible -- the speed of light. At this speed, you could fly around the world seven times in just one second. Now you can zip past the Moon in about a second and arrive at the Sun in just eight minutes. Even at this speed, to reach the nearest star would take over four years. The stars are much further away than the Moon or Sun.

Irish Independent Supplement

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