When David Gill was a teenager, he helped to set up an observatory in Aberdeen University. By carefully observing the transition of stars across the stellar meridian he was able to calibrate a clock in the observatory with great accuracy. He rigged the clock to generate an electrical pulse once per second. He then ran cables over 3 kilometres to take the signal to his father’s shop. And then connected a clock in the shop to synchronise it with the master clock.
Navigating ships was a difficult problem on the 19th Century.
Knowing your longitudinal position was difficult and needed two things.
A clock that keeps accurate time on a ship and an accurate reference to set the clock to.
English clockmaker John Harrison solved the first part of that problem - a sophisticated clock that could handle the movements of ships, even in stormy seas.
From 1829, various balls were installed on roof top poles in British Port towns to provide a visible time indicator to ships in port or nearby. The one in Edinburgh still drops at 13:00 each day. But it wasn’t very accurate – and accuracy matters in navigation – 20 seconds inaccuracy could create a 500 metre position error.
Gill’s shop became renowned as being the place to go to get accurate time - far superior to Edinburgh’s one o’clock gun! Ship’s Captains would make a pilgrimage to Gill’s to share a story and reset their watches.
One of Gill’s key astronomy tools was the heliometer, an instrument capable of splitting an image of a celestial object to measure angular separations with extreme precision. Gill used this instrument during his 1877 expedition to Ascension Island to observe the parallax of Mars. Parallax is the difference in angles when objects are viewed from different positions.
This procedure had first been proposed by an earlier Aberdonian, James Gregory, in 1663.
He calculated the parallax angle of Mars and, from that, the dimensions of the Astronomical Unit – the distance between Earth and the Sun. His measurements were 50 times more accurate than anyone before. An accurate AU allowed the distances to, and positions of, the other planets to be better defined.
If you want to see Gill's heliometer - take a look in Provost Skene's House.
(Our trail on Museums, Galleries and Monuments will give you more information.)
Provost Skene’s House is open April - October, Thursday to Monday 10am-5pm, (Sunday 11am-4pm) Closed Tuesday, Wednesday. Free entry, no pre-booking required.
Watchmaking Skills
The craft of watchmaking demands exceptional precision and the ability to work with intricate mechanical systems. These skills translated directly to Gill’s ability to design, calibrate, and use precision instruments for astronomy, such as heliometers and telescopes. His watchmaking experience likely enhanced his ability to understand and improve the fine mechanical adjustments required for these devices.
Gill also excelled in measuring stellar parallax, the apparent shift in a star’s position due to Earth’s motion around the Sun.
It was the inability to see any star parallax that led some Ancient Greek astronomers to discount the idea of a heliocentric centric solar system.
Using photographic methods, he achieved measurements far more precise than his contemporaries. By photographing stars and comparing their positions against a fixed background over time, he reduced human error and increased measurement reliability. His work revealed the distances to nearby stars with an accuracy that revolutionised our understanding of the universe’s scale.
One of Gill’s most notable achievements was his work at the Royal Observatory in Cape Town, South Africa, where he served as Her Majesty’s Astronomer from 1879 to 1907. Gill revolutionised celestial observation by using photography to create detailed and precise star maps. Gill’s photographic atlas of the Milky Way was one of the first to showcase the distribution of stars, revealing the vast structure of our galaxy.
Gill’s work marked a turning point in astronomical methodology. By integrating photographic techniques and precision instruments, he enabled more reliable data collection and expanded the scope of celestial studies.
David Gill was also a pioneer of lunar photography, achieving remarkable results in an era when others struggled to capture celestial objects with precision. His success stemmed from his innovative use of technology, meticulous planning, and collaboration with skilled individuals. Gill collaborated with the renowned optical instrument maker Howard Grubb, who specialised in designing high-precision telescopes and instruments. Together, they developed advaned photographic techniques and instruments specifically suited for lunar imaging.
The Measure of the Man
In a lecture to the British Association for the Advancement of Science (BAAS) in 1884, Aberdeen watchmaker and astronomer David Gill proposed that metric units should be based on natural measures.
Gill suggested that the metric units, particularly the metre, should be linked to natural constants, such as the wavelength of light, rather than to a physical object like the standard metre bar. (Which was supposed to be one ten-millionth of the distance from the equator to the North Pole – but wasn’t really!)
People supported the concept – but it took a while…
So how were the units of the metric system originally defined?
The metric system was first developed during the French Revolution in the late 18th century to create a universal and rational system of measurement. The initial definitions of its basic units were based on natural phenomena and supposed to create readily reproducible standards.
1. Metre (m) – Unit of Length: Initial definition (1791):
Defined as one ten-millionth (1/10,000,000) of the distance from the equator to the North Pole along a meridian through Paris (i.e., a quadrant of the Earth’s circumference). This was an attempt to base the unit on the Earth’s size, making it a natural and universal reference. In 1799 a platinum prototype bar was made and used as the standard.
2. Kilogram (kg) – Unit of Mass: Initial definition (1795):
Defined as the mass of one litre (cubic decimetre) of pure water at its melting point (0°C). In 1799 a platinum cylinder—was created as the standard.
3. Second (s) – Unit of Time
Already in use before the metric system. Defined as 1⁄86,400 of a mean solar day (the average time it takes for the Earth to rotate once relative to the sun). (The second was not part of the original metric system but later adopted into SI in 1960 as one of the base units.)
The other SI units were added later.
4. Ampere (Electric current): Introduced 1881
Originally defined by the electromagnetic force between parallel one-metre long conductors in a vacuum.
5. Candela (Luminous intensity): Introduced 1948
Originally based on the luminous intensity of a black body at the temperature of melting platinum.
6. Kelvin (Thermodynamic temperature): Introduced 1954
Based on absolute zero and the triple point of water. Originally, 0 K was absolute zero, and 273.16 K was the triple point of water.
7.Mole (Amount of substance): Introduced 1971
Originally defined as the number of molecules in 12 grams of carbon-12.
Impact of Gill’s Proposal
Although Gill’s idea did not immediately replace the physical metre bar standard of the time, it started a movement toward defining units based on natural constants.
In 1960 the internationally agreed definition for One Metre was changed to a set number of wavelengths of a spectral line of Krypton-86.
In 1967 the definition of One Second changed to frequency of emitted radiation from Caesium-133.
Over 50 years later, and 135 years after the initial idea, on 20th May 2019 Gill’s idea was eventually implemented in full when the final physical prototype was abandoned - the kilogram.
Gill's concerns with measurement units began with the metre - he recognised that a small variation in the length of the metre could scale up to enormous differences for astronomical measurements. So it is fitting that in 2019 the definition of the metre was changed to the distance that light will travel in a vacuum in 1/299,792,458 of a second (1/c where c = the speed of light). Gill's professor, James Clerk Maxwell was the first to calculate the speed of light and confirming that it was a close match to the measured value by various pioneers.
Now all 7 fundamental SI units are based on naturally occurring measures rather than man-made objects.
Unit | Definition | (Cheeky) Comment |
Metre | Speed of light in vacuum | People went to great lengths to define this |
Kilogram | Planck constant | We had to weight for long enough for this definition |
Second | Transitions of Caesium-133 atoms | About time. |
Ampere | Elementary charge (e) | This is the current definition. |
Kelvin | Boltzmann constant | Absolute zero is OK. |
Mole | Avogadro’s number | You have to dig deep to understand this. |
Candela | Luminous efficacy of a specific frequency of light | Bright sparks will know this definition. |
Legacy of the Idea
Gill’s advocacy for natural measures contributed to the broader scientific discussion that culminated in the adoption of natural constants as the basis for modern measurement systems. Gill’s emphasis on light wavelengths as a basis for metric units played a pivotal role in advancing this concept within the astronomical and metrological communities.
The SI system is used worldwide although older measurement systems such as imperial units are used alongside metric measures in some countries. The base unit of length in the imperial system, the “inch”, was defined as being 25.4 mm in 1959 - all other imperial units of distance, area and volume are derived from this. Two countries have not formally adopted the metric system – Myanmar, where traditional units are used and the USA, apthough metric units are widely used there.
Taking things to extremes
Between 1793 and 1806, France adopted a decimal calendar. 10 hours in a day, 100 minutes in an hour, 100 seconds in a minute. 12 months of 30 days (3 decimal weeks) in a year with 5 or 6 added days at the end. It wasn’t popular and eventually abandoned in 1806.
