--- title: How we know: measuring the sky — a lesson of Mova's school · trees and pragmatics description: «How we know: measuring the sky», a lesson of the space course of Mova's primary school, sentence by sentence with dependency trees and pragmatics. It begins: Nobody has flown to a star with a tape measure, yet we know how far stars are and what they are made of: astronomers are detectives who read --- [Mova](https://md.mova.elamur.ai/free/en/) · [this page in HTML](https://mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/) [home](https://md.mova.elamur.ai/free/en/) / [corpora](https://md.mova.elamur.ai/free/en/corpora/) / [school](https://md.mova.elamur.ai/free/en/corpora/school/) / [Space](https://md.mova.elamur.ai/free/en/corpora/school/space/) / How we know: measuring the sky # How we know: measuring the sky Lesson 28 of the space course of Mova's primary school: 17 sentences, each with its syntax tree and pragmatics. The lessons are written for Mova, and their agreed trees are part of Mova corpus 0.4 (CC BY-SA 4.0). How Mova does on its tests: [the lesson at school](https://md.mova.elamur.ai/free/en/school-run/space/28-how-we-know-measuring-the-sky/). - [0](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/0/) — Nobody has flown to a star with a tape measure, yet we know how far stars are and what they are made of: astronomers are detectives who read clues in light. - [1](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/1/) — Hold a finger at arm's length and wink one eye, then the other: the finger jumps against the wall. - [2](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/2/) — Stars do the same as Earth moves round the Sun, and the smaller the jump, the farther the star. - [3](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/3/) — This is parallax. - [4](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/4/) — Split starlight into a rainbow with a prism and you see thin dark lines, like a barcode: each element (a basic kind of stuff, like hydrogen, helium or iron) leaves its own lines, so we can tell what a star is made of. - [5](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/5/) — Light from a galaxy rushing away is stretched towards red, like a siren dropping its pitch as an ambulance drives off: the bigger the stretch, the faster it goes. - [6](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/6/) — Some answers came first from pure reasoning: Neptune was found in 1846 exactly where maths said a hidden planet must be pulling Uranus. - [7](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/7/) — Astronomers measure distances to nearby stars by parallax: a star seems to shift slightly against far-off stars as Earth moves round the Sun; the farther apart the two places you look from, the bigger the shift. - [8](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/8/) — In 1838 Friedrich Bessel measured the parallax of the star 61 Cygni, the first star distance ever measured. - [9](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/9/) — The European Space Agency's Gaia spacecraft measured the positions and motions of nearly two billion stars before it was switched off in 2025. - [11](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/11/) — Splitting starlight into a rainbow, called a spectrum, shows dark lines that reveal which elements the star is made of. - [12](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/12/) — Helium was discovered in the Sun's spectrum in 1868, more than 25 years before it was found on Earth, and its name comes from the Greek word for Sun. - [13](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/13/) — In 1925 Cecilia Payne showed from star spectra that stars are made mostly of hydrogen and helium, a finding other astronomers at first refused to accept. - [14](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/14/) — Light from a galaxy moving away from us is stretched to redder colours, called redshift, and the amount tells how fast it is moving. - [15](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/15/) — Henrietta Leavitt found in 1912 that certain pulsing stars called Cepheids pulse more slowly the brighter they really are, letting astronomers measure great distances: if a Cepheid looks dimmer than its pulse says it really is, it must be farther away. - [16](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/16/) — Neptune was found in 1846 where Urbain Le Verrier predicted it should be, from the way Uranus was being pulled off course. - [17](https://md.mova.elamur.ai/free/en/corpora/school/space/28-how-we-know-measuring-the-sky/17/) — Astronomers can weigh the Sun by using how fast Earth goes round it and how far away it is: the heavier the Sun, the harder it pulls and the faster Earth must go round. - ← previous: [Telescopes and light](https://md.mova.elamur.ai/free/en/corpora/school/space/27-telescopes-and-light/) - next →: [The history of space exploration](https://md.mova.elamur.ai/free/en/corpora/school/space/29-the-history-of-space-exploration/) - ↑ back to [Space](https://md.mova.elamur.ai/free/en/corpora/school/space/)