Parsec distance lightyears11/9/2023 ![]() Although parsecs are used for the shorter distances within the Milky Way, multiples of parsecs are required for the larger scales in the universe, including kiloparsecs (kpc) for the more distant objects within and around the Milky Way, megaparsecs (Mpc) for mid-distance galaxies, and gigaparsecs (Gpc) for many quasars and the most distant galaxies. Partly for this reason, it is the unit preferred in astronomy and astrophysics, though the light-year remains prominent in popular science texts and common usage. ![]() The word parsec is a portmanteau of "parallax of one second" and was coined by the British astronomer Herbert Hall Turner in 1913 to simplify astronomers' calculations of astronomical distances from only raw observational data. That means we are viewing Alpha Centauri as it was four years ago.A parsec is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond (not to scale) 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. And why Star Wars nerds have spent countless hours trying to explain away this broken piece of techno-babble dialogue. Perhaps this is why Han Solo talks about parsecs as if they are units of time, not distance. The parsec is what astronomers use every day but, being 206,265 AU and ending in "sec", it isn't the most easily understood unit of distance. 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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