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A SURVEY OF THE WHOLE SURFACE OF THE HEAVENS

Between 1786 and 1802, William Herschel published catalogs listing more than 1,000 new objects in the night sky. Following his death in 1822, William’s son John continued his work, but expanded its scope and ambition to carry out a complete survey of the night sky. William’s observations had all been made from southern England, and so were limited to objects down to around 33° below the celestial equator. To survey the rest of the sky, his son’s observations would have to be made from somewhere in the southern hemisphere.

Herschel settled on South Africa, then a part of the British Empire. He moved there in 1833, taking with him his wife and young family, an assistant, and his father’s 20-ft (6-m) focal length telescope. This was the same instrument that had been used to survey the northern skies, and Herschel chose it to ensure that the new information gathered from the southern hemisphere was comparable to that already produced. The family set themselves up in a house near the base of Table Mountain, far enough away to avoid the clouds that often gathered on its summit, and Herschel spent the next four years completing his survey.

The Milky Way’s core is clearest from the southern hemisphere. The dark regions are where starlight is blocked by interstellar dust.

The southern skies

The Magellanic clouds are two dwarf galaxies close to the Milky Way, and are only visible from the southern hemisphere. They can be seen by the naked eye, but Herschel’s telescopic surveys provided the first detailed observations available to astronomers. He compiled a list of more than 1,000 stars, star clusters, and nebulae within these galaxies.

Herschel also made careful observations of the distributions of stars within the Milky Way. Due to the orientation of the solar system within the Milky Way, the brightest section of it, which is now known to be the core of the galaxy, is only visible low on the horizon from the northern hemisphere during the summer when nights are short. From the southern hemisphere, the brighter core is visible higher in the sky and during the darker months of the year, allowing easier and more detailed observations.

The end result of Herschel’s labors, The General Catalog of Nebulae and Clusters of Stars, listed more than 5,000 objects in total. These included all the objects observed by John and his father, and also many discovered by others such as Charles Messier, since it was intended to be a complete catalog of the stars.

“The stars are the landmarks of the universe.” John Herschel

JOHN HERSCHEL

John Herschel left Cambridge University in 1816, already a renowned mathematician. He worked with his father, William, and continued his work after William’s death in 1822. Herschel became one of the founders of the Royal Astronomical Society and served as president for three separate terms. He married in 1826 and fathered 12 children. Herschel had numerous interests in addition to astronomy. While in South Africa, Herschel and his wife produced a portfolio of botanical illustrations. He also made important contributions to photography, experimenting with color reproduction, and published papers on meteorology, telescopy, and other subjects.

AN APPARENT MOVEMENT OF THE STARS

Parallax is the apparent movement of a nearby object against distant objects due to the changing position of the observer. According to this phenomenon, nearby stars should appear to change position against the background of more distant stars as Earth moves around its orbit. The idea that it might be possible to measure the distance to nearby stars using parallax dates back to ancient Greece. However, it was not achieved until the 19th century, due to the distances involved being far greater than anyone supposed.

Much of German astronomer Friedrich Bessel’s career had been dedicated to the accurate determination of the positions of stars and finding their proper motion (changes in position due to the motion of the star, rather than changes in apparent position due to the time of night or the season). By the 1830s, with improvements in the power of telescopes, there was a race to carry out the first accurate measurement of stellar parallax. In 1838, Bessel measured parallax with an angle of 0.314 arc seconds for the star 61 Cygni, which indicated that it was 10.3 light-years away. The current estimate is 11.4 light-years, giving Bessel’s measurement an error of just under 10 percent.

Due to the effects of parallax, a nearby star’s apparent position against distant background stars moves from b in June to a in December.

SUNSPORTS APPEAR IN CYCLES

Sunspots are cooler areas on the sun’s surface caused by changes in its magnetic field. The first written observations of sunspots date from about 800 BCE, in China, but it was not until 1801 that the British astronomer William Herschel made the connection between sunspots and changes in Earth’s climate.

See also: Observing Uranus • The properties of sunspots • Carrington (Directory)
Samuel Schwabe, a German astronomer, started observing sunspots in 1826. He was looking for a new planet thought to orbit closer to the sun than Mercury, provisionally named Vulcan. It would have been very difficult to observe such a planet directly, but Schwabe thought he might see it as a dark spot moving in front of the sun. He did not find Vulcan, but he did discover that the number of sunspots varied over 11-year cycles. Swiss astronomer Rudolf Wolf studied Schwabe’s and other observations, including some from as far back as Galileo, and numbered the cycles starting at 1 for the 1755–66 cycle. Eventually, he saw that there are long periods in each cycle when the number of sunspots is low. Herschel had not noticed the pattern because he was observing during what is now called the Dalton Minimum, when overall numbers of sunspots were low.

Sunspots can last from a few days to several months. The largest can be the size of Jupiter.

A SPIRAL FORM OF ARRANGEMENT WAS DETECTED

In the 1840s, a British aristocrat named William Parsons, Lord Rosse, decided to commit some of his considerable wealth to building the world’s largest reflecting telescope. Rosse was curious to reexamine some of the nebulae listed by John Herschel in the early 19th century, in particular those nebulae that did not appear to be clusters of stars.

To re-observe these nebulae, Rosse needed to build a larger and better telescope than that used by Herschel. He experimented for many years with methods for casting a 36-inch (0.9-m) mirror. Mirrors at the time were made from a metal called speculum, an alloy of copper and tin—a brittle material that was prone to cracking as it cooled.

Despite this difficulty, by 1845 Rosse had succeeded in casting a mirror that was 72 in (1.8 m) in diameter. He mounted it in his telescope at Birr Castle, near Parsonstown in Ireland, where it became known as the Leviathan of Parsonstown. This telescope remained the world’s largest reflecting type until the 100-in (2.5-m) reflector was built at Mount Wilson in California in 1917.

Central Ireland proved a far from ideal place to build a telescope, as overcast or windy conditions often prevented viewing. The telescope itself had limited mobility, meaning that only a small area of the sky could be examined. Nonetheless, when the weather was clear, Rosse was able to use the huge instrument to observe and record the spiral nature of some nebulae—now called spiral galaxies—for the first time. The first of these spirals that Rosse identified was M51, later known as the Whirlpool galaxy. Today, about threequarters of all the galaxies that have been observed are spiral galaxies. However, these are thought ultimately to transform into elliptical galaxies. Formed of older stars, elliptical galaxies are dimmer and much harder to spot, but astronomers believe that they are probably the most common galaxy type in the universe.

The nebular hypothesis

In the mid-19th century, astronomers debated whether nebulae consisted of gas or stars. In 1846, Rosse found numerous stars in the Orion nebula, and so for a time, the idea of gaseous nebulae was rejected. However, although the stars were real, their presence did not mean that there was no gas. The gaseous nature of some nebulae was not demonstrated until spectroscopic analysis was used by William Huggins in 1864.

“The light by which we recognize the nebulae now must be merely that which left their surfaces a vast number of years ago … phantoms of processes completed long in the Past.” Edgar Allen Poe

The Leviathan Telescope at Parsonstown held a mirror weighing 3.3 tons (3 metric tons), inside a 54 ft- (16.5 m-) long tube. The whole structure weighed about 13 tons (12 metric tons).

LORD ROSSE

William Parsons was born in Yorkshire in 1800 and became Third Earl of Rosse on the death of his father in 1841. He was educated at Trinity College, Dublin, and at Oxford University, where he was awarded a first-class degree in mathematics. He married in 1836, but only four of his 13 children survived to adulthood. Lord Rosse’s estates were in Ireland, and this is where he built his telescopes.

In 1845, after he made public his findings on nebulae, Rosse was criticized by John Herschel, who was convinced that nebulae were gaseous in nature. Both men accused each other of using flawed instruments. Ultimately, however, neither succeeded in demonstrating sufficient scientific evidence to resolve conclusively the question of whether nebulae were composed of gas or stars.

THE PLANET WHOSE POSITION YOU HAVE POINTED OUT ACTUALLY EXISTS

In the months following William Herschel’s discovery of Uranus in 1781, astronomers found irregularities, or perturbations, in its orbit. Most perturbations in orbits are caused by the gravitational effects of other large bodies, but with Uranus there were no known planets that could cause the observed motion. This led some astronomers to suggest that there must be a planet orbiting beyond Uranus.

Calculations of Uranus’s predicted orbit took into account the gravitational effects of the sun, Jupiter, and Saturn. However, the observed orbit deviated from the calculations in a way that suggested the pull of another massive body farther out from the sun

Searching for the invisible

Frenchman Urbain Le Verrier tackled the problem of the perturbations of Uranus by assuming the location of an undiscovered planet and using Newton’s law of gravity to work out what its effect might be on Uranus. This prediction was compared to observations of Uranus, and the position was revised according to the planet’s movements. After many repetitions of this process, Le Verrier established the likely position of an unknown planet. He presented his ideas before the Académie des Sciences in 1846, and he also sent his predictions to Johann Galle (1812–1910) at the Berlin Observatory.

Galle received Le Verrier’s letter on the morning of September 23, 1846, and obtained permission to look for the planet. Working with his assistant Heinrich D’Arrest, he located an unknown object within 1° of the predicted position that same night. Observations on subsequent nights showed that the object was moving against the background of stars and was, indeed, a planet—one that would later be named Neptune at Le Verrier’s suggestion. Galle later gave the credit for the discovery to Le Verrier.

Independent discovery

At the same time as Le Verrier was calculating the position of the unknown planet, British astronomer John Couch Adams (1819–92) was also looking at the cause of the perturbations in the orbit of Uranus. He arrived at a similar conclusion to Le Verrier, completely independently, but his results were not published until after Galle had observed the new planet. There was some controversy over who should have the credit for the discovery, but Adams always acknowledged that Le Verrier had the better claim.

Galle was not the first person to observe Neptune. Once the orbit of Neptune had been worked out, it was possible to go through old records and find that others had already observed it without realizing it was a planet, including both Galileo and John Herschel. Later, Le Verrier used a similar technique to analyze the orbit of Mercury and found that perturbations in its orbit could not be explained by Newtonian mechanics. He suggested that this might be due to the influence of another planet even closer to the sun, provisionally named Vulcan. This speculation ended when Einstein explained the perturbations using his general theory of relativity.

URBAIN LE VERRIER

Urbain Le Verrier studied at the École Polytechnique, near Paris. After graduating, his initial interests were in chemistry, before he switched to astronomy. His astronomical work was focused on celestial mechanics—the description of the movements of the bodies in the solar system using mathematics. Le Verrier obtained a position at the Paris Observatory and spent most of his life there, acting as director from 1854. However, his management style was not popular and he was replaced in 1870. He took up the position again in 1873 after his successor drowned, and held it until his own death in 1877.

Le Verrier spent his early career building on Pierre-Simon Laplace’s work on the stability of the solar system. He later went on to study periodic comets before turning his attention to the puzzle of Uranus’s orbit.

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