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The Scale of Things: Angles, Speeds and Distances in Astronomy

© Wesley Colley

One idea that is hard for astronomers to convey to the public is the overall scale of celestial objects in the Universe. Astronomers like to use units of arcseconds, parsecs and kilometers per second.

An arcsecond is an angular size, 1/3600 of a degree. That's about the width of a period in a newspaper article as seen from 100 yards away. Objects which subtend one arcsecond are about 200,000 times as distant as they are wide. The Hubble Space Telescope can distinguish objects separated by only 0.1 arcseconds; it could tell your height and shape from 200 kilometers away!

A parsec is the distance at which the radius of the earth's orbit around the sun subtends an angle of one arcsecond. Earth's orbit is 150 million kilometers in radius, so a parsec is 200,000 x 150 million = 30 trillion kilometers, so far that a light beam takes more than three years to get there, and that's still not as far as the closest star to the Sun!

A kilometer per second is a typical velocity in astronomy. That's about the speed of a bullet leaving a gun, or of a jet going at Mach 3. The earth's travels around the sun at 30 km/s (Mach 90). The sun is travelling around the galaxy at 220 km/s (Mach 660), and the galaxy is moving in the Universe at around 500 km/s (Mach 1500). Traveling at Mach 1000 (300 km/s), you would need 3000 years to traverse one parsec. The nearest galaxy is about one million parsecs away.

Since light can only travel so far so fast (300,000 km/s, or Mach One Million), looking back in distance becomes looking back in time. The light just takes that long to get here from that far away. By the time we look at something 3 billion parsecs (10 billion light-years) away, we are looking back 10 billion years into the past, which is roughly the epoch of the Creation of the Universe. For this reason, 3 gigaparsecs is called the "Hubble Length" and 10 billion years is called the "Hubble Time," after Edwin Hubble who first understood that the Universe is expanding. Since we clearly cannot see further back than Creation, this distance of 3 gigaparsecs sets the scale of the observable Universe.