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2019 Guide to the Night Sky: Bestselling month-by-month guide to exploring the skies above Britain and Ireland

2019 Guide to the Night Sky: Bestselling month-by-month guide to exploring the skies above Britain and Ireland
2019 Guide to the Night Sky: Bestselling month-by-month guide to exploring the skies above Britain and Ireland Wil Tirion Storm Dunlop Royal Observatory Greenwich “THIS IS A GREAT GUIDE TO THE NIGHT SKY AT A GREAT PRICE.” Astronomy NowA comprehensive handbook to the planets, stars and constellations visible from the northern hemisphere. 6 pages for each month covering January–December 2019.This practical guide is both an easy introduction to astronomy and a useful reference for seasoned stargazers. Now includes a section on comets and a map of the moon.Designed for Britain and Ireland but usable anywhere in the world between 40°N and 60°N, covering most of Europe, southern Canada and the northern United States.Written and illustrated by astronomical experts, Storm Dunlop and Wil Tirion, and approved by the astronomers of the Royal Observatory Greenwich.Content includes:• Advice on where to start looking.• Easy-to-use star maps for each month with descriptions of what to see.• Positions of the moon and visible planets.• Details of objects and events you might see in 2019.• Diagrams of notable events visible from Britain.Also available: A month-by-month guide to exploring the skies south of the Equator ISBN 978-0-00-829499-1 andA month-by-month guide to exploring the skies above North America ISBN 978-0-22-810105-5. Copyright (#ulink_f92ea3b5-bb56-5551-b4da-ada1acb7e34f) Published by Collins An imprint of HarperCollins Publishers Westerhill Road Bishopbriggs Glasgow G64 2QT www.harpercollins.co.uk (http://www.harpercollins.co.uk) In association with Royal Museums Greenwich, the group name for the National Maritime Museum, Royal Observatory Greenwich, Queen’s House and Cutty Sark www.rmg.co.uk (http://www.rmg.co.uk) © HarperCollins Publishers 2018 Text and illustrations © Storm Dunlop and Wil Tirion Photographs © see acknowledgements here (#litres_trial_promo) Collins ® is a registered trademark of HarperCollins Publishers Ltd All rights reserved under International and Pan-American Copyright Conventions. By payment of the required fees, you have been granted the non-exclusive, non-transferable right to access and read the text of this e-book on screen. No part of this text may be reproduced, transmitted, downloaded, decompiled, reverse engineered, or stored in or introduced into any information storage and retrieval system, in any form or by any means, whether electronic or mechanical, now known or hereafter invented, without the express written permission of HarperCollins. The contents of this publication are believed correct at the time of printing. Nevertheless the publisher can accept no responsibility for errors or omissions, changes in the detail given or for any expense or loss thereby caused. HarperCollins does not warrant that any website mentioned in this title will be provided uninterrupted, that any website will be error free, that defects will be corrected, or that the website or the server that makes it available are free of viruses or bugs. For full terms and conditions please refer to the site terms provided on the website. A catalogue record for this book is available from the British Library eBook Edition © Sep 2018 ISBN 9780008311643 Version: [2018-07-14] Contents Cover (#u0e0976e4-942e-5a6c-bd45-c0943f31e63f) Title Page (#u5259c494-d8cb-5e09-be1b-4b13c5cadce6) Copyright (#ulink_92924074-489e-53bb-9719-95471fcb0c89) Introduction (#ulink_d36dac5f-d4d3-5eab-be64-1999befa3a42) The Constellations (#ulink_9d8bdcb7-9ff7-55cb-812c-074a45cdb598) The Northern Circumpolar Constellations (#ulink_9d8bdcb7-9ff7-55cb-812c-074a45cdb598) Comets and the Moon (#ulink_009baf63-9a1c-5768-9775-c3a9b40dbe7a) Map of the Moon (#ulink_f80fc6d0-d02d-58be-8a1c-4b36f472ab44) Introduction to the Month-by-Month Guide (#litres_trial_promo) Month-by-Month Guide (#litres_trial_promo) January (#litres_trial_promo) February (#litres_trial_promo) March (#litres_trial_promo) April (#litres_trial_promo) May (#litres_trial_promo) June (#litres_trial_promo) July (#litres_trial_promo) August (#litres_trial_promo) September (#litres_trial_promo) October (#litres_trial_promo) November (#litres_trial_promo) December (#litres_trial_promo) Glossary and Tables (#litres_trial_promo) Acknowledgements (#litres_trial_promo) Further Information (#litres_trial_promo) About the Publisher (#litres_trial_promo) Introduction (#ulink_744b447d-f717-5ef1-ba4c-8b1c3121f6d3) The aim of this Guide is to help people find their way around the night sky, by showing how the stars that are visible change from month to month and by including details of various events that occur throughout the year. The objects and events described may be observed with the naked eye, or nothing more complicated than a pair of binoculars. The conditions for observing naturally vary over the course of the year. During the summer, twilight may persist throughout the night and make it difficult to see the faintest stars. There are three recognized stages of twilight: civil twilight, when the Sun is less than 6° below the horizon; nautical twilight, when the Sun is between 6° and 12° below the horizon; and astronomical twilight, when the Sun is between 12° and 18° below the horizon. Full darkness occurs only when the Sun is more than 18° below the horizon. During nautical twilight, only the very brightest stars are visible. During astronomical twilight, the faintest stars visible to the naked eye may be seen directly overhead, but are lost at lower altitudes. As the diagram shows, during June and most of July full darkness never occurs at the latitude of London, and at Edinburgh nautical twilight persists throughout the whole night, so at that latitude only the very brightest stars are visible. 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 may seriously interfere 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 observation. 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. Similarly, the celestial equator lies in the same plane as the Earth’s equator, and divides the sky into northern and southern hemispheres. Because this Guide is written for use in Britain and Ireland, the area of the sky that it describes includes the whole of the northern celestial hemisphere and those portions of the southern that become visible at different times of the year. Stars in the far south, however, remain invisible throughout the year, and are not included. The duration of twilight throughout the year at London and Edinburgh. 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 the (invisible) one below one’s feet as 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. In this book, reference is frequently 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. Measuring altitude and azimuth on the celestial sphere. 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 charts in this book are produced for the latitude of 50°N, so the North Celestial Pole (NCP) is 50° above the northern horizon. The fact that the NCP is fixed relative to the horizon means that all the stars within 50° of the pole are always above the horizon and may, therefore, always be seen at night, regardless of the time of year. This northern circumpolar region is an ideal place to begin learning the sky, and ways to identify the circumpolar stars and constellations will be described shortly. 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 spring) equinox, which occurs on March 20. At this time (and at the 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 Guide. The altitude of the North Celestial Pole equals the observer’s latitude. The Sun crossing the celestial equator in spring. 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 constellations Since ancient times, the celestial sphere has been divided into various constellations, most dating back to antiquity and usually associated with certain myths or legendary people and animals. Nowadays, the boundaries of the constellations have been fixed by international agreement and their names (in Latin) are largely derived from Greek or Roman originals. Some of the names of the most prominent stars are of Greek or Roman origin, but many are derived from Arabic names. Many bright stars have no individual names and, for many years, stars 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 (abbreviated α UMi). The Greek alphabet is shown here (#litres_trial_promo) with a list of all the constellations that may be seen from latitude 50°N, together with abbreviations, their genitive forms and English names. Other naming schemes exist for fainter stars, but are not used in this book. Asterisms Apart from the constellations (88 of which cover the whole sky), certain groups of stars, which may form a part of a larger constellation or cross several constellations, 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’, the common name for the seven brightest stars in the constellation of Ursa Major, the Great Bear. The names and details of some asterisms mentioned in this book are given in the list here (#litres_trial_promo). Magnitudes The brightness of a star, planet or other body is frequently given in magnitudes (mag.). This is a mathematically defined scale where larger numbers indicate a fainter object. The scale extends beyond the zero point to negative numbers for very bright objects. (Sirius, the brightest star in the sky is mag. -1.4.) Most observers are able to see stars to about mag. 6, under very clear skies. The Moon Although the daily rotation of the Earth carries the sky from east to west, the Moon gradually moves eastwards by approximately its diameter (about half a degree) in an hour. Normally, in its orbit around the Earth, 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 disk); 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. 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), 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. 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. 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 positions of the planets are shown for every month on a special chart of the ecliptic. 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. (Some details of the fainter superior planets, Uranus and Neptune, are included in this Guide, and special charts for both are given here (#litres_trial_promo) and here (#litres_trial_promo).) Inferior planet. The inferior planets are most readily seen at eastern or western elongation, when their angular distance from the Sun is greatest. For superior planets, they are best seen at opposition, when they are directly opposite the Sun in the sky, and cross the meridian at local midnight. 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 Plough. 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 a 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 three and eight 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 here (#litres_trial_promo). Superior planet. 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.) 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 by the naked eye, but all those given in the list 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. Meteor shower (showing the April Lyrid radiant). Measuring angles in the sky. Some interesting objects. The Northern Circumpolar Constellations (#ulink_1c2c3fa3-7e7c-5c3d-b862-ef58dcc83c27) The northern circumpolar stars are the key to starting to identify the constellations. For anyone in the northern hemisphere they are visible at any time of the year, and nearly everyone is familiar with the seven stars of the Plough – known as the Big Dipper in North America – an asterism that forms part of the large constellation of Ursa Major (the Great Bear). Ursa Major Because of the movement of the stars caused by the passage of the seasons, Ursa Major lies in different parts of the evening sky at different periods of the year. The diagram below shows its position for the four main seasons. The seven stars of the Plough remain visible throughout the year anywhere north of latitude 40°N. Even at the latitude (50°N) for which the charts in this book are drawn, many of the stars in the southern portion of the constellation of Ursa Major are hidden below the horizon for part of the year or (particularly in late summer) cannot be seen late in the night. Polaris and Ursa Minor The two stars Dubhe and Merak (α and β Ursae Majoris, respectively), farthest from the ‘tail’ are known as the ‘Pointers’. A line from Merak to Dubhe, extended about five times their separation, leads to the Pole Star, Polaris, or α Ursae Minoris. All the stars in the northern sky appear to rotate around it. There are five main stars in the constellation of Ursa Minor, and the two farthest from the Pole, Kochab and Pherkad (β and ϒ Ursae Minoris, respectively), are known as ‘The Guards’. Cassiopeia On the opposite of the North Pole from Ursa Major lies Cassiopeia. It is highly distinctive, appearing as five stars forming a letter ‘W’ or ‘M’ depending on its orientation. Provided the sky is reasonably clear of clouds, you will nearly always be able to see either Ursa Major or Cassiopeia, and thus be able to orientate yourself on the sky. To find Cassiopeia, start with Alioth (ε Ursae Majoris), the first star in the tail of the Great Bear. A line from this star extended through Polaris points directly towards ϒ Cassiopeiae, the central star of the five. Cepheus Although the constellation of Cepheus is fully circumpolar, it is not nearly as well-known as Ursa Major, Ursa Minor or Cassiopeia, partly because its stars are fainter. Its shape is rather like the gable end of a house. The line from the Pointers through Polaris, if extended, leads to Errai (ϒ Cephei) at the ‘top’ of the ‘gable’. The brightest star, Alderamin (α Cephei) lies in the Milky Way region, at the ‘bottom right-hand corner’ of the figure. Draco The constellation of Draco consists of a quadrilateral of stars, known as the ‘Head of Draco’ (and also the ‘Lozenge’), and a long chain of stars forming the neck and body of the dragon. To find the Head of Draco, locate the two stars Phecda and Megrez (ϒ and δ Ursae Majoris) in the Plough, opposite the Pointers. Extend a line from Phecda through Megrez by about eight times their separation, right across the sky below the Guards in Ursa Minor, ending at Grumium (ξ Draconis) at one corner of the quadrilateral. The brightest star, Eltanin (ϒ Draconis) lies farther to the south. From the head of Draco, the constellation first runs northeast to Altais (δ Draconis) and ε Draconis, then doubles back southwards before winding its way through Thuban (α Draconis) before ending at λ Draconis between the Pointers and Polaris. The stars and constellations inside the circle are always above the horizon, seen from our latitude. The path of comet 46P/Wirtanen from January 1 to March 3, 2019. The chart above shows stars down to magnitude 8.5 and the chart on the left shows stars to magnitude 10.0. Comet C/2014 Q2 Lovejoy, which reached naked-eye visibility, photographed on 20 December 2014, when in Columba, by Damian Peach. Comets and the Moon (#ulink_192260b3-0c8e-5055-987f-d63e37ce8da0) Comet C/2006 P1 McNaught, imaged on 20 January 2007, from Lawlers Gold Mine, Western Australia (Photographer: Sjbmgrtl). Comets Although comets may occasionally become very striking objects in the sky, their occurrence and particularly the existence or length of any tail and their overall magnitude are notoriously difficult to predict. Naturally, it is only possible to predict the return of periodic comets (whose names have the prefix ‘P’). Many comets appear unexpectedly (these have names with the prefix ‘C’). Bright, readily visible comets such as C/1995 Y1 Hyakutake & C/1995 O1 Hale-Bopp or C/2006 P1 McNaught (sometimes known as the Great Comet of 2007) are rare. Comet Hale-Bopp, in particular, was visible for a record 18 months and was a prominent object in northern skies. Comet McNaught was notable for its multiple tail structure. Most periodic comets are faint and only a very small number ever become bright enough to be readily visible with the naked eye or with binoculars. One comet, 46P/Wirtanen, was expected to become visible in binoculars in October 2018. It is well placed in the northern sky in the first few months of 2019. The accompanying charts show its path at its brightest during early 2019 until March, when it is expected to fade below magnitude 10. 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. 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. 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