Practical Astronomer

Practical Astronomer

by Brian Jones

View All Available Formats & Editions

From planets and asteroids to the expanding universe, The Practical Astronomer is the perfect introduction to the fascinating world of astronomy.

It not only shows you what to look for throughout the whole yearly cycle of the sky, but exactly how to go about looking for it. Based on the latest findings in astronomical research, the book explains the


From planets and asteroids to the expanding universe, The Practical Astronomer is the perfect introduction to the fascinating world of astronomy.

It not only shows you what to look for throughout the whole yearly cycle of the sky, but exactly how to go about looking for it. Based on the latest findings in astronomical research, the book explains the practical keys to successful observation - whether you are observing with the naked eye, binoculars, or a telescope.

This unique and indispensable guide will help you make the most of this fascinating hobby. Contents include:

The Solar System - The Milky Way - The Expanding Universe - Binoculars and Telescopes - Making a Telescope - Astrophotography - Drawing at the Telescope - Equipping an Amateur Observatory - Guide to the Moon - Observing the Sun - Planets and Asteroids - Gas Giants, Comets, Meteors and Meteorites - Artificial Satellites

With more than 275 photographs and diagrams, including month-by-month skysearch charts.

Product Details

Publication date:
Product dimensions:
7.50(w) x 9.78(h) x 0.48(d)

Read an Excerpt


The Earth is one of nine major planets orbiting the Sun. Along with the planets the solar system contains numerous smaller bodies, including minor planets, comets, meteoroids and copious amounts of interplanetary dust.

The planetary members of the solar system are split into two distinct groups, each with its own basic characteristics. The four inner planets (Mercury, Venus, Earth, and Mars) are all relatively small, rocky in composition, and clustered together close to the Sun. These are the terrestrial planets (from the Latin terra, meaning "earth"), a name indicating that they resemble the Earth in composition. In contrast, the next four planets out from the Sun (Jupiter, Saturn, Uranus, and Neptune) all have large diameters, gaseous compositions, and orbits that are quite well spread out. These are the gaseous, or Jovian planets, so called because of their resemblance to Jupiter.

The formation of the solar system

The present-day layout of the solar system offers a number of clues as to how the Sun and its family came into being. From observational evidence gleaned by astronomers we have pieced together a theory as to the formation of the solar system that we believe to be correct, at least in general terms.

The solar system formed from a rotating cloud comprising gas and particles of dust and ice. This cloud is referred to as the solar nebula. Through the effect of gravity, the material in the solar nebula congregated toward the central region, resulting in an increase in density, temperature and pressure and the formation of a "protostar".

At the same time, the rotation of the irregular cloud caused it to flatten out into a disk. It was from the central region of this disk that the Sun was formed, the outer parts eventually forming the planets. This theory ties in quite well with observation, which shows us that the planetary orbits are more or less in the same plane.

It was from the heavier elements, that the four inner planets were formed. The outer, gaseous planets formed primarily from the much lighter elements, mainly hydrogen and helium, that existed nearer the outer edges of the cloud. Any hydrogen and helium that may have been present in the cloud's inner regions were driven away by the heat from the young Sun, resulting in a scarcity of these elements in the four inner planets.

Particles within the solar nebula began to collide with each other. This resulted in their sticking together and accumulating into small objects with diameters of the order of a few tens of kilometers. These so-called planetesimals then began to bump into each other and, by a similar process, built up into still larger bodies known as protoplanets. Eventually the four inner planets we see today formed as more and more planetesimals collided with the protoplanets. These objects were rocky worlds, rich in the heavier elements such as iron, silicon, aluminium and so on.

At this time these bodies consisted of gas and molten rock, primarily through the heat generated by the constant impact of the sizable planetesimals. This had the effect of causing all the heavier materials to sink towards the central regions of the newly formed planets, leaving lighter, less dense material nearer their surfaces. We see the legacy of this process today in the form of the iron-rich cores and rocky outer regions of the terrestrial planets, their thick early atmospheres dispersed by the hot, young sun.

The outer planets formed in a similar way with the building up of planetesimals. An initial rocky core collected the gas that abounded in the outer regions of the solar nebula. The end results were huge gaseous planets with atmospheres rich in hydrogen, enveloping rocky cores comparable in size to the terrestrial planets.

In the meantime, things were happening to the protostar at the center of the nebula. Here, the temperature and pressure at its core increased until nuclear reactions were initiated. The Sun as we know it then came into being. This turning-on of the Sun occurred at the same time as the Earth itself was finally formed. The newborn Sun shed its outermost layers into surrounding space, the material ejected sweeping through the interplanetary medium and clearing it of much of the remaining debris, leaving the solar system much as it is now. Many of the remaining larger particles impacted onto the surfaces of the newly formed planets, modeling the rugged, crater-strewn surfaces that exist today.


Containing around 98 per cent of the total mass of the solar system, the Sun dominates out region of space. As well as making life on our planet possible, the light and heat from out parent star greatly influence the other objects in our neighborhood. Yet, bright as the Sun appears to us, it ranks as just an average star when compared to many of the other stars scattered throughout the Galaxy. The Sun has a diameter of 1,392,000 km (865,000 mlles) and, classed as a yellow dwarf, it is a typical main-sequence star (see p.38).

The temperature of the visible surface of the Sun is of the order of 6,000°C (11,000°F). This region of the Sun is called the photosphere, a name that means "sphere of light," and it is from here that all the Sun's light and heat are emitted. Yet the prodigious amounts of energy that leave the Sun are created deep within its globe. Down at the core, the temperature and pressure are so high that nuclear reactions are taking place. A temperature of over 15 million°C (27 million°F) combines with a pressure of around 340,000 million atmospheres to sustain a reaction known as hydrogen burning, in which four hydrogen nuclei are fused together to form one nucleus of helium. In the process, a tiny amount of mass is converted to energy and carried to the surface, where it escapes as light and heat. This mass loss is equal to 0.7 per cent of the total amount of hydrogen used in the reaction -- amounting to 4 million tonnes per second!

Close examination of the photosphere shows it to have a mottled appearance. This so-called granulation is a result of the eruption of energy at the solar surface. The turbulence set up by energy erupting from within the Sun creates bright granules that measure around 1,000 km (625 miles) across. A darker boundary is seen to surround each granule. Observation has shown that the bright areas are regions where hot gases are emerging, only to cool by some 300°C (540°F) and spill down through the darker boundaries.

Other features visible on the photosphere are sunspots, depressions in the photosphere that appear as dark patches when seen against the brighter and hotter background. Sunspots come in a wide variety of sizes, although typically they are several tens of thousands of kilometers across.

The average lifetime of a typical sunspot is a few days and its temperature is around 4,000°C (7,000°F). Sunspots have a dark central region known as the umbra, which is surrounded by a lighter region, the penumbra. Although sunspots can appear on the solar disk at any time, there is a definite cycle of sunspot activity. This cycle was first noted in 1843 by the German astronomer Heinrich Schwabe, who pointed out that sunspot activity reached a maximum once every 11 years. During the early part of a cycle sunspots appear some 30° north or south of the solar equator. As the cycle progresses they develop nearer and nearer the equator.


Sunspots are associated with intense magnetic fields, and these give rise to a number of phenomena that occur near the spots. Faculae are highly luminous areas above which are seen regions where sunspots are about to form. Some of the most spectacular sunspot-related features, however, are prominences, huge columns of gas that appear above sunspots. Prominences are of two types. Eruptive prominences are the result of material being lifted away from the Sun at colossal speeds of up to 1,000 km (600 miles) per second. Their form can change from minute to minute. Quiescent prominences, on the other hand, are relatively stable and have been known to hang above the solar surface for periods of up to several months.

Flares are the most active of solar phenomena. These arise within complex sunspot groups as bright filaments of hot gas, the temperatures of which can soar to several millions of degrees within a very short time. Flares can give rise to vast increases in the amount of charged particles emanating from the Sun, resulting in a sometimes dramatic increase in the frequency and intensity of auroral displays in the Earth's atmosphere.

Above the photosphere lies the chromosphere. This is a relatively cool region through which solar energy passes on its way from the Sun. It is in this region that the flares described above are seen. Above the chromosphere is the corona, the outermost region of the Sun's atmosphere. The corona reaches out from just above the chromosphere to a distance of several million kilometers. It is within the inner regions of the corona that prominences appear. Its outer regions stretch out until it becomes the stream of energized particles called the solar wind.



Orbiting the Sun once every 87.97 days, at a mean distance of 58 million km (36 million miles), Mercury is the innermost planet. It is similar in size to the Moon, with a diameter of only 4,878 km (3,031 miles). It is also similar to the Moon in appearance. The Mariner 10 mission, launched in November 1973, made several flybys, revealing a surface covered in craters, mountains, ridges and valleys. There were fewer of the dark plains that dominate the lunar surface, however, the largest of those on Mercury being named the Caloris Basin.

Early observers believed that Mercury had a captured, or synchronous, rotation period, with one hemisphere permanently facing the Sun. However, radar measurements carried out in the early 1960s showed that the true axial rotation period was 58.65 days. This means that the planet spins exactly three times during two revolutions of the Sun. All parts of Mercury's surface therefore receive sunlight at some time or another. The result of this so-called spin orbit coupling is that the Mercurian solar day (sunrise to sunrise) is 176 Earth days long, or two Mercurian years.

One curious aspect of the relationship between Mercury's axial rotation period, its year, and the Earth's year is that the same hemisphere is presented to Earth every time the planet is best placed for observation. It is this that led early observers to believe that Mercury had a captured rotation.

The Mercurian atmosphere is so rarefied that it betters the best vacuum capable of being produced in a laboratory on Earth. A major reason is the fact that Mercury's escape velocity is only 4.3 km (2.67 miles) per second. Mariner 10 instruments detected traces of hydrogen and helium, probably originating from the Sun, near the Mercurian surface. Spectroscopic observations made from Earth in 1985 led to the discovery of sodium, which appears to be the most abundant component of the atmosphere. The temperature on Mercury can reach some 425°C (800°F) on the equator at noon, although this can drop to -180°C (-292°F just before sunrise. The virtual lack of an insulating atmosphere is a major contributor to this vast range in surface temperature.


Venus is the second planet out from the Sun, which it orbits every 225 days at a mean distance of 108 million km (67 million miles). It is the planet which can come closest to the Earth and the brightest object in our sky apart from the Sun and Moon. The reason for its brilliance is the fact that it is covered in dense white clouds, which reflect 79 per cent of the sunlight received by the planet. There are no features on the visible disk that are permanent enough for accurate measurements of the rotation period to be made. However, in 1961, radar signals were bounced from the surface of the planet. Those received back from the approaching side were of a higher frequency than those on the receding side, owing to the Doppler effect. These observations, coupled with subsequent measurements, have led to the determination of an axial rotation period of 243.02 days. Remarkably, the rotation is retrograde (opposite to the direction of orbital movement).

The principal constituent of the Venusian atmosphere is carbon dioxide, although traces of many other materials have been found, including hydrogen sulphide, carbon monoxide, water vapor, sulphur dioxide, argon, krypton, and xenon. The atmosphere extends to a height of around 250 km (150 miles) above the Venusian surface, although 90 per cent of it is concentrated within 28 km (17 miles) of the surface. The result is a surface pressure of around 90 times that on the Earth. The temperature is well over 400°C (750°F). This extreme temperature is due to a runaway greenhouse effect: the heat received from the Sun at the surface is trapped by carbon dioxide and is unable to escape back into space.

All our knowledge of the Venusian surface has come from American and Soviet space probes. Radar mapping has shown us that most of the Venusian surface consists of rolling plains, although there are several highland regions. These include the two areas Aphrodite Terra, which straddles the equator, and Ishtar Terra in the north. The latter contains several mountains, including Maxwell Montes, the highest mountains on Venus, situated at the eastern end of Ishtar and rising to around 11 km (7 mlles) above the mean surface of Venus. Near Maxwell Montes is Cleopatra Patera, an impact crater 1.5 km (1 mlle) deep, 100 km (60 miles) in diameter, which has a smaller crater 1 km (1/2 mile) deep and 55 km (35 miles) in diameter at its center. Ishtar itself is about the size of Australia and has an average height of almost 3 km (2 miles). Aphrodite is comparable in size to Africa.

Another highland region is Beta Regio, which contains Rhea Mons and Theia Mons, two large and possibly active shield volcanoes, 4 km (2 1/2 miles) high. Another feature of note is Diana Chasma, a huge valley comparable in size to the Valles Marineris system on Mars. Diana Chasma is the deepest fracture on Venus, with a depth of around 2 km (1 1/4 miles) and a width of nearly 300 km (180 mlles).


The Earth is the third planet out from the Sun. It is very unlike Venus, with its covering of corrosive cloud and inferno-like surface conditions, and the fourth planet Mars, with its barren, desert landscape and tenuous atmosphere. Earth's nitrogen-rich atmosphere plays host to constantly changing weather patterns. It enshrouds a world covered primarily by seas and oceans with a variety of land masses, some ice-covered, others basking in tropical temperatures. It is a world ideally suited to the needs of mankind, its dominant species.

Plate tectonics

Even a casual look at a map of the Earth will reveal some interesting facts. It can be seen that the continents of Africa and South America would, if not for the intervention of the Atlantic Ocean, fit together like a huge jigsaw. The same applies to North America and Europe. This was noticed earlier this century by the German meteorologist and geophysicist Alfred Lothar Wegener. He was of the opinion that an original supercontinent, which he called Pangaea, broke up some 200 million years ago and began to drift apart. This theory has since been refined. It is now thought that the original land mass first of all broke into two pieces, a northern Laurasia and a southern Gondwanaland. Laurasia then divided into North America, Asia (except India) and Europe, while Gondwanaland became America, South America, India, Australia, and Antarctica.

The idea of drifting continents was initially scorned, but current research has verified it. The problem with the original idea was that nobody knew just what was pushing these huge land masses across the Earth's surface. However, the discovery of volcanoes on the floor of the oceans solved the problem. The new material that is being ejected from inside the Earth along such formations as the Mid-Atlantic Ridge causes the ocean floor to expand and spread out, with the result that the continents on either side are moving apart at the rate of several centimetres (about an inch) per year. This so-called seafloor spreading has been linked with continental drift to formulate "plate tectonics."

Examination of seismic activity has led to the realization that the Earth's crust and upper, solid mantle is a region called the lithosphere, which is divided into a number of major and minor plates. Continents and ocean floors are of much lighter material and are simply carried around by the plates; the plates float on a layer of liquid, lower mantle rock called the asthenosphere. Beneath this layer extends the planet's molten outer core.

Currents within the Earth's mantle result in the expulsion of material through joint lines between plates. These joint lines are the ocean ridges, the effect of this upwelling of material being to force the two adjoining plates away from each other. Where plates collide, one can be dragged down below the other, resulting in the formation of an oceanic trench. The deepest known example is the Mariana Trench in the northwest Pacific Ocean, which has a maximum depth of over 11,000 m (36,000 ft). Mountain ranges can also be formed, such as the Himalayas, which have arisen through the collision of the Indian and Asian plates.

Air and water

The Earth's atmosphere differs greatly from those of its two neighboring planets. Both the Venusian and Martian atmospheres are composed almost entirely of carbon dioxide, while the Earth's atmosphere contains very little. The dominant material in our atmosphere is nitrogen, which constitutes 77 per cent of the total atmospheric content. This is coupled with around 21 per cent oxygen, a gas which is almost nonexistent on either Venus or Mars. Why is our atmosphere so different? The answer lies with the abundance of life that populates the Earth's surface. Lire has existed in one form or another on our planet for over three billion years, and the mechanisms of life, including processes such as photosynthesis, are mainly responsible for the atmosphere we have today. Our planet also has an abundance of water. This covers over 70 per cent of the Earth's surface, and has a total area of around 363 million sq. km (140 million sq. mlles). Again, this is in stark contrast to either Mars or Venus, both of which are extremely arid and inhospitable worlds.

Copyright © 1990 by Quarto Publishing plc

Customer Reviews

Average Review:

Write a Review

and post it to your social network


Most Helpful Customer Reviews

See all customer reviews >