Magazine article UNESCO Courier

The Scale of Space

Magazine article UNESCO Courier

The Scale of Space

Article excerpt

For the relatively short distances within the Solar System, astronomers can use the kilometre as a unit. But even the distance from the Earth to the Sun, 150 million km, is becoming a little too large to work with. Solar system astronomers often take this distance as their unit and call it "one astronomical unit". In this way, the distance to Pluto, the outermost planet, becomes a convenient 39 astronomical units.

Another measure of an astronomical distance is given by the time taken for light to traverse it. The Earth-Sun distance is 8-1/3 light-minutes; the diameter of Pluto's orbit is 11 light-hours. The latter dimension, while minuscule on the scale of the Galaxy, is similar to the size of the compact powerhouse within a quasar.

The nearest star, Proxima Centauri, lies 7,000 further off than Pluto, at 4.2 light-years. The brightest stars in the sky lie at a variety of distances, Sirius at 8.6 light-years, for example, and Arcturus at 36; Deneb (in Cygnus) is among the most distant of the "first magnitude" stars, at 1,500 light-years away. But this is just on our doorstep in the whole Galaxy: it is 30,000 light-years from the Sun to the Galaxy's centre; and the Milky Way is some 100,000 light-years in diameter. …

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