The light from APM 08279+5255 took 12 billion years to reach Earth and started its journey when the universe was just a tenth of its current age. This quasar is more than 12 billion light-years from Earth. Barely detectable to the human eye was the faint glow of the quasar APM 08279+5255. One evening, at Mount Lemmon Observatory in Arizona, I used an eyepiece and 1.5-m telescope to peer across the universe. The Large Magellanic Cloud, which can be seen on very dark nights, is 160,000 light-years away!Īstronomers regularly observe galaxies that are billions of light-years away, and I have even seen such distant objects with my own eyes. Credit: Sloan Digital Sky SurveyĮven nearby galaxies are at vast distances compared to the stars we see at night. Quasar APM 08279+5255, the yellow dot at the centre of this image, is 12 billion light-years from Earth. That means we are viewing Alpha Centauri as it was four years ago. ![]() Alpha Centauri, the brightest of "The Pointers" near the Southern Cross, is over four light-years away. The light-year is the distance light travels in a year in the emptiness of space, and is just shy of 10 trillion kilometres (or 10 13 kilometres).ĭespite a light-year being a vast distance, even the nearest stars are several light-years away. When we look at the moon, we are seeing it as it was a second ago, because the light takes a second to travel from the moon to us. Our eyes and telescopes are time machines for looking into the past. That means the further we look, the further we look back in time. Light travels almost a million times faster than sound, a little under 300,000 kilometres every second, or just over a billion kilometres an hour. The speed of sound is roughly 340 metres every second, or 1,200 kilometres an hour. There is a flash of lightning and some seconds later we hear the roll of thunder. ![]() We are comfortable with the finite speed of sound. The light-year is a more comprehensible measure of distance than the parsec – the catch being (and it is a big one) that you need to accept the finite speed of light and time machines. Last month, the Cassini spacecraft took a photo of Earth from Saturn, and our home was reduced to a mere dot: Ten is a reassuringly small number, but remember 10 AU is 1.5 billion kilometres. Saturn, the most distant planet we can easily see, is at a distance of 10 AU. The planet Venus, which is now the brightest "star" in the early evening sky, can be a mere 0.28 AU from Earth. ![]() Our solar system fits snugly within tens of Astronomical Units, with Neptune being roughly 30 AU away. By construction, that's almost the exact distance from Earth to the sun. It corresponds to the (almost) comprehensible distance of 149.6 million kilometres. The starting point is the Astronomical Unit, often abbreviated to AU. So, to cope with the overwhelming bigness of space, astronomers have devised a series of really big units. Could you memorise that the Large Magellanic Cloud is 1.5x10 18 kilometres away? I start losing track somewhere around the 14 th nought. And if you aren't familiar with scientific notation, that's 40,000,000,000,000 kilometres, or 40 trillion kilometres.Īlmost everyone struggles with numbers so unbelievably big, including astronomers. The distance to the nearest stars? They are more than 4x10 13 kilometres away. The distance to the sun? Just under 150 million kilometres.
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