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Large Distances

What is the largest distance we can measure? It is definitely a stretch of one’s imagination to think more on how our perception of length changes with scale. How far we can go? Measuring the very, very big and the very, very small requires using units that can be hard to imagine. Keep these rough comparisons in mind next time you come across a mind-bending scale.

A parsec

A parsec is a unit of length that measures long, long distances. It's typically used for objects far beyond our own Solar System. The word parsec is a portmanteau for 'parallax arcsecond', and describes the distance from the Sun to an object that seems to shift by a set amount (called an arcsecond) against a background of stars as the Earth orbits. It's an equivalent of just over 30,000,000,000,000 kilometres, or nearly 20,000,000,000,000 miles. To get an idea of how far that is, the star nearest to the Sun - Proxima Centauri - is about 1.3 parsecs away.

If you're still not sure how big that distance is, it'd take around 3.8 million years to fly this distance in an Airbus. Be sure to pack extra socks if you do.

A light-year

Confusingly, a light-year sounds more like a unit of time than a distance. It is in fact the distance light covers in one year as it moves unobstructed through a vacuum. It's a number large enough to be useful for astronomical distances on the scale of stars. Since light zips along at an astonishing 299,792,458 metres per second, it can cover a top distance of 9,460,700,000,000 kilometres (about 5,878,600,000,000 miles) in 365 (and a quarter) days.

Travel a single light-year from here, and you'd find yourself somewhere near the Solar System's very icy outer fringes of ice and dust called the Oort Cloud, or roughly a quarter of the way towards our closest neighbouring star. If you could Uber your way out there, it would take your driver 10 billion years to cover that single light-year, which would set you back

about $80 if it's off peak.

An astronomical unit (AU)

Astronomical units are used to describe distances on the scale of planets and solar systems. It's roughly the distance from Earth to the Sun, which amounts to around 150,000,000 kilometres, or 93,000,000 miles … give or take. For sticklers over precision, since 2012 it's been defined as exactly 149,597,870,700 metres.

If you started cycling towards the Sun now, you could make it to its photosphere, about 1AU away, by around the year 3000. Maybe sooner if you don't skip leg day at the gym.

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