From Hipparchus to Hipparcos: A sonification of stellar catalogues
22 September 2016
ESA's Gaia mission recently released its first batch of data, based on observations obtained with the satellite during its first 14 months of science operations. This ambitious mission is the culmination of over two thousand years of astrometry – the science of charting the sky. A new sonification demonstrates in a novel way the remarkable progress that has been made in the lead up to the first data release from Gaia. |
| Astrometry through the ages. Credit: ESA |
Astronomy is one of the first natural sciences developed by early civilisations across the globe, and astrometry is one of the oldest branches of astronomy. Over the centuries, as technical advances were made and new instruments became available, the history of mapping the heavens gradually unfolded. Astronomers could see fainter and more distant celestial objects, measuring the position, brightness and colour of each star ever more accurately to determine their distance, their velocity through the Galaxy and their physical composition.
To convey the progress made in the field of astrometry, a new representation has been created using sonification – the practice of turning data into sound – by Jamie Ferguson in collaboration with ESA. Jamie first became interested in sonification during his Masters Degree studies in computer science, and he has recently applied this technique to the challenge of communicating astronomy to visually impaired students. He is soon to begin his PhD studies on using sonification in the context of astronomy research, education and outreach.
"The human ear is incredibly efficient at recognising patterns in sounds, making sonification an ideal tool for locating patterns and anomalies in extensive datasets that are common in astronomy – such as the enormous quantity of data contained in stellar catalogues," said Jamie.
"Sonification is not only an incredibly useful research tool in astronomy, but it is beginning to allow the field to be more open and engaging to people who are visually impaired and cannot interact with the common visual supports that accompany astronomy in the media."
After analysing a number of ancient and modern catalogues of stars, Jamie created a software to map the various astronomical parameters provided in each catalogue into different dimensions in a soundscape. The result is a short audio clip, From Hipparchus to Hipparcos: A sonification of stellar catalogues, which can be listened to and downloaded from ESA's SoundCloud channel.
This project was developed in the weeks leading to the first Gaia data release, so it portrays the status of astrometric catalogues prior to the advent of Gaia. An extension of the sonification, including data from the first Gaia release, will follow.
From Hipparchus to Hipparcos starts with a sonification of the ancient star catalogue compiled by Greek astronomer Hipparchus of Nicaea in the second century BCE and proceeds through several other historical catalogues, before ending with the Hipparcos Catalogue, published in 1997 and based on data from ESA's Hipparcos satellite, Gaia's predecessor and the first space mission dedicated to astrometry.
The improvement in the quantity and precision of data, as well as the increased information content and dimensions contained in each catalogue, are palpable as the sound clip evolves from the ancient Hipparchus to the modern Hipparcos.
For the most ancient five catalogues (see list below), which only list stellar positions and brightnesses, Jamie mapped each star in a virtual sound space based on its 2D position on the sky, and used the brightness of each star to modulate the pitch of each sound. He also added a background of white noise to convey the accuracy of the measurements – this becomes less audible for the more modern and accurate catalogues.
In the case of the first three catalogues (Hipparchus, Ulugh Beg and Tycho Brahe), the sound corresponding to each star is played individually; for the later, larger catalogues (Flamsteed, Lalande, the Yale Trigonometric Parallax Catalogue and the Hipparcos Catalogue), the sounds corresponding to more than one star are played simultaneously to compress the great number of stars contained in these catalogues in the same time length.
Since the distances to the stars were not known until the nineteenth century, the sounds all have the same volume in the first five catalogues. Volume is then used as an additional parameter in the final two catalogues (the Yale Trigonometric Parallax Catalogue and the Hipparcos Catalogue) to convey the stellar distances measured via parallax – the annual shift of a star's position in the sky, caused by Earth's motion around the Sun and inversely proportional to the star's distance.
The modern catalogues contain a wealth of information beyond stellar positions and distances, so Jamie had to create additional sounds to convey the extra dimensions probed by these catalogues. He added a 'hiss' sound, which is played once for every 5 iterations of the main sound, to represent the stellar colours (which are, in turn, indicative of stellar temperatures). The frequency of the hiss is lower for bluer/hotter stars and higher for redder/cooler ones.
For the Hipparcos Catalogue, at the end of the clip, Jamie encoded information about the proper motion of the stars – the projection on the sky of the star's true motion through space – in the duration of the hiss, which is short for fast-moving stars and longer for slower-moving stars.
More information about the sampled catalogues can be found below.
From Hipparchus to Hipparcos: A sonification of stellar catalogues - details of the audio file
The sonification of each catalogue lasts about 15 seconds in the sound clip.
Instruments: Naked eye observations, astrolabes, sextants and quadrants
1) Hipparchus of Nicaea compiled the oldest known comprehensive catalogue of stellar positions in the second century BCE. His work, based on even more ancient observations from Assyro-Babylonian astronomers, was handed down three hundred years later by Ptolemy, in his second-century treatise known as the Almagest, 'The Greatest'.
Hipparchus's catalogue was compiled with naked-eye observations and the few instruments available at the time (gnomons, astrolabes, and armillary spheres). It lists the positions of 850 stars with a precision of less than one degree [1].
Parameters used to create the sound: Pitch (star brightness) and location in the sound space (star position: right ascension and declination); background noise to convey accuracy of measurements. Each star is played individually.
2) In the fifteenth century, Ulugh Beg of the Timurid dynasty created a catalogue of 994 stars. Ruling over Central Asia, he was also an astronomer and mathematician, and constructed an enormous sextant with a radius of 36 metres in Samarkand, located in present-day Uzbekistan. Ulugh Beg's catalogue has a precision slightly better than that of Hipparchus.
Parameters used to create the sound: Pitch (star brightness) and location in the sound space (star position: right ascension and declination); background noise to convey accuracy of measurements. Each star is played individually.
3) In the late sixteenth century, Danish astronomer Tycho Brahe measured the positions of about 1000 stars with a precision of about one arcminute [2], using large quadrants and sextants at the Uraniborg observatory on the island of Hven (in present-day Sweden). His catalogue was completed in 1598 and published in 1627.
Parameters used to create the sound: Pitch (star brightness) and location in the sound space (2D position: right ascension and declination); background noise to convey accuracy of measurements. Each star is played individually.
Instruments: Telescope, quadrants, sextants
4) In 1725, English astronomer John Flamsteed published the first stellar catalogue compiled with the aid of a telescope, listing the positions of almost 3000 stars with a precision of 10-20 arcseconds.
Parameters used to create the sound: Pitch (star brightness) and location in the sound space (2D position: right ascension and declination); background noise to convey accuracy of measurements. Groups of two stars are played simultaneously.
5) In 1801, French astronomer Jérôme Lalande published an even greater catalogue with the position of 50 000 stars and a precision of around three arcseconds.
Parameters used to create the sound: Pitch (star brightness) and location in the sound space (2D position: right ascension and declination); background noise to convey accuracy of measurements. Groups of three stars are played simultaneously.
Instruments: Telescope, photography
6) The Yale Trigonometric Parallax Catalogue was published in 1995 by American astronomer William van Altena, who completed the work started by Frank Schlesinger in 1924 (who had made a huge step forward in the field thanks to progresses in the use of photographic plates) and continued by Louise Freeland Jenkins in 1952. It is the largest catalogue of stellar parallaxes compiled with ground-based telescopes, listing positions and parallaxes for over 8000 stars, with a precision of about 0.01 arcseconds.
Parameters used to create the sound: Pitch (star brightness) and location in the sound space (2D position: right ascension and declination); volume (distance); hiss (colour); background noise to convey accuracy of measurements. Groups of five stars are played simultaneously.
Instrument: Space telescope
7) ESA's Hipparcos mission, operating from 1989 to 1993, was the first space telescope devoted to measuring stellar positions. The Hipparcos catalogue, released in 1997, contains the position, parallax and proper motion of 117 955 stars with a precision of 0.001 arcseconds, allowing astronomers to probe stellar distances out to over 300 light-years.
Parameters used to create the sound: Pitch (star brightness) and location in the sound space (2D position: right ascension and declination); volume (distance); hiss (colour); length of hiss (proper motion). The very high accuracy of this catalogue is conveyed by there being no background noise. Groups of ten stars are played simultaneously.
[1] One degree is twice the angular size of the full Moon, or the width of one's little finger seen at arm's length.
[2] Minutes, or seconds, of arc are used to describe very small angles. An arcminute is 1/60 of a degree and an arcsecond is 1/60 of an arcminute. One arcminute is roughly the width of the edge of a bank card seen from a distance of 2.5m; one arcsecond is the width of a human hair seen from a distance of 10 m.
Read more in the History of Astrometry series: http://sci.esa.int/gaia/history-of-astrometry
For further information, please contact:
Jamie Ferguson
Email: j.ferguson.4
research.gla.ac.uk
Website: http://jfergusoncompsci.co.uk
