Copyright

Published by Collins

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© HarperCollins Publishers 2020

Text and illustrations © Storm Dunlop and Wil Tirion

Cover illustrations © Julia Murray

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eBook Edition © October 2020

ISBN 9780008403614

Version: 2020-09-11

Contents

Cover

Title Page

Copyright

Foreword

Introduction

Some Interesting Objects

Major Events in 2021

The Moon

Map of the Moon

The Circumpolar Constellations

The Monthly Maps

World Map

January

February

March

April

May

June

July

August

September

October

November

December

Dark Sky Sites

Twilight Diagrams

Glossary and Tables

Searchable Terms

Acknowledgements

About the Publisher

Foreword

For thousands of generations, your ancestors have marvelled at the night sky above them. They pondered its meaning and distance, tracing patterns and telling stories with the stars; they watched with great interest as the Moon and a handful of bright, wandering lights traversed the sky independently; sometimes they looked on in awe as a new visitor drifted above, with a long tail in tow; at other times they stayed up, alert, as stars appeared to streak across the sky with alarming speed; perhaps they cowered in fear when, on rare occasions, the Moon turned blood red or challenged the daylight by taking a bite from the Sun.

Gradually, superstition gave way to understanding. Comprehension of the night sky’s cyclical nature proved foundational to an inevitable agricultural revolution, which propelled the progress of civilisation forward, forever cementing astronomy’s influence on our lives. The stars have also permeated into our wider cultural heritage, appearing in religious texts and great works of art – visual, poetic and musical. The Moon, our unfailing celestial companion, altered the course of life on Earth through its mastery of the tides, and has since become a frontier of exploration for our species – a stepping stone towards our spacefaring future. It is remarkable how far we have come since humans first turned their attention to the stars over 50,000 years ago.

Today, our lives are very different from theirs, but the curiosity driving the earliest stargazers has not left us. It has only become muffled by the increasing intrusion of artificial light pollution that has brightened our skies. The stars, planets and Moon still play overhead as they always have, and each year many annual meteor showers and other events go underappreciated as fewer of us enjoy regular access to the night sky. Yet, with only a little guidance you can rekindle this deeply ingrained fascination and, armed with the extensive knowledge of the modern astronomical age, explore the universe we call home.

This Almanac invites you to reconnect with your ancestral stargazing roots, by following the progress of the constellations throughout the seasons. With the aid of its monthly calendars and maps, you will chart the rhythm of the lunar phases, discover events that light up the sky for brief periods, and explore the rich tapestry of characters that adorn the starry canvas overhead. You can delve as deeply as you like, or follow your own favourite subject throughout the year. The treasures of the night sky will be waiting for you.

Tom Kerss

Introduction

The aim of this book is to help people to find their way around the night sky and to understand what is visible every month, from anywhere in the world. The stars that may be seen depend on where you are on Earth, but even if you travel widely, this book will show you what you can see. The night sky also changes from month to month and these changes, together with some of the significant events that occur during the year are described and illustrated.

The charts that are used differ considerably from those found in most astronomy books, and have been specifically designed for use anywhere in the world. A full description of how to use and understand the monthly charts is given here to here.

Sunrise, sunset and twilight

The conditions for observing naturally vary over the course of the year and one’s location on Earth. Sunrise and sunset vary considerably, depending in particular on one’s latitude. Sunrise and sunset times are given each month for nine different locations around the world. These places are shown in a bold typeface on the world map here and here. Sunrise and sunset times are given for the first and last days in every month, for these specific locations. Another factor that influences what may be seen is twilight at dusk and dawn. Again, this varies considerably with one’s latitude on Earth. The diagrams here to here show how this varies for the nine locations, which have been chosen to show the range of variation, rather than just for the importance of the places that have been included. The different stages of twilight and how they affect observing are also explained there.

Moonlight

Yet another factor that affects the visibility of objects is the amount of moonlight in the sky. At Full Moon, it may be very difficult to see some of the fainter stars and objects, and even when the Moon is at a smaller phase it seriously interferes with visibility if it is near the stars or planets in which you are interested. A full lunar calendar is given for each month and may be used to see when nights are likely to be darkest and best for observing.

The celestial sphere

All the objects in the sky (including the Sun, Moon, and stars) appear to lie at some indeterminate distance on a large sphere, centred on the Earth. This celestial sphere has various reference points and features that are related to those of the Earth. If the Earth’s rotational axis is extended, for example, it points to the North and South Celestial Poles, which are thus in line with the North and South Poles on Earth. As shown in the diagrams, the altitude of the celestial pole is equal to the observer’s latitude, whether in the north or south. Similarly, the celestial equator lies in the same plane as the Earth’s equator, and divides the sky into northern and southern hemispheres.

The altitude of the Celestial Pole equals the observer’s latitude.

It is useful to know some of the special terms for various parts of the sky. As seen by an observer, half of the celestial sphere is invisible, below the horizon. The point directly overhead is known as the zenith, and this point is shown on the monthly charts for several different latitudes, where it is an important reference point. The (invisible) point below one’s feet is the nadir. The line running from the north point on the horizon, up through the zenith and then down to the south point is the meridian. This is an important invisible line in the sky, because objects are highest in the sky, and thus easiest to see, when they cross the meridian in the south. Objects are said to transit, when they cross this line in the sky.

Measuring altitude and azimuth on the celestial sphere.

In this book, reference is sometimes made in the text and in the diagrams to the standard compass points around the horizon. The position of any object in the sky may be described by its altitude (measured in degrees above the horizon) and its azimuth (measured in degrees from north, 0°, through east, 90°, south, 180°, and west, 270°). Experienced amateurs and professional astronomers also use another system of specifying locations on the celestial sphere, but that need not concern us here, where the simpler method will suffice.

The celestial sphere appears to rotate about an invisible axis, running between the north and south celestial poles. The location (i.e., the altitude) of the celestial poles depends entirely on the observer’s position on Earth or, more specifically, their latitude.

The ecliptic and the zodiac

Another important line on the celestial sphere is the Sun’s apparent path against the background stars – in reality the result of the Earth’s orbit around the Sun. This is known as the ecliptic. The point where the Sun, apparently moving along the ecliptic, crosses the celestial equator from south to north is known as the vernal (or northern spring) equinox, which occurs around March 21. At this time (and at the northern autumnal equinox, on September 22 or 23, when the Sun crosses the celestial equator from north to south) day and night are almost exactly equal in length. (There is a slight difference, but that need not concern us here.) The vernal equinox is currently located in the constellation of Pisces, and is important in astronomy because it defines the zero point for a system of celestial coordinates, which is, however, not used in this book.

The Moon and planets are to be found in a band of sky that extends 8° on either side of the ecliptic. This is because the orbits of the Moon and planets are inclined at various angles to the ecliptic (i.e., to the plane of the Earth’s orbit). This band of sky is known as the zodiac, and when originally devised, consisted of twelve constellations, all of which were considered to be exactly 30° wide. When the constellation boundaries were formally established by the International Astronomical Union in 1930, the exact extent of most constellations was altered, and nowadays, the ecliptic passes through thirteen constellations. Because of the boundary changes, the Moon and planets may actually pass through several other constellations that are adjacent to the original twelve.

The smallest constellation is Crux. It covers an area of just 68 square degrees.

The constellations

In the western astronomical tradition, the celestial sphere has always been divided into various constellations, most dating back to antiquity and usually associated with certain myths or legendary people and animals. Nowadays, 88 constellations cover the whole sky, and their boundaries have been fixed by international agreement. Their names (in Latin) are largely derived from Greek or Roman originals. (A full list of the constellations is given here to here, with their names in English, their abbreviations, and genitive forms.) Some of the names of the most prominent stars are of Greek or Roman origin, but many are derived from Arabic names. Some bright stars have no individual names, and for many years, they were identified by terms such as ‘the star in Hercules’ right foot’. A more sensible scheme was introduced by the German astronomer Johannes Bayer in the early seventeenth century. Following his scheme – which is still used today – most of the brightest stars are identified by a Greek letter followed by the genitive form of the constellation’s Latin name. An example is the Pole Star, also known as Polaris and α Ursae Minoris. The Greek alphabet is shown here.

The largest constellation is Hydra. It covers an area of 1303 square degrees, and more than 7 hours of Right Ascension (105°).

Asterisms

Apart from the constellations, certain groups of stars, which may form a small part of a larger constellation, are readily recognizable and have been given individual names. These groups are known as asterisms, and the most famous (and well-known) is the ‘Plough’ or ‘Big Dipper’, the common name for the seven brightest stars in the constellation of Ursa Major, the Great Bear. The names and identifications of some popular asterisms are given in the list here.

The Moon

As it passes across the sky from west to east in its orbit around the Earth, the Moon moves by approximately its diameter (about half a degree) in an hour. Normally, in its orbit, the Moon passes above or below the direct line between Earth and Sun (at New Moon) or outside the area obscured by the Earth’s shadow (at Full Moon). Occasionally, however, the three bodies are more-or-less perfectly aligned to give an eclipse: a solar eclipse at New Moon, or a lunar eclipse at Full Moon. Depending on the exact circumstances, a solar eclipse may be merely partial (when the Moon does not cover the whole of the Sun’s disc); annular (when the Moon is too far from Earth in its orbit to appear large enough to hide the whole of the Sun); or total. Total and annular eclipses are visible from very restricted areas of the Earth, but partial eclipses are normally visible over a wider area. Two forms of solar eclipse occur this year, and are described in detail in the appropriate month.

Precautions must always be taken when viewing even partial phases of a solar eclipse to avoid damage to your eyes. Only ever use proper eclipse glasses, or a proper solar filter over the full objective of a telescope. The glass ‘solar filters’ sometimes provided with cheap telescopes should never be used. They are unsafe.

When the Moon passes through the Earth’s shadow (top), a lunar eclipse occurs. When it passes in front of the Sun (below) a solar eclipse occurs.

Somewhat similarly, at a lunar eclipse, the Moon may pass through the outer zone of the Earth’s shadow, the penumbra (in a penumbral eclipse, which is not generally perceptible to the naked eye); pass so that just part of the Moon is within the darkest part of the Earth’s shadow, the umbra (in a partial eclipse); or completely within the umbra (in a total eclipse). Unlike solar eclipses, lunar eclipses are visible from large areas of the Earth. Again, these are described in detail in the relevant month.

Occasionally, as it moves across the sky, the Moon passes between the Earth and individual planets or distant stars, giving rise to an occultation. As with solar eclipses, such occultations are visible from restricted areas of the world, but certain significant occultations are described in detail.

The Planets

Because the planets are always moving against the background stars, they are treated in some detail in the monthly pages and information is given when they are close to other planets, the Moon, or any of five bright stars that lie near the ecliptic. Such events are known as appulses or, more frequently, as conjunctions. (There are technical differences in the way these terms are defined – and should be used – in astronomy, but these need not concern us here.) The term conjunction is also used when a planet is either directly behind or in front of the Sun, as seen from Earth. (Under normal circumstances it will then be invisible.) The conditions of most favourable visibility depend on whether the planet is one of the two known as inferior planets (Mercury and Venus) or one of the three superior planets (Mars, Jupiter and Saturn) that are covered in detail. Brief details of the fainter superior planets, Uranus and Neptune, are given, especially when they come to opposition.

The inferior planets are most readily seen at eastern or western elongation, when their angular distance from the Sun is greatest. For superior planets and minor planets, they are best seen at opposition, when they are directly opposite the Sun in the sky, and cross the meridian at local midnight.

Inferior planet.

Superior planet.

Measuring angles in the sky.

It is often useful to be able to estimate angles on the sky, and approximate values may be obtained by holding one hand at arm’s length. The various angles are shown in the diagram, together with the separations of the various stars in the asterism, known as the Plough or Big Dipper, and also for stars around the constellation of Orion.

Events

A number of interesting events are shown in diagrams for each month. They involve the planets and the Moon, sometimes showing them in relation to specific stars. Events have been chosen as they will appear from one of three different locations: from London; from the central region of the USA; or from Sydney in Australia. Naturally, these events are visible from other locations, but the appearance of the objects on the sky will differ slightly from the diagrams. A list of major astronomical events in 2021 is given here and here.

Meteor shower (showing the April Lyrid radiant).

Meteors

At some time or other, nearly everyone has seen a meteor – a ‘shooting star’ – as it flashed across the sky. The particles that cause meteors – known technically as ‘meteoroids’ – range in size from that of grain of sand (or even smaller) to the size of a pea. On any night of the year there are occasional meteors, known as sporadics, that may travel in any direction. These occur at a rate that is normally between 3 and 8 in an hour. Far more important, however, are meteor showers, which occur at fixed periods of the year, when the Earth encounters a trail of particles left behind by a comet or, very occasionally, by a minor planet (asteroid). Meteors always appear to diverge from a single point on the sky, known as the radiant, and the radiants of major showers are shown on the charts.

Meteors that come from a circular area, 8° in diameter, around the radiant are classed as belonging to the particular shower. All others that do not come from that area are sporadics (or, occasionally, from another shower that is active at the same time). A list of the major meteor showers is given on the next page. Although the positions of the various shower radiants are shown on the charts, looking directly at the radiant is not the most effective way of seeing meteors. They are most likely to be noticed if one is looking about 40–45° away from the radiant position. (This is approximately two hand-spans as shown in the diagram for measuring angles here.)

Shower | Dates of activity 2021 | Date of maximum 2021 | Possible hourly rate
Quadrantids | Dec. 28 to Jan. 12 | Jan. 3–4 | 120
α-Centaurids | Jan.31 to Feb. 20 | Feb. 8 | 6
γ-Normids | Feb. 24 to Mar. 24 | Mar. 14–15 | 6
April Lyrids | Apr. 13–29 | Apr. 22 | 18
π-Puppids | Apr. 14 to Apr. 27 | Apr. 23–24 | var.
η-Aquariids | Apr. 18 to May 27 | May 6 | 40
Piscis Austrinids | Jul. 14 to Aug. 27 | Jul. 28 | 5
α-Capricornids | Jul. 2 to Aug. 14 | Jul. 30 | 5
Southern δ-Aquariids | Jul. 13 to Aug. 24 | Jul. 30 | < 25
Perseids | Jul. 16 to Aug. 23 | Aug. 12–13 | 150
α-Aurigids | Aug. 28 to Sep. 5 | Sep. 1 | 6
Southern Taurids | Sep. 10 to Nov. 20 | Oct. 10 | < 5
Draconids | Oct. 7–11 | Oct. 8–9 | var.
Orionids | Oct. 1 to Nov. 6 | Oct. 21 | 25
Northern Taurids | Oct. 20 to Dec. 10 | Nov. 12 | < 5
Leonids | Nov. 5–29 | Nov. 17–18 | < 15
Phoenicids | Nov. 22 to Dec. 9 | Dec. 2 | var.
Puppid Velids | Nov. 30 to Dec. 14 | Dec. 7 | 10
Geminids | Dec. 3–16 | Dec. 14 | 120+
Ursids | Dec. 17–26 | Dec. 22–23 | < 10

Other objects

Certain other objects may be seen with the naked eye under good conditions. Some were given names in antiquity – Praesepe is one example – but many are known by what are called ‘Messier numbers’, the numbers in a catalogue of nebulous objects compiled by Charles Messier in the late-eighteenth century. Some, such as the Andromeda Galaxy, M31, and the Orion Nebula, M42, may be seen faintly by the naked eye, but all those given in the list here will benefit from the use of binoculars.

Apart from galaxies, such as M31, which contain thousands of millions of stars, there are also two types of cluster: open clusters, such as M45, the Pleiades, which may consist of a few dozen to some hundreds of stars; and globular clusters, such as M13 in Hercules, which are spherical concentrations of many thousands of stars. One or two gaseous nebulae, consisting of gas illuminated by stars within them are also visible. The Orion Nebula, M42, is one, and is illuminated by the group of four stars, known as the Trapezium, which may be seen within it by using a good pair of binoculars. A list of interesting objects is given on the next page.

In 1781, Charles Messier (26 June 1730 – 12 April 1817) published the final version of his catalogue of 110 nebulous objects and faint star clusters that might be confused with comets. The objects in this catalogue are still known as the Messier objects and are always quoted as ‘M’ numbers.

Eta Carinae (η Carinae) is one of the most massive and luminous stars known. It is estimated to have a mass between 120 and 150 times that of the Sun, and be between four and five million times as luminous.

Dates and time

Astronomers, worldwide, use a standardized method of expressing the date and time. This prevents confusion in comparing observations made by different observers. The various elements are given in descending order: year, month (three-letter abbreviation to prevent confusion over using numbers), day, hour, minutes, seconds. (In extreme cases, fractions of minutes or seconds may be used.) The date and time are that on the Greenwich meridian (GMT), and ignore any changes for Summer Time / Daylight Saving Time (DST), and any adjustments for local time at the observer’s location. This standard is known as Coordinated Universal Time (UTC). In this book and in many others this is generally given as Universal Time (UT). All times given in this book are in UT.

Some Interesting Objects

Messier IC / NGC | Name | Type | Constellation
— | 47 Tucanae | globular cluster | Tucana
— | Hyades | open cluster | Taurus
— | Double Cluster | open cluster | Perseus
— | Melotte 111 | open cluster | Coma Berenices
M3 | — | globular cluster | Canes Venatici
M4 | — | globular cluster | Scorpius
M8 | Lagoon Nebula | gaseous nebula | Sagittarius
M11 | Wild Duck Cluster | open cluster | Scutum
M13 | Hercules Cluster | globular cluster | Hercules
M15 | — | globular cluster | Pegasus
M22 | — | globular cluster | Sagittarius
M27 | Dumbbell Nebula | planetary nebula | Vulpecula
M31 | Andromeda Galaxy | galaxy | Andromeda
M35 | — | open cluster | Gemini
M42 | Orion Nebula | gaseous nebula | Orion
M44 | Praesepe | open cluster | Cancer
M45 | Pleiades | open cluster | Taurus
M57 | Ring Nebula | planetary nebula | Lyra
M67 | — | open cluster | Cancer
IC 2602 | Southern Pleiades | open cluster | Carina
NGC 752 | — | open cluster | Andromeda
NGC 3242 | Ghost of Jupiter | planetary nebula | Hydra
NGC 3372 | Eta Carinae Nebula | gaseous nebula | Carina
NGC 4755 | Jewel Box | open cluster | Crux
NGC 5139 | Omega Centauri | globular cluster | Centaurus

To avoid problems over the changes involved in moving to and from Summer Time / Daylight Saving Time (and the complications over the beginning and end dates) and also the adjustments for local time, experienced astronomers set a (cheap) watch or clock to Universal Time and keep it that way. Smartphone users may use a simple world clock app, provided they lock it to the time (GMT) on the Greenwich Meridian.

Similarly, the date given for an event is the date as it applies at the Greenwich meridian, i.e., in UT. Occasionally this may differ from the date as given by your local time. An event that occurs (say) late in the night in Europe may seem to occur on the previous day to an observer to the west (such as in the USA), when local time is taken into account. This is another complication that is avoided by using the Universal Time standard.

In 1801, the Italian astronomer Giuseppe Piazzi discovered what appeared to be a new planet orbiting between Mars and Jupiter, and named it Ceres. William Herschel proved it to be a very small object, calculating it to be only 320 km in diameter, and not a planet. He proposed the name asteroid, and soon other similar bodies were found. We now know that Ceres is 952 km in diameter. It is now considered to be a dwarf planet.

The Moon

The Moon at First Quarter.

The Moon

The monthly pages include diagrams showing the phase of the Moon for every day of the month, and also indicate the day in the lunation (or age of the Moon), which begins at New Moon. The diagrams showing the Moon’s phase are repeated for southern-hemisphere observers who will see the Moon, south up. Although the main features of the surface – the light highlands and the dark maria (seas) – may be seen with the naked eye, far more features may be detected with the use of binoculars or any telescope. The many craters are best seen when they are close to the terminator (the boundary between the illuminated and the non-illuminated areas of the surface), when the Sun rises or sets over any particular region of the Moon and the crater walls or central peaks cast strong shadows. Most features become difficult to see at Full Moon, although this is the best time to see the bright ray systems surrounding certain craters. Accompanying the Moon map on the following pages is a list of prominent features, including the days in the lunation when features are normally close to the terminator and thus easiest to see. A few bright features such as Linné and Proclus, visible when well illuminated, are also listed. One feature, Rupes Recta (the Straight Wall) is readily visible only when it casts a shadow with light from the east, appearing as a light line when illuminated from the opposite direction.

The dates of visibility vary slightly through the effects of libration. Because the Moon’s orbit is inclined to the Earth’s equator and also because it moves in an ellipse, the Moon appears to rock slightly from side to side (and nod up and down). Features near the limb (the edge of the Moon) may vary considerably in their location and visibility. (This is easily noticeable with Mare Crisium and the craters Tycho and Plato.) Another effect is that at crescent phases before and after New Moon, the normally non-illuminated portion of the Moon receives a certain amount of light, reflected from the Earth. This Earthshine may enable certain bright features (such as Aristarchus, Kepler and Copernicus) to be detected even though they are not illuminated by sunlight.

Moon features

The numbers on the following page indicate the age of the Moon when features are usually best visible.

Abulfeda | 6:20
Agrippa | 7:21
Albategnius | 7:21
Aliacensis | 7:21
Alphonsus | 8:22
Anaxagoras | 9:23
Anaximenes | 11:25
Archimedes | 8:22
Aristarchus | 11:25
Aristillus | 7:21
Aristoteles | 6:20
Arzachel | 8:22
Atlas | 4:18
Autolycus | 7:21
Barrow | 7:21
Billy | 12:26
Birt | 8:22
Blancanus | 9:23
Bullialdus | 9:23
Bürg | 5:19
Campanus | 10:24
Cassini | 7:21
Catharina | 6:20
Clavius | 9:23
Cleomedes | 3:17
Copernicus | 9:23
Cyrillus | 6:20
Delambre | 6:20
Deslandres | 8:22
Endymion | 3:17
Eratosthenes | 8:22
Eudoxus | 6:20
Fra Mauro | 9:23
Fracastorius | 5:19
Franklin | 4:18
Gassendi | 11:25
Geminus | 3:17
Goclenius | 4:18
Grimaldi | 13–14:27–28
Gutenberg | 5:19
Hercules | 5:19
Herodotus | 11:25
Hipparchus | 7:21
Hommel | 5:19
Humboldt | 3:15
Janssen | 4:18
Julius Caesar | 6:20
Kepler | 10:24
Landsberg | 10:24
Langrenus | 3:17
Letronne | 11:25
Linné | 6
Longomontanus | 9:23
Macrobius | 4:18
Mädler | 5:19
Maginus | 8:22
Manilius | 7:21
Mare Crisium | 2–3:16–17
Maurolycus | 6:20
Mercator | 10:24
Metius | 4:18
Meton | 6:20
Mons Pico | 8:22
Mons Piton | 8:22
Mons Rümker | 12:26
Montes Alpes | 6–8:21
Montes Apenninus | 8
Orontius | 8:22
Pallas | 8:22
Petavius | 3:17
Philolaus | 9:23
Piccolomini | 5:19
Pitatus | 8:22
Pitiscus | 5:19
Plato | 8:22
Plinius | 6:20
Posidonius | 5:19
Proclus | 14:18
Ptolemaeus | 8:22
Purbach | 8:22
Pythagoras | 12:26
Rabbi Levi | 6:20
Reinhold | 9:23
Rima Ariadaeus | 6:20
Rupes Recta | 8
Saussure | 8:22
Scheiner | 10:24
Schickard | 12:26
Sinus Iridum | 10:24
Snellius | 3:17
Stöfler | 7:21
Taruntius | 4:18
Thebit | 8:22
Theophilus | 5:19
Timocharis | 8:22
Triesnecker | 6–7:21
Tycho | 8:22
Vallis Alpes | 7:21
Vallis Schröteri | 11:25
Vlacq | 5:19
Walther | 7:21
Wargentin | 12:27
Werner | 7:21
Wilhelm | 9:23
Zagut | 6:20

Map of the Moon

The Circumpolar Constellations
The northern circumpolar constellations

Learning the patterns of the stars, the constellations and asterisms is not particularly difficult. You need to start by identifying the various constellations that are circumpolar where you live. These are always above the horizon, so you can generally start at any time of the year. The charts here and here show the northern and southern circumpolar constellations, respectively. The fine, dashed lines indicate the areas that are circumpolar at different latitudes.

A single day on Venus lasts 243.025 days. This rotation is longer than its orbital period, which is 224.701 days. Its rotation is retrograde: in the opposite direction to that of all the other planets.

The key constellation when learning the pattern of stars in the northern sky is Ursa Major, in particular the seven stars forming the asterism, known to more-or-less everyone, as ‘the Plough’ or to people in North America, as the ‘Big Dipper’. As the chart shows, this is just circumpolar for anyone at latitude 40°N, except for Alkaid (η Ursae Majoris), the last star in the ‘tail’. Even so, the asterism of the Plough is low on the northern horizon between September and November, so it will be much easier to make out at other times of the year.

The two stars Dubhe and Merak (α and β Ursae Majoris) are known as the ‘Pointers’, because they indicate the position of Polaris, the Pole Star (α Ursae Minoris), at about a distance of five times their separation. Following this line takes you to the constellation of Ursa Minor, the ‘Little Bear’ or ‘Little Dipper’, where Polaris is at the end of the ‘tail’ or ‘handle’. On the far side of the Pole is the constellation of Cassiopeia, which is highly distinctive, with its five main stars forming the letter ‘M’ or ‘W’, depending on its orientation. Cassiopeia is circumpolar for observers at latitude 40°N or closer to the North Pole, although at times it is near the northern horizon and more difficult to see. (But at such times Ursa Major is clearly visible.) To find Cassiopeia from Ursa Major, start at Alioth (ε Ursae Majoris) and extend a line from that star to Polaris and beyond. It points to the central star of the five. Moving anticlockwise from Cassiopeia, we come to Cepheus, which has been likened to the gable end of a house, with its base in the Milky Way. The line from the Pointers to Polaris, if extended points to Errai (γ Cephei), at the top of the ‘gable’. Continuing in the same direction, we come to Draco, which wraps around Ursa Minor. The quadrilateral of stars that forms the ‘head’ of Draco is just circumpolar for observers at 40°N latitude, although it is brushing the horizon in January. On the opposite side of the sky to the head of Draco, the whole of the faint constellation of Camelopardalis is visible.

The northern circumpolar constellations.

For observers slightly farther north, say at 50°N, additional constellations become circumpolar. The most important of these are Perseus, not far from Cassiopeia, and most of which is visible and, farther round, the northern portion of Auriga, with bright Capella (α Aurigae). On the other side of the sky is Deneb, the brightest star in Cygnus, although it is often close to the horizon, especially during the early night during the winter months. Vega (α Lyrae) another of the three stars that form the Summer Triangle is even farther south, often brushing the northern horizon, and only truly circumpolar and clearly seen at any time of the year for observers at 60°N.

Such far northern observers will also find that Castor (α Geminorum) is actually circumpolar, although at times it is extremely low on the horizon. The other bright star in Gemini, Pollux (β Geminorum) is slightly farther south and cannot really be considered circumpolar.

The southern circumpolar constellations

Just as Ursa Major is the key constellation in the northern sky, so is Crux (the Southern Cross) an easily recognized feature of the southern circumpolar sky, although at times it may be brushing the horizon for observers at 30°S – roughly the latitude of Sydney in Australia. This is particularly true in the southern spring. Northeners, new to the southern sky, sometimes mistake the ‘False Cross’, which consists of two stars from each of the constellations of Vela and Carina for the true Southern Cross. Crux itself is accompanied by the clearly visible dark cloud of the ‘Coalsack’ and also the two brightest stars in Centaurus: Rigil Kentaurus and Hadar (α and β Centauri, respectively). Together, the four stars of Crux and the two from Centaurus act as principal guides to the southern constellations.

The southern circumpolar constellations.

Unfortunately, unlike the situation in the north, there is no star conveniently located at the South Celestial Pole (SCP), which lies in a relatively empty region of sky in the faint, triangular constellation of Octans. Octans itself is perhaps best found by using the stars of Pavo as guides. A line from Peacock (α Pavonis) through the second brightest star (β) in that constellation, if extended by about the same amount as the distance between the two stars, points to the ‘base’ of the triangle of Octans.

The main ‘upright’ of Crux, if extended and curving slightly to the right, does point in the approximate direction of the Pole, passing through Musca and the tip of Chamaeleon. However, a better way is to start at Hadar (the star in the bright pair that is closest to Crux), turn at right-angles at Rigil Kentaurus, and following an imaginary line through the brightest star in the small constellation of Circinus and then right across the sky, brushing past the outlying star of Apus, and the star (δ) at the apex of Octans itself. Centaurus is a straggling constellation, with many stars well north of Rigil Kentaurus and Hadar, and some fainter ones that partially enclose Crux. Starting at Crux, and moving clockwise (in the same direction as the sky rotates), we come to the stars of Carina and Vela, both originally part of the large, now obsolete constellation of Argo Navis. Past the False Cross, we come to Canopus (α Carinae), the second brightest star in the sky, which is just circumpolar for observers at 40°S, although occasionally, especially in June, very low over the northern horizon. Lying between Canopus and the SCP is the Large Magellanic Cloud (LMC), a satellite galaxy to our own. It lies across the boundaries of the constellations of Dorado and Mensa.

Olympus Mons, on Mars, is the highest mountain (volcano) in the Solar System. It is about 27 km high (measured as 21,287.4 metres above a datum level) and is roughly the size of Spain.

Continuing round from Canopus we pass the constellations of Dorado, the small constellation of Reticulum and the undistinguished constellation of Horologium, beyond which is Achernar (α Eridani) the brightest star in the long, winding constellation of Eridanus, which actually starts far to the north, close to Rigel in Orion. Between Achernar and the SCP, lies the triangular constellation of Hydrus, next to the constellation of Tucana which contains the Small Magellanic Cloud (SMC).

For observers farther south (at say, 50°S) there are the constellations of Phoenix, followed by the roughly cross-shaped constellation of Grus and the rather faint Indus. For observers at 40°S, the whole of the constellation of Pavo is visible, including its brightest star, Peacock. Farther round there is the constellation of Ara and, for observers farther north at 30°S, Triangulum Australe is fully visible.

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