52 Weeks of Historical How-To’s, Week 51: How to discover a planet

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Astronomer Johannes Hevelius at his quadrant from his Machina coelestis (1679)

Humans have been looking to the night sky for millennia in awe and wonder of the heavens above us, or in contemplation of the complex forces that guide the seemingly clockwork nature of our universe. The basic make-up of our solar system (Mercury, Venus, the Moon, Mars, Jupiter and Saturn) has been understood since the age of the Babylonian astronomers; however observation of the night skies in search of new planets through the aid of a telescope is a practice with its roots in the 17th century. Luckily for us, the vaults of Special Collections are chock full of some of the greatest works of astronomical observation during this period.

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Some of the greatest astronomical works of the 16th and 17th century held in our collections include: (from left to right) Copernicus’s De revolutionibus (1543); Galileo’s Siderius Nuncius (1610); Kepler’s Astronomia nova (1609); Huygens’s Systema Saturnium (1659)

Instead of trying to work out how new planets are discovered in today’s astronomical world, we’ve decided to consult with some local experts. Less than a quarter of a mile from our special collections store sits a complex of five observatory domes and the heart of astronomical observations at the University of St Andrews. Sitting quietly among these is not only the largest telescope in Scotland but the largest working optical telescope in the UK, the 37 inch James Gregory Telescope (JGT); and, in truth, this blog post was just an excuse to go visit this wonderful resource and learn more about its history and its function today.

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Set of five telescopes located at the west end of St Andrews.

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The UK’s largest optical telescope, the James Gregory Telescope and two smaller observatories with their domes open. Image credit Joe Llama.

Astronomical observation has gone through exceptional changes since the early 1990s. Up to that point astronomers were still using photographic glass plate negatives and view-finders to track the slightest changes in the sky. A skilled telescope operator would spend hours in the dark of the night exposing and handling these delicate plates, ensuring quality images. With the onset of more light-sensitive digital imaging technology not only has the capturing of images from a large-format telescope become much easier and more efficient, but also immediate circulation of data has become possible. Now nearly everything is run through computers and connected to a global network of other observatories all collecting and sharing data.


James Gregory Telescope, University of St Andrews.

Some telescopes here at St Andrews are used solely for teaching. However, the largest of these is the JGT and it is used for both teaching and research.

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Inside the dome of the James Gregory Telescope.

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A view up the James Gregory Telescope to the heavens. Image credit Jan Bölsche.

Today almost all astronomical stargazing undertaken for research purposes is carried out with the aid of digital cameras, computers and world-wide networks of telescopes.

The primary research tasks for the JGT are to: locate and track space debris (i.e. orbiting rubbish) and to assist in the discovery of new planets. As space debris is a relatively new phenomenon, on a Special Collections time-scale that is, we really couldn’t think of a creative ‘Historical How-To’. But the works we hold relating to early planetary observations are extensive.

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The first illustration of Christiaan Huygens discovery of Titan, a moon of Saturn, first observed on 25 March 1655. From his Systema Saturnium (1659)

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Portrait of James Gregory

Our collections of 17th century scientific works are strong for two reasons:

  1. Mathematics and astronomy came into acute focus in St Andrews towards the end of the 17th century when James Gregory was appointed the first Regius Professor in Mathematics by Charles II. One of Gregory’s goals at St Andrews was to establish and build an observatory; this was never completed, as Gregory was poached by the University of Edinburgh in 1674; however by then the University had purchased several instruments and had begun to stock a library full of the foremost publications on astronomical observations and equipment building. Some of these books still remain in the 17th century reserve collections of the University and include Huygens’ Horologium oscillatorium (1673), several of Hevelius’s works and observations, and Brahe’s monumental Astronomiae (1602).
  2. In 1929, George Forbes, FRS, presented to the University the library of his father, James David Forbes (1809-1868), a glaciologist and also Principal of the United Colleges from 1859. J.D. Forbes’s collecting interests were largely in the field of scientific history and he had a particularly good eye for quality copies of some of the most famous works in this genre. Because of this collection, St Andrews possesses copies of the first editions of Copernicus’s De revolutionibus orbium caelestium (1543), a signed copy of Galileo’s Difesa (1607) as well as his Sidereus nuncius (1610), Kepler’s Astronomia nova (1609) Newton’s Principia (1687), etc. etc.
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Diagrams of Galileo’s observations of the moons of Jupiter, described initially as stars, from his 1610 Sidereus Nuncius.

The most significant works we have in our book collection in regards to planetary discovery date from the 17th century, the great era of new planet and satellite discovery. Galileo was the first western observer to construct a telescope (30x magnification) and point it towards the heavens; he published his first findings of astronomical observations, which included the discovery of the four Galilean moons of Jupiter, in his Sidereus Nuncius (1610).  Christiaan Huygens Systema Saturnium (1659) not only synthesises the past 50 years of observing the ringed planet of Saturn, to which Hevelius’s findings contributed to heavily, but also his discovery of Titan, a moon of Saturn.

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A folding plate from Herschel’s initial report on his observation of a comet that would ultimately be identified as a new planet in our solar system, Uranus. From the 1781 volume of the Philosophical Transactions of the Royal Society.

The next planet in our solar system to be discovered was Uranus, first observed by William Herschel in 1781, first thought to be a comet and then later confirmed as a planet. His initial findings were read before the Royal Society and published in the Society’s Philosophical Transactions in the same year (you can find a pdf of the original piece here).

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Stark contrast between the new and old. Control unit for the telescope and dome dates to the 1960s. The digital camera and computer links now replace what once held a plate holder for the photographic glass plates. Left hand image of controls at night by Jan Bölsche.

Although extensive research is still going on, scientists have discovered and observed most of the planetary bodies in our solar system. However, things really changed in the 1990s when we started discovering new planets outside of our solar system – so called extrasolar planets.

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Our host for the evening Director of the Observatory Dr Aleks Scholz shows us how the planetary search is done today at St Andrews.

Considering how far away these stars and potential planets actually are; you cannot simply look for a tiny dark spot orbiting around a faraway star. Or can you? Well as it turns out you can; of course it is not nearly as simple as that. Stars have a set amount of light that they give off and when that light dims, ever so slightly, it could be the result of a small dark mass (i.e. a planet) eclipsing a very tiny portion of the star. This is called a ‘transit’, you can see a video of our own ‘transit of Venus’ back in 2012 here:

The image seen by the JGT operators would not be as clear as was the transit of Venus, but astronomically speaking, with the right instruments you can pick up a very minor shift in brightness.

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Here is a graph created by the University of St Andrews Observatory where you can see a slight dip in the brightness of an observed star in November 2013 during the transit of planet WASP-33b.

When you look up to the sky and think about how many stars there are in our galaxy you quickly realise that it would be terribly inefficient to pick a star randomly then sit back and wait. So there is an incredible network of astronomers who coordinate their efforts in the search for new planets.

Telescopes scanning larger areas of the sky, such as those on La Palma, are part of the Wide Angle Search for Planets (Super-WASP), search for and create a list of ‘candidate stars’. That list is then shared with observatories all over the world. Astronomers, such as those here at St Andrews, examine the data and are able to pick out a preferred, likelier candidate: at which point they turn their telescopes to it and wait for the possible-planet to block out just a few beams of light and thus make itself seen.

The research then continues at other institutions that can measure the mass and composition of the planet.

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The opening lines of William Herschel’s first observations of what would be later named Uranus, from the 1781 volume of the Philosophical Transactions of the Royal Society.

Like Galileo, Kepler and Huygens, today’s astronomers require several checks and validations of their data; however instead of relying solely on written correspondence, or publications in scholarly journals today’s scientists can communicate fluidly and quickly to confirm or deny the existence of new heavenly bodies. The means of capturing data and communicating may have changed wildly in the past 400 years, but the methods of these great figures in scientific history still remain the same.


With special thanks to Dr Aleks Scholz for his time at the Observatory. If you’d like to learn more about the St Andrews Observatory you can visit their website.


2 responses to “52 Weeks of Historical How-To’s, Week 51: How to discover a planet

  1. Pingback: Whewell's Ghost·

  2. Pingback: 52 Weeks of Historical How-To’s: Reflections and Visual Index | Echoes from the Vault·

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