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eBook Edition © August 2020

ISBN 9780008399764

Version: 2020-06-26

Contents

Cover

Title Page

Copyright

Introduction

The Constellations

The Northern Circumpolar Constellations

The Winter Constellations

The Spring Constellations

The Summer Constellations

The Autumn Constellations

The Moon and the Planets

The Moon

Map of the Moon

Eclipses

The Planets

Minor Planets

Comets

Introduction to the Month-by-Month Guide

Month-by-Month Guide

January

February

March

April

May

June

July

August

September

October

November

December

Dark Sky Sites

Glossary and Tables

Further Information

Acknowledgements

About the Publisher

Introduction

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.

Measuring altitude and azimuth on the celestial sphere.

The altitude of the North 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 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.

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 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 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.

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 are given here.)

Inferior planet.

Superior 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.

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.

Messier / NGC | Name | Type | Constellation | Maps (months)
— | Hyades | open cluster | Taurus | Sep. – Mar.
— | Double Cluster | open cluster | Perseus | All year
— | Melotte 111 (Coma Cluster) | open cluster | Coma Berenices | Jan. – Aug.
M3 | — | globular cluster | Canes Venatici | Jan. – Sep.
M4 | — | globular cluster | Scorpius | May – Aug.
M8 | Lagoon Nebula | gaseous nebula | Sagittarius | Jun. – Aug.
M11 | Wild Duck Cluster | open cluster | Scutum | May – Oct.
M13 | Hercules Cluster | globular cluster | Hercules | Feb. – Nov.
M15 | — | globular cluster | Pegasus | Jun. – Dec.
M22 | — | globular cluster | Sagittarius | Jun. – Sep.
M27 | Dumbbell Nebula | planetary nebula | Vulpecula | May – Dec.
M31 | Andromeda Galaxy | galaxy | Andromeda | All year
M35 | — | open cluster | Gemini | Oct. – May
M42 | Orion Nebula | gaseous nebula | Orion | Nov. – Mar.
M44 | Praesepe | open cluster | Cancer | Nov. – Jun.
M45 | Pleiades | open cluster | Taurus | Aug. – Apr.
M57 | Ring Nebula | planetary nebula | Lyra | Apr. – Dec.
M67 | — | open cluster | Cancer | Dec. – May
NGC 752 | — | open cluster | Andromeda | Jul. – Mar.
NGC 3242 | Ghost of Jupiter | planetary nebula | Hydra | Feb. – May

Some interesting objects.

The Northern Circumpolar Constellations

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 γ 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 γ 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 (γ 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 (γ 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 (γ 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 Giausar (λ Draconis) between the Pointers and Polaris.

The stars and constellations inside the circle are always above the horizon, seen from our latitude.

The Winter Constellations

The winter sky is dominated by several bright stars and distinctive constellations. The most conspicuous constellation is Orion, the main body of which has an hour-glass shape. It straddles the celestial equator and is thus visible from anywhere in the world. The three stars that form the ‘Belt’ of Orion point down towards the southeast and to Sirius (α Canis Majoris), the brightest star in the sky. Mintaka (δ Orionis), the star at the northeastern end of the Belt, farthest from Sirius, actually lies just slightly south of the celestial equator.

A line from Bellatrix (γ Orionis) at the ‘top right-hand corner’ of Orion, through Aldebaran (α Tauri), past the ‘V’ of the Hyades cluster, points to the distinctive cluster of bright blue stars known as the Pleiades, or the ‘Seven Sisters’. Aldebaran is one of the five bright stars that may sometimes be occulted (hidden) by the Moon. Another line from Bellatrix, through Betelgeuse (α Orionis), if carried right across the sky, points to the constellation of Leo, a prominent constellation in the spring sky.

Six bright stars in six different constellations: Capella (α Aurigae), Aldebaran (α Tauri), Rigel (β Orionis), Sirius (α Canis Majoris), Procyon (α Canis Minoris) and Pollux (β Gemini) form what is sometimes known as the ‘Winter Hexagon’. Pollux is accompanied to the northwest by the slightly fainter star of Castor (α Gemini), the second ‘Twin’.

In a counterpart to the famous ‘Summer Triangle’, an almost perfect equilateral triangle, the ‘Winter Triangle’, is formed by Betelgeuse (α Orionis), Sirius (α Canis Majoris) and Procyon (α Canis Minoris).

Several of the stars in this region of the sky show distinctive tints: Betelgeuse (α Orionis) is reddish, Aldebaran (α Tauri) is orange and Rigel (β Orionis) is blue-white.

The Spring Constellations

The most prominent constellation in the spring sky is the zodiacal constellation of Leo, and its brightest star, Regulus (α Leonis), which may be found by extending a line from Megrez and Phecda (δ and γ Ursae Majoris, respectively) – the two stars on the opposite side of the bowl of the Plough from the Pointers – down to the southeast. Regulus forms the ‘dot’ of the ‘backward question mark’ known as ‘the Sickle’. Regulus, like Aldebaran in Taurus is one of the bright stars that lie close to the ecliptic, and that are occasionally occulted by the Moon. The same line from Ursa Major to Regulus, if continued, leads to Alphard (α Hydrae), the brightest star in Hydra, the largest of the 88 constellations.

The shape formed by the body of Leo is sometimes known as the ‘Spring Trapezium’. At the other end of the constellation from Regulus is Denebola (β Leonis), and the line forming the back of the constellation through Denebola points to the bright star Spica (α Virginis) in the constellation of Virgo. A saying that helps to locate Spica is well-known to astronomers: ‘Arc to Arcturus and then speed on to Spica.’ This suggests following the arc of the tail of Ursa Major to Arcturus and then on to Spica. Arcturus (α Boötis) is actually the brightest star in the northern hemisphere of the sky. (Although other stars, such as Sirius, are brighter, they are all in the southern hemisphere.) Overall, the constellation of Boötes is sometimes described as ‘kite-shaped’ or ‘shaped like the letter P’.

Although Spica is the brightest star in the zodiacal constellation of Virgo, the rest of the constellation is not particularly distinct, consisting of a rough quadrilateral of moderately bright stars and some fainter lines of stars extending outwards.

The Summer Constellations

On summer nights, the three bright stars Deneb (α Cygni), Vega (α Lyrae) and Altair (α Aquilae) form the striking ‘Summer Triangle’. The constellations of Cygnus (the Swan) and Aquila (the Eagle) represent birds ‘flying’ down the length of the Milky Way. This part of the Milky Way contains the Great Rift, an elongated dark region, where the light from distant stars is obscured by intervening dust. The dark Rift is clearly visible even to the naked eye.

The most prominent stars of Cygnus are sometimes known as the ‘Northern Cross’ (as a counterpart to the ‘Southern Cross’ – the constellation of Crux – in the southern hemisphere). The central line of Cygnus through Albireo (β Cygni), extended well to the southwest, points to Sabik (η Ophiuchi) in the large, sprawling constellation of Ophiuchus (the Serpent Bearer) and beyond to Antares (α Scorpii) in the constellation of Scorpius. Like Cepheus, the shape of Ophiuchus somewhat resembles the gable-end of a house, and the brightest star Rasalhague (α Ophiuchi) is at the ‘apex’ of the ‘gable’.

A line from the central star of Cygnus, Sadr (γ Cygni) through δ Cygni, in the northwestern ‘wing’ points towards the Head of Draco, and is another way of locating that part of the constellation. Another line from Sadr to Vega indicates the central portion, ‘The Keystone’, of the constellation of Hercules. An arc through the same stars, in the opposite direction, points towards the constellation of Pegasus, and more specifically to the ‘Great Square of Pegasus’.

Aquila is less conspicuous than Cygnus and consists of a diamond shape of stars, representing the body and wings of the eagle, together with a rather faint star, λ Aquilae, marking the ‘head’. Lyra (the Lyre) mainly consists of Vega (α Lyrae) and a small quadrilateral of stars to its southeast. Continuation of a line from Vega through Altair indicates the zodiacal constellation of Capricornus.

The Autumn Constellations

During the autumn season, the most striking feature is the ‘Great Square of Pegasus’, an almost perfect rectangle on the sky, forming the main body of the constellation of Pegasus. However, the star at the northeastern corner, Alpheratz, is actually α Andromedae, and part of the adjacent constellation of Andromeda. A line from Scheat (β Pegasi) at the northeastern corner of the Square, through Matar (η Pegasi), points in the general direction of Cygnus. A crooked line of stars leads from Markab (α Pegasi) through Homam (ζ Pegasi) and Biham (θ Pegasi) to Enif (ε Pegasi). A line from Markab through the last star in the Square, Algenib (γ Pegasi) points in the general direction of the five stars, including Menkar (α Ceti) that form the ‘tail’ of the constellation of Cetus (the Whale). A ring of seven stars lying below the southern side of the Great Square is known as ‘the Circlet’, part of the constellation of Pisces (the Fishes).

Extending the line of the western side of the Great Square towards the south leads to the isolated bright star, Fomalhaut (α Piscis Austrini), in the Southern Fish. Following the line of the eastern side of the Great Square towards the north leads to Cassiopeia while, in the other direction, it points towards Diphda (β Ceti), which is actually the brightest star in Cetus.

Three bright stars leading northeast from Alpheratz form the main body of the constellation of Andromeda. Continuation of that line leads towards the constellation of Perseus and Mirfak (α Persei). Running southwards from Mirfak is a chain of stars, one of which is the famous variable star Algol (β Persei). Farther east, an arc of stars leads to the prominent cluster of the Pleiades, in the constellation of Taurus.

Between Andromeda and Cetus lie the two small constellations of Triangulum (the Triangle) and Aries (the Ram).

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. 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.

The Moon phases. During its orbit around the Earth we see different portions of the illuminated side of the Moon’s surface.