Parsecs

When 'vast' is an understatement. (Vol. 2; Issue 49)

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The Stop

On Earth, measuring long distances is accomplished by using either miles (if you’re sticking with Imperial units) or kilometres (if you’ve joined the 21^st century and are using metric). However, the vastness of the space beyond our planet means these everyday units of measurement become so large that they are unwieldy and impractical. Astronomers have solved this problem by using other forms of measurement: astronomical units, light-years and parsecs.¹

Galaxy NGC4414 | ca. 48 million light-years in diameter | ca. 62 million light-years from Earth

The astronomic unit (AU) is a useful measurement for distances within the solar system. Defined as the distance from the Sun to the Earth’s orbit – about 93 million miles/150 million kilometres – the AU allows for large distances to be compressed and made more manageable. For example, there are 484 million miles/740.79 million km between the Sun and Jupiter. Rather than writing these numbers out when referencing this distance, the shorthand of the astronomical unit allows it to be expressed as 5.2 AU. Similarly, instead of writing out the distance between the Sun and the body-formerly-known-as-a-planet Pluto as 3,700,000,000 miles/5,900,000,000 km, it is far easier to express this as 39.8 AU.

At greater interstellar distances, astronomical units become unwieldy, so light-years are used in their place. Defined as the distance a photon of light can travel in a vacuum at its constant speed of 186,000 miles/300,000 km per second in a year (about 6 trillion miles/9 trillion km), a single light-year corresponds to about 63,000 AU. By compressing this distance into another unit, the light-year allows expression of much greater distances to be more manageable. For example, the star system nearest to ours is the triple-star system Alpha Centauri. This system is roughly 25 trillion miles/40 trillion km from Earth, a distance of about 272,900 AU – a number far simpler to compress and write as 4.3 light-years.²

However, even light-years quickly become impractical as units of measurement when their number climbs into the high thousands or millions. For these purposes, astronomers use parsecs – a measurement equivalent to 3.26 light-years, or around 19 trillion miles/31 trillion km. Using this unit, the 4.3 light-years between Earth and Alpha Centauri become 1.3 parsecs and the distance to Sirius becomes 2.6 parsecs instead of the more cumbersome 8.6 light-years or 197,800 AU.³

Nevertheless, due to the vastness of the universe, even the parsec must be adapted: for measurements within galaxies, astronomers use ‘kiloparsecs’ – 1,000 parsecs – and for distances between galaxies and galaxy clusters, ‘megaparsecs’ – one million parsecs – are employed. As an illustration, the Sun is around 8.3 kiloparsecs (27,058 light-years) from the centre of the Milky Way, which is itself over 31 kiloparsecs (101,060 light-years) across. These are certainly large distances, but they pale in comparison to the distance from Earth to the Andromeda Galaxy (2.54 million light-years, or about 0.77 megaparsec) or the Sombrero Galaxy (29.35 million light-years, or about 9 megaparsecs), and even these are a drop in the proverbial bucket when compared to the farthest galaxies and quasars whose distances exceed 13 billion light-years - enormous numbers best expressed in the compressed form of ‘about 4,000 megaparsecs.’ Vast distances, indeed.

The Detour

Today’s Detour - for the first time in The Bus Volume 2 - is to a text article on the web, but I found it interesting and worth the read: ‘Why Thomas Pynchon’s ‘Gravity’s Rainbow’, now 50 years old, is the ‘Iliad’ of our times.’ Originally published on The Conversation, Julien Murphet⁴ argues that the notoriously difficult novel is worthy of inclusion amongst literature’s classics. As a fan of the book - I’ve struggled with it for years, have made many failed starts, and have only completed it once⁵ - I tend to think it’s one of the finest ever written. This doesn’t mean it’s easy, that it’s perfect or that it’s to everyone’s taste - but why does that matter? Food for thought.

'Gravity's Rainbow' - the 'Iliad' of our times.

The Recommendation

Today’s recommendation is Stanley Kubrick’s 2001: A Space Odyssey (1968). Written by Kubrick and science fiction author Arthur C. Clarke, the film is considered one of the greatest ever made. Using minimal dialogue, long sequences accompanied only by music, pioneering special effects and a slavish dedication to a scientifically accurate depiction of spaceflight, the film addresses themes including technology, human evolution, artificial intelligence and the possibility of extraterrestrial life. It is a truly amazing film - and highly recommended.

2001: A Space Odyssey (1968) Trailer

2001: A Space Odyssey streams on various platforms.

The Sounds

Today’s playlist is composed of five tracks that might be appropriate if you’re stuck on a journey over a parsec or two: ‘Novocaine for the Soul’ (Eels, 1996), ‘Flying’ (The Beatles, 1967), ‘Float On’ (Modest Mouse, 2004), ‘Feels Like We Only Go Backwards’ (Tame Impala, 2012) and ‘Smile the Day After Today’ (Black Moth Super Rainbow, 2009). Enjoy!

The Thought

Today’s Thought is from the American novelist Kurt Vonnegut (1922-2007):⁶

‘We are here on Earth to fart around, and don’t let anybody tell you different.’

If you have a thought on this Thought - or any part of today’s issue - please leave a comment below:

And that’s the end of this Stop - I hope you enjoyed the diversion!

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Until the next Stop …

1. Now, I remain a big fan of the original Star Wars films (especially The Empire Strikes Back), but the science doesn’t stand up to much scrutiny. Not that they need to – that was never their point – but occasionally there’s an error that makes you wonder why no one noticed it. Such as when Han Solo boasts to Luke Skywalker and Obi Wan Kenobi that the Millennium Falcon is fast because it ‘made the Kessel Run in under 12 parsecs.’ Which sounds great but is impossible – because a parsec is a unit of distance, not time. Now, if he’d said the Falcon had done 12 parsecs in under a light-year .... Anyway, though I have an interest in mathematics, I’m in no way a mathematician - and, despite checking my conversions, I’m not completely sure they’re error free. Nevertheless, a good bit of reading backs up today’s issue, and the sources include: BBC Science Focus, Parsec (Britannica), Astronomical Unit ... Overhaul (Scientific American), Light-year (Britannica) and Cox, Brian and Andrew Cohen. Wonders of the Universe. London: Harper Collins, 2011. To calculate very large numbers, check out: Big Number Calculator (calculator.net).

2. Light-years also provide another perspective on interstellar distances. For example, the Sun is about eight light-minutes from Earth (there are also light-seconds and light-hours), so the sunlight tanning you as you sit by the pool is as it was when it left the Sun eight minutes earlier. Similarly, when Jupiter was visible in the night sky a few weeks ago, the light seen from Earth was reflected off the planet between 35 – 52 minutes earlier (depending on whether the Earth is on the same side of the Sun as Jupiter) – and the light seen through a telescope trained on Alpha Centauri is at it was when it left that system 4.3 years ago.

3. And far less cumbersome than writing out 18,385,400,000,00 miles/29,670,000,000,000 km.

4. Murphet is Jury Professor of English and Language and Literature at the University of Adelaide, Australia.

5. Though I’m still none the wiser as to most of what’s going on, I found Steven C. Weisenburger’s A Gravity’s Rainbow Companion indispensable. If you’re not the type who enjoys a novel that requires a handbook, GR probably isn’t for you - but it doesn’t reduce the book’s importance and place amongst canonical literature.

6. Vonnegut - most famous for his novel Slaughterhouse Five; or, The Children’s Crusade (1969) - is quite the character, and the topic of an issue in the upcoming Volume 3 of The Bus. In the meantime, for more information see: Vonnegut (Britannica).