Mercury, the innermost planet, is also the smallest. It always stays in the same part of the sky as the Sun, and can therefore never be seen against a really dark background, and is not a conspicuous naked-eye object, though at its best it is actually brighter than any star. Its quick movements led to its being named after Hermes (Mercury), the fleet-footed Messenger of the Gods.

Data for Mercury are given in Table 4.1.

VULCAN

It was once thought that a planet existed closer to the Sun than the orbit of Mercury. It was even given a name – Vulcan, after the blacksmith of the gods. Only in the twentieth century was it finally found to be non-existent and relegated to the status of a ghost.

The story of Vulcan really goes back to 1781, when William Herschel discovered a new planet, Uranus, moving far beyond the orbit of Saturn. Over the years it was found that Uranus was not moving quite as it was expected to do; something was perturbing it, and mathematicians began to suspect that there might be yet another planet still further from the Sun. From these tiny perturbations a leading French astronomer, U. J. J. Le Verrier, worked out the position of the unknown world, and in 1846 J. Galle and H. D'Arrest, at the Berlin Observatory, discovered Neptune, very close to the position given by Le Verrier.

From BC 4000, constellation patterns were drawn up. All these were different; the Chinese and Egyptian constellations, for example, are quite unlike ours (for example, our Draco seems to correspond with the Egyptian hippopotamus). Our system is derived from that of Ptolemy (Table 28.1); it may originally have been Cretan, though opinions differ. In all, 88 separate constellations are now in use. A list of these is given in Table 28.2.

Ptolemy gave a list of 48 constellations: 21 northern, 12 zodiacal and 15 southern (Table 28.1). All these are to be found on modern maps, although in many cases their boundaries have been altered – and the huge Argo Navis, the Ship Argo, has been chopped up into a Keel (Carina), sails (Vela) and a poop (Puppis).

Surviving post-Ptolemaic constellations are given in Table 28.3. Many of the original names have been shortened: thus Pisces Volant, the Flying Fish, has become simply Volant, while Mons Mensæ, the Table Mountain, has become Mensa. Pisces Australis may also be called Pisces Austrians, while Scorpius is often incorrectly called Scorpio. There was some confusion over two of Bayer's constellations, Apis (the Bee) and Avis Indica (the Bird of Paradise); modern maps give it as Apus. There were also two Musca, one formed by Lacaille to replace Bayer's Apis, and the other (rejected) formed by Bode out of stars near Aries.

Double stars are of two types: optical pairs (that is to say line-of-sight effects) and binaries (physically associated pairs). Binaries are much the more frequent. They range from contact pairs, where the components are almost or quite touching, to very distant pairs separated by at least a light-year. In a binary system the components move round their common centre of gravity. For visual binaries the shortest period is that of Wolf 630 Ophiuchi (1.725 years), but shorter periods are known: the record-holder is X-1820–303, an X-ray star in the globular cluster NGC 6623, distance 30 000 light-years. Its period is 685 s or 11 min. It was discovered in 1987 by the aptly named L. Stella and collaborators with the Exosat satellite. It is impossible to say which is the binary with the longest period, and all we can say is that very widely separated components share a common motion through space. Table 21.1 lists prominent double stars.

EARLY OBSERVATIONS

The term ‘double star’ was first used by Ptolemy, who wrote that η Sagittarii was ‘διπλυοζ’. There are of course several doubles which can be separated with the naked eye, so that presumably they have been known since antiquity; of these the most celebrated is Mizar (ζ Ursæ Majoris), which makes a naked-eye pair with Alcor (80 Ursæ Majoris). The Arabs described it – although they regarded Alcor as a rather difficult object. This is not true today, but it is most unlikely that there has been any real change.

To give every date of importance in the history of astronomy would be a mammoth undertaking. What I have therefore tried to do is to make a judicious selection, separating out purely space-research advances and discoveries.

It is impossible to say just when astronomy began, but even the earliest men capable of coherent thought must have paid attention to the various objects to be seen in the sky, so that it may be fair to say that astronomy is as old as Homo sapiens. Among the earliest peoples to make systematic studies of the stars were the Mesopotamians, the Egyptians and the Chinese, all of whom drew up constellation patterns. (There have also been suggestions that the constellations we use as a basis today were first worked out in Crete, but this is speculation only.) It seems that some constellation-systems date back to 3000 BC, probably earlier, but of course all dates in these very ancient times are uncertain.

The first essential among ancient civilisations was the compilation of a good calendar. Probably the first reasonably accurate value of the length of the year (365 days) was given by the Egyptians. (The first recorded monarch of all Egypt was Menes. who seems to have reigned around 3100 BC; he was eventually killed by a hippopotamus – possibly the only sovereign ever to have met with such a fate!) They paid great attention to the star Sirius (Sothis), because of its ‘heliacal rising’, or date when it could first be seen in the dawn sky, gave a reliable clue to the time of the annual flooding of the Nile, upon which the Egyptian economy depended.

Le Verrier's correct prediction of the position of Neptune was a personal triumph for him, there have been suggestions that there was a large element of luck about it, but it cannot be denied that the actual discovery, by Galle and D'Arrest, was due entirely to Le Verrier.

The discovery of Pluto

Despite the fact that the new planet did not move at the distance required by Bode's law, then generally believed to be significant rather than coincidental, the Solar System could well be regarded as complete. Yet there was no proof of this, and as early as 1846 Le Verrier himself suggested that there might well be a planet moving beyond the orbit of the recently discovered Neptune. The first systematic search was made in 1877 by David Peck Todd, from the US Naval Observatory. From perturbations of Uranus, he predicted a planet at a distance of 52 a.u. from the Sun, with a diameter of 80 000 km. He conducted a visual search, using the 66-cm USNO reflector with powers of ×400 and ×600, hoping to detect an object showing a definite disc. He continued the hunt for 30 clear, moonless nights between 3 November 1877 and 5 March 1878, but with negative results. A second investigation was conducted in 1879 by the French astronomer Camille Flammarion, who based his suggestion upon the fact that several comets appeared to have their aphelia at approximately the same distance, well beyond the orbit of Neptune.

Neptune is the third most massive planet in the Solar System and, like Uranus, may be described as an ‘ice giant’. It is too faint to be seen with the naked eye, but binoculars show it easily, and a small telescope will reveal its pale bluish disc. Data are given in Table 12.1.

MOVEMENTS

Neptune is a slow mover; it takes almost 165 years to complete one journey round the Sun so that it was discovered less than one ‘Neptunian year’ ago. Opposition dates are given in Table 12.2. Some close planetary conjunctions involving Neptune are listed in Table 12.3: occultations by the Moon can of course occur.

EARLY OBSERVATIONS

Neptune was observed on several occasions before being identified as a planet. The first observation seems to have been made by Galileo on 27 December 1612. While drawing Jupiter and its four satellites, he recorded a ‘star’ which was certainly Neptune. He again saw it twice in January 1613, and noted its movement but, not surprisingly, mistook it for a star. His telescope had a magnification of ×18 and a resolving power of 190 arcsec, with a field of view 17 arcmin in diameter. Neptune's magnitude was 7.7, and Galileo often plotted stars fainter than that. Jupiter actually occulted Neptune in 1613.

The next telescopic observation was made in May 1795 by J. J. de Lalande, but again Neptune was mistaken for a star.

The stars show a tremendous range in luminosity, though much less in mass. Some known stars are millions of times more luminous than the Sun, while others are remarkably feeble. At its peak, in the 1840s, the erratic variable η Carinæ was estimated to be 6 000 000 times as powerful as the Sun; S Doradûs, in the Large Magellanic Cloud, has an absolute magnitude of –8.9, so that it is at least a million times as luminous as the Sun – yet because of its great distance (170 000 light-years) it cannot be seen with the naked eye. At the other end of the scale is MH18, discovered in 1990 by M. Hawkins at what was then the Royal Observatory, Edinburgh, from plates taken with the UK Schmidt telescope in Australia. It has 1/20 000 the luminosity of the Sun, and is presumably a brown dwarf (see below). Its mass is 5% that of the Sun, and its distance is 68 light-years.

The first attempt to classify the stars according to their spectra was made by the Italian Jesuit astronomer, Angelo Secchi, in 1863–7. He divided the stars into four main types:

1. (1) White or bluish stars: with broad, dark lines of hydrogen but obscure metallic lines. Example: Sirius.

2. (2) Yellow stars: hydrogen lines less prominent, metallic lines more so. Examples: Capella, the Sun.

3. (3) Orange stars: complicated, banded spectra. Examples: Betelgeux, Mira. The class included many long-period variables.

4. (4) Red stars: with prominent carbon lines; all below magnitude 5. Example: R Cygni. This class also included many variables.

The Sun, the controlling body of the Solar System, is the only star close enough to be studied in detail. It is 270 000 times closer than the nearest stars beyond the Solar System, those of the α Centauri group. Data are given in Table 2.1.

DISTANCE

The first known estimate of the distance of the Sun was made by the Greek philosopher Anaxagoras (500–428 BC). He assumed the Earth to be flat, and gave the Sun's distance as 6500 km (using modern units), with a diameter of over 50 km. A much better estimate was made by Aristarchus of Samos, around 270 BC. His value, derived from observations of the angle between the Sun and the exact half Moon, was approximately 4 800 000 km; his method was perfectly sound in theory, but the necessary measurements could not be made with sufficient accuracy. (Aristarchus also held the belief that the Sun, not the Earth, is the centre of the planetary system.) Ptolemy (c. AD 150) increased the distance to 8 000 000 km, but in his book published in AD 1543 Copernicus reverted to only 3 200 000 km. Kepler, in 1618 gave a value of 22 500 000 km.

The first reasonably accurate estimate of the Earth–Sun distance (the astronomical unit) was made in 1672 by Giovanni Cassini, from observations of the parallax of Mars. Some later determinations are given in Table 2.2.

Comets are the most erratic members of the Solar System. They may sometimes look spectacular, but they are not nearly so important as they then seem, and by planetary standards their masses are very low indeed. In most cases, though not all, their orbits round the Sun are highly eccentric. A comet has been aptly described as a dirty ice-ball.

COMET PANICS

In earlier times comets were not classed as being celestial bodies, and were put down as atmospheric phenomena, although it is true that around 500 BC the Greek philosopher Anaxagoras regarded them as being due to clusters of faint stars. They were always regarded as unlucky. Recall the lines in Shakespeare's Julius Cæsar:

When beggars die, there are no comets seen:

In 1578, the Lutheran bishop Andreas Calichus went further, and described comets as being ‘the thick smoke of human sins rising every day, every moment, full of stench and horror before the face of God’. However, his Hungarian contemporary, Andreas Dudith, sagely pointed out that in this case the sky would never be comet-free! The first proof that comets were extraterrestrial came from the Danish astronomer Tycho Brahe, who found that the comet of 1577 showed no diurnal parallax, and must therefore be at least six times as far away as the Moon (actually, of course, it was much more remote than that).

Nebulæ are gas-clouds. They are of very different types. Bright nebulæ, such as M 42 in the Sword of Orion, are stellar nurseries, while planetary nebulæ are dying stars, and there are also supernova remnants, such as the Crab Nebula in Taurus. The galaxies were once called ‘spiral nebulæ’, but this name for them is now obsolete.

PLANETARY NEBULÆ

Planetary nebulæ are very inappropriately named. They were so called by William Herschel because their pale, often greenish discs make them look superficially not unlike Uranus or Neptune when seen through a small telescope, but they are not true nebulæ, and have absolutely nothing to do with planets.

A proto-planetary nebula marks the brief period in a star's history when it has left the asymptotic giant branch of the Hertzsprung–Russell (HR) diagram but has not reached the planetary nebula stage – it has been ‘caught in the act’, so to speak. These nebulæ are faint and rare; a selected list is given in Table 24.1.

(Do not confuse a proto-planetary nebula with a proto-planetary disc. ‘Preplanetary nebula’ has been proposed as an alternative name, and this would be a distinct improvement.)

The Red Rectangle is one of the most notable of the proto-planetaries. It takes the form of a symmetric, bipolar nebula with X-shaped spikes; the central star is a close binary, apparently surrounded by a thick dust torus which forces the otherwise spherical outflow into tip-touching cone shapes.

The Sun is a member of a system of stars known as the Galaxy. The system is popularly called the Milky Way, but this name is more properly restricted to the luminous band stretching across the sky. The name is the translation of the Latin Via Lactea; the Greek name was ‘Galaxies’.

The Milky Way has been known since very ancient times – on a dark, clear night it cannot be overlooked – and there are many legends about it. In Chinese tales there are two lovers, Chuc Nu and Nguu Lang, at the court of the Jade Emperor, who neglected their official duties and were banished to the sky as the stars Vega and Altair, on opposite sides of the Milky Way river. They are allowed to meet once a year, on the seventh day of the seventh month, when flocks of birds form a bridge across the river to enable the lovers to cross. In Finland, birds are again involved; it was thought that migratory birds used the Milky Way as a guideline to their home, Lintukoto – and there may be some truth in this! To the people of the Kalahari Desert the Milky Way was due to embers from a fire lit by a girl who wanted to find her way around on a starless night. The Aborigines of South Australia had a different idea; the Milky Way was Woodlipari, a celestial river where there are caves in which live dangerous creatures called yura (the caves themselves are yurakauwe, ‘homes of monsters’).

Patrick Moore has inspired generations of astronomers. He has done unparalleled service, through his handbooks, lectures and articles – not to mention his BBC programme The Sky at Night.

Over his prolific career, Patrick has witnessed, recorded and expounded a huge enlargement of our cosmic knowledge. To see this, one need only compare the present book with one of its precursors: the Guinness Book of Records in Astronomy published more than 50 years ago, at the dawn of the space age.

We owe this progress to sophisticated telescopes on the ground, and to a flotilla of instruments launched into space. The planets and moons of our Solar System are now better mapped that some parts of our Earth were before the twentieth century. An unsuspected population of trans-Neptunian objects has been revealed – telling us that the Solar System is more complex and extensive than thought hitherto. Even more important, planets have been detected around hundreds of other stars. The study of ‘extra-solar’ planets is proceeding apace: within a decade we will have discovered thousands of planetary systems, and will for the first time have evidence on just how unusual our Solar System is.

Novel technology has not only led to more powerful optical telescopes, but also to space telescopes that observe the cosmos in other wavebands out to distances exceeding 10 billion light years. We inhabit a much vaster Universe than was envisaged 50 years ago; we understand a surprising amount about how it evolved and what it contains.

When setting out to discuss the origin and evolution of the universe, we are at once confronted with three clear-cut alternatives. They are:

1. The universe began at a definite moment. This was also the beginning of time, so that there was no ‘before’. It will also cease to exist at a definite moment, so that there will be no ‘after’.

2. The universe has always existed, in which case we must accept a period of time which extends back for ever. It will always exist, so that there will be no ‘end’.

3. The universe began at a definite instant, which was also the beginning of time, so that there was no ‘before’. It will continue to exist for ever, so that there will be no ‘end’.

To explain any of these precepts in plain English is not easy, even for someone with the brain of a Newton or an Einstein!

TIMESCALE

The timescale of the universe was not appreciated until comparatively modern times. The age of the Earth itself was wildly underestimated by almost all scientists of the nineteenth century, and only gradually did the evidence provided by fossils and radiometric dating show that our world must be thousands of millions of years old.

James Ussher, Archbishop of Armagh (1581–1656) held very definite views. By basing his chronology upon key events recorded in the Bible, he found that the moment of the Creation was nine o'clock on Sunday, 23 October 4004 BC. Even today we find people who genuinely believe that everything in the Bible is literally true – and Creationism, re-named Intelligent Design, is so widespread in parts of the United States that some schools teach it as a serious alternative to Darwinian evolution!

Selecting a limited number of astronomers for short biographical notes may be somewhat invidious. However, the list given here includes most of the great pioneers and researchers. No astronomers still living at the time of writing are included. All dates are AD unless otherwise stated.

Abul Wafa, Mohammed. 959–88. Last of the famous Baghdad school of astronomers. He wrote a book called Almagest, a summary of Ptolemy's great work, also called the Almagest, in Arabic.

Adams, John Couch. 1819–92. English astronomer, born in Lidcot, Cornwall. He graduated brilliantly from Cambridge in 1843, but had already formulated a plan to search for a new planet by studying the perturbations of Uranus. By 1845, his results were ready, but no quick search was made, and the actual discovery was due to calculations by U. Le Verrier. Later he became Director of the Cambridge Observatory, and worked upon lunar acceleration, the orbit of the Leonid meteor shower, and upon various other investigations.

Airy, George Biddell. 1801–92. English astronomer. Born in Northumberland, he graduated from Cambridge 1823 and was Professor of Astronomy there (1826–35). On becoming Astronomer Royal (1835–81) he totally reorganised the Greenwich Observatory and raised it to its present eminence. He re-equipped the Observatory and ensured that the best use was made of its instruments; it is ironical that he is probably best remembered for his failure to instigate a prompt search for Neptune when receiving Adams' calculations.

Clusters and nebulæ are among the most striking of stellar objects. Several are easily visible with the naked eye. Few people can fail to recognize the lovely star cluster of the Pleiades or Seven Sisters, which has been known since prehistoric times and about which there are many old legends. The nebula in the Sword of Orion, the Sword-Handle in Perseus, Præsepe in Cancer and the Jewel Box cluster in Crux are other objects easily visible without optical aid. Keen-sighted people have little difficulty in locating the great Andromeda Spiral and the globular cluster in Hercules, while in the far south there are the two Clouds of Magellan which cannot possibly be overlooked, as well as the bright globular clusters ω Centauri and 47 Tucanæ.

The most famous of all catalogues of nebulous objects was compiled by the French astronomer Charles Messier, and published in 1781. Ironically, Messier was not interested in the objects he listed: he was a comet-hunter, and merely wanted a quick means of identifying misty patches which were non-cometary in nature. In 1888, J. L. E. Dreyer, Danish by birth (although he spent much of his life in Ireland, and finally in England) published his New General Catalogue (NGC), augmented in 1898 and again in 1908 by his Index Catalogue (I or IC).

Uranus, the seventh planet in order of distance from the Sun, was the first to be discovered in telescopic times, by William Herschel, in 1781. It is a giant world, but it and the outermost giant, Neptune, are very different from Jupiter and Saturn, both in size and in constitution. It is probably appropriate to refer to Jupiter and Saturn as gas giants and to Uranus and Neptune as ice giants.

Data for Uranus are given in Table 11.1.

MOVEMENTS

Since Uranus' synodic period is less than five days longer than our year, Uranus comes to opposition every year. Opposition dates for 2010–2020 are given in Table 11.2. The opposition magnitude does not vary a great deal; under good conditions the planet can just be seen with the naked eye. The most recent aphelion passage was that of 27 February 2007; Uranus was at its minimum distance from the Earth (21.09 a.u.) on 13 March of that year. The last perihelion passage was on 20 May 1966; Uranus was closest to the Earth (17.29 a.u.) on 9 March of that year. The next perihelion will be that of 13 August 2050.

In June 1989, Uranus reached its greatest southerly declination (–23.7°). Greatest northern declination had been reached in March 1950.

Close planetary conjunctions involving Uranus are listed in Table 11.3. It is interesting to note that between January and March 1610 Uranus was within 3° of Jupiter.

Variable star research is an important branch of modern astronomy – amateur observers make very valuable contributions. Variable stars are of many types; elaborate systems of classifying them have been proposed, and the data given here are not intended to be more than a general guide. Seven major categories are now recognised.

1. (1) Eclipsing stars (more properly eclipsing binaries, because they are not intrinsically variable).

2. (2) Pulsating variables: either radial or non-radial pulsations.

3. (3) Eruptive variables, where the changes are caused by flares or the ejection of shells of material.

4. (4) Cataclysmic variables, where the changes are due to explosions in the star or in an accretion disc round it. Novæ dwarf novæ and supernovæ come into this category.

5. (5) Rotating variables, where the changes are caused by star-spots, non-spherical shape or magnetic effects.

6. (6) X-ray variables, usually inherent in the neutron star or black hole companion of a binary.

7. (7) Unclassifiable stars, which do not fit into any accepted category.

We have already noted what are termed secular variables: stars which have permanently brightened or faded in historic times. Thus Ptolemy ranked β Leonis and θ Eridani as of the first magnitude, whereas today they are below magnitude 2 and 3 respectively: α Ophiuchi was ranked of magnitude 3, but is now 2.1. However, these changes must be regarded as highly suspect. It is unwise to trust the old observations too far.