There are about $400\,000\,\mathrm m$ by car from Copenhagen to Aalborg^{1} and $2\,600\,000\,\mathrm s$ in a month,^{2} while the thickness of a pencil stroke is $0.001\,\mathrm m$ and a lightning strike lasts $0.00003\,\mathrm s$.^{[2]} Let’s rather write these in

**kilo**metres:^{3}$400\,\mathrm{\mathbf km}$,**mega**seconds:^{4}$2.6 \,\mathrm{\mathbf Ms}$,**milli**metres:^{5}$1 \,\mathrm{\mathbf mm}$ and**micro**seconds:^{6}$30 \,\mathrm{\mathbf{\mu} m}$.^{7}

Big or small numbers become easier to write with such a **prefix** representing a thousand, a thousandth, a million, a millionth etc.

Prefixes^{[4]} |
||
---|---|---|

$\vdots$ | ||

Giga... | Billion | $1\,\mathrm G\ldots=1000\,000\,000\ldots$ |

Mega... | Million | $1\,\mathrm M\ldots=1000\,000\ldots$ |

Kilo... | Thousand | $1\,\mathrm k\ldots=1000\ldots$ |

Milli... | Thousandth | $1\,\mathrm m\ldots=0.001\ldots$ |

Micro... | Millionth | $1\,\mathrm \mu\ldots=0.000\,001\ldots$ |

Nano... | Billionth | $1\,\mathrm n\ldots=0.000\,000\,001\ldots$ |

$\vdots$ |

Stepping up/down a prefix makes the number a thousand times larger or smaller. This severally changes the number’s influence or “magnitude”. Let’s call each prefix step an **order of magnitude**. Saying ‘it is two orders of magnitude larger’ means something with 6 more zeros; something a million times larger. It’s a huge change.

For a feeling of scale in the case of size and time:

Size scale | |
---|---|

$\vdots$ | |

Gigametres | The Sun^{[9]} |

Megametres | The Earth^{[8]} |

Kilometres | Mountains^{[7]} |

Millimetres | Insects |

Micrometres | Bacteria^{[6]} |

Nanometres | Molecules^{[5]} |

$\vdots$ |

Time scale | |
---|---|

$\vdots$ | |

Gigaseconds | A lifetime^{11} |

Megaseconds | A vacation^{10} |

Kiloseconds | A movie^{9} |

Milliseconds | Blink of an eye^{[11]} |

Microseconds | A lightning strike^{[2]} |

Nanoseconds | Computer memory^{8} |

$\vdots$ |

It may sound odd to say ‘megametres’ and ‘kiloseconds’. We could also say for example ‘kilo-euros’. They are perfectly fine words; we just aren’t used to them.

You have surely heard about other prefixes as well such as **deci**metre and **centi**metre and maybe even **pico**second. Many more have been invented and collected into a standardised **international system of units**, abbreviated SI.^{12} A complete list of SI prefixes is found in Resource: Prefixes.

References:

- ‘
**Route Copenhagen-Aalborg**’ (web page), Google Maps, goo.gl/maps/GyeGdhyS4o41SWFT7 (accessed May 2nd, 2019) - ‘
**An Explanation of Lightning**’ (web page),*Dennis Mollet & James Kistner*, Arizona State University, www.public.asu.edu/~gbadams/lightning/lightning.html (accessed May 2nd, 2019) - ‘
**Online Etymology Dictionary**’ (dictionary),*Douglas Harper*, www.etymonline.com - ‘
**The International System of Units (SI)**’ (book),*B. Inglis, J. Ullrich and others*, Bureau International des Poids et Mesures (BIPM), 9th ed., 2019, www.bipm.org/utils/common/pdf/si-brochure/SI-Brochure-9.pdf, chapter 3 - ‘
**Biotechnology**’ (book),*P. David Clark & Nanette J. Pazdernik*, Newnes, 2nd ed., 2015, books.google.de/books?id=osqcBAAAQBAJ, ISBN 9780123850164 - ‘
**Small but Mighty: Cell Size and Bacteria**’ (article),*Petra Anne Levin (Department of Biology at Washington University) & Esther R. Angert (Department of Microbiology at Cornell University)*, Cold Spring Harbor Laboratory Press Biol, 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4484965/pdf/cshperspect-CSZ-a019216.pdf, pages 2-3 - ‘
**Highest Mountain in the World**’ (web page), Geology.com, www.geology.com/records/highest-mountain-in-the-world.shtml (accessed May 3rd, 2019) - ‘
**How Big Is Earth?**’ (web page),*Tim Sharp*, Space.com, 2017, www.space.com/17638-how-big-is-earth.html (accessed May 3rd, 2019) - ‘
**The Sun**’ (web page), Nasa, www.nasa.gov/sun (accessed May 3rd, 2019) - ‘
**RAM (Random Access Memory)**’ (web page),*Margaret Rouse, Stacey Peterson and others*, Tech Target: Search Storage, searchstorage.techtarget.com/definition/RAM-random-access-memory (accessed May 7th, 2019) - ‘
**Blink frequency and duration during perimetry and their relationship to test-retest threshold variability**’ (article),*Yanfang Wang (School of Biomedicine at Manchester University), Sonia S. Toor (School of Psychology at University of Reading) and others*, Investigative Ophthalmology and Visual Science, vol. 52, issue 7, 2011, iovs.arvojournals.org/article.aspx?articleid=2188061, DOI 10.1167/iovs.10-6553, pages 4546–4550 - ‘
**Guide for the Use of the International System of Units (SI)**’ (article),*Ambler Thompson & Barry N. Taylor*, National Institute of Standards and Technology, 2008, physics.nist.gov/cuu/pdf/sp811.pdf