The history of sky mapping

https://www.visualcinnamon.com/2020/04/designing-the-hubble-skymap/

We're going to look at the history of humanity's efforts to map the skies above them today.

• In which era do you think we'll start?

• What are some possible reasons why humans might have started tracking the skies above them?

Prehistory and the Bronze Age

The earliest sky-watchers left no writing behind, only buildings and objects. Ancient stone monuments such as Stonehenge in England and Newgrange in Ireland are built to face the sunrise on the shortest or longest day of the year. The Nebra sky disc, a bronze object from Germany made around 1600 BCE, seems to show a group of stars called the Pleiades, together with a system for connecting the calendar of the moon with the calendar of the sun.

Babylon: the first records

From about 1000 BCE, writers in Babylon began keeping records of eclipses and the positions of the moon and planets. They wrote them on clay tablets and continued for hundreds of years. They divided the sky into the twelve signs of the zodiac and created mathematical rules that could predict where a planet would appear. They had no theory about why the planets moved this way, but their predictions worked. Later civilisations built on this information.

Discussion

Have you heard the term pre-history before? What does it mean?

Most people are familiar with Stonehenge. Have you heard of Newgrange or any other similar structures?

The earliest attempts at mapping the skies were, it seems, attempts at making some kind of calendar. Why do you think these people wanted / needed a calendar?

Do you know anything about the Babylonians?

Do you find it remarkable that their twelve signs of the zodiac have survived into the modern day?

Where do you think we're going next?

Greek geometry

The Greeks wanted to know what shape the universe had. Around 130 BCE, Hipparchus made the first detailed catalogue of the stars. By comparing his results with the older Babylonian records, he discovered that the Earth's axis slowly moves in a circle, taking about 26,000 years to complete one turn. Around 150 CE, Ptolemy wrote the Almagest, which gave the position of about 1,000 stars. In his model, the Earth stood at the centre and the planets moved in small circles inside bigger circles. This idea was wrong, but its predictions were accurate enough to be used for the next 1,400 years. The Antikythera mechanism, a machine with bronze gears from about 100 BCE, shows that people were also building this knowledge into machines.

China, India and the Islamic world

Chinese astronomers kept records for longer than anyone else, writing down comets, new stars and spots on the sun for two thousand years. The Dunhuang star atlas, from around 700 CE, is the oldest star map on paper that still exists. In India, astronomers combined Greek geometry with their own mathematics. From the 9th century, scholars writing in Arabic, such as al-Battānī and al-Ṣūfī, corrected Ptolemy's numbers and made new star catalogues. They also improved the astrolabe, a metal instrument that worked like an early computer for the sky. The observatory of Ulugh Beg in Samarkand, built in the 1420s, produced measurements that nobody improved on for over a century.

Discussion

Have you heard of the names Hipparchus or Ptolemy before?

What are some of the other big names in Ancient Greece?

Do you have any idea how the Antikythera mechanism or the Astrolabe might have worked?

How surprised are you to see Samarkand (Uzbekistan) mentioned in this lesson?

The Chinese, famously, are known for their many firsts. Can you think of other examples of things that the Chinese came up with?

Where do you think we're going next?

The European revolution

In 1543, Copernicus put the sun, not the Earth, at the centre. This made the mathematics simpler, but at first it did not match the observations any better. In the 1580s and 1590s, Tycho Brahe measured the sky with his eyes alone, and did it more accurately than anyone before him. His careful measurements allowed Kepler, in 1609, to show that the planets move in ellipses rather than circles. At almost the same time, Galileo pointed a telescope at the sky and found moons going around Jupiter, and discovered that the Milky Way is made of separate stars.

Mapping becomes an industry

In the 18th and 19th centuries, sky mapping became large-scale, organised work. Measurements became so accurate that in 1838 Bessel calculated the distance to a star for the first time. From the 1880s, photography and an international project called the Carte du Ciel turned the sky into a permanent record that could be measured again and again. Scientists also began splitting starlight into its colours, which told them what stars are made of and how fast they are moving. Using these methods, Hubble showed in the 1920s that the other galaxies exist and are moving away from us.

Discussion

How familiar are you with the names Copernicus, Brahe, Kepler and Galileo?

"Galileo pointed a telescope at the sky and found moons going around Jupiter." Personally, I was blown away by that sentence. Were you, too?

How important do you think photography was for the sky mapping industry?

Only in the 1920s did we discover that other galaxies exist. Is it reasonable to assume that future generations will discover other universes?

The modern map

Today we can also study types of light that the human eye cannot see, such as radio waves and X-rays. Each one shows us a different sky. Satellites have removed the problem of the Earth's atmosphere. The European Gaia mission has now measured almost two billion stars in three dimensions, including how they move, so the map can be run forwards and backwards in time.

The main idea

The purpose of a sky map has changed over time. First it was a calendar. Then it became a tool for prediction. Later it became a model of how the universe works. Today it is a three-dimensional record of where everything is and how it moves. Most of these steps happened because measurement improved, not because someone had a new idea. Kepler needed Tycho's numbers, and modern astronomy needed photography.

Discussion

"The European Gaia mission has now measured almost two billion stars in three dimensions, including how they move, so the map can be run forwards and backwards in time." Again, how mindblowing uis this? Can you even picture what this might look like?

Every generation has probably thought, "Right, now we've reached the limit of what is possible to know" and have have been wrong. We are probably no different. Where do you think sky mapping will go from here?

Equipment

Gnomon (c. 3000 BCE) — A simple vertical stick. Its shadow gave the time of day, the direction of north, and the date of the solstices. Almost every later instrument grew out of it.

Antikythera mechanism (c. 100 BCE) – a box of bronze gears that calculated the positions of the sun and moon and predicted eclipses.

Astrolabe (from c. 800 CE) — A flat brass disc that modelled the turning sky. It was portable, and it could solve many different problems: the time, the direction of a city, the height of a star. It spread across the Islamic world and then into Europe.

The great graduated instruments (1420s–1590s) — Enormous quadrants and sextants, some built into walls or the ground, from Ulugh Beg in Samarkand to Tycho Brahe in Denmark. Bigger meant finer scales, and Tycho reached about one arcminute with his eyes alone. Kepler's ellipses came directly from these numbers.

Telescope (1608) — Suddenly the sky contained objects nobody had ever seen: the moons of Jupiter, the phases of Venus, and stars inside the Milky Way. It changed not just how accurately we could measure, but what there was to measure.

Meridian circle and pendulum clock (1650s–1690s) — A telescope fixed to point only north–south, plus a clock accurate enough to time stars crossing it. Together they made star catalogues far more accurate, and made distance measurement possible: the first parallax followed in 1838.

Spectroscope (widely used from the 1860s) — By splitting starlight into its colours, astronomers could tell what stars are made of and how fast they are moving towards or away from us. Astronomy stopped being only about position.

Photographic plate (standard from the 1880s) — The sky became a permanent record. A plate could collect light for hours, catching objects too faint for the eye, and it could be measured again years later by someone else.

Radio telescope (from 1932) — The first tool to observe light the eye cannot see. It opened the way to infrared, X-ray and gamma-ray astronomy, each revealing a different sky above the same stars.

The digital space telescope (Hubble 1990, Gaia 2013) — Electronic sensors replaced film in the 1970s, and satellites removed the blurring atmosphere completely. Gaia has now measured almost two billion stars in three dimensions, including their motion — a map that runs both forwards and backwards in time.