Moongazing: Beginner’s guide to exploring the Moon

Astrophotography (#litres_trial_promo)

Lunar Astrophotography (#litres_trial_promo)

Afocal Smartphone Photography (#litres_trial_promo)

Wide-Angle and Telephoto Lunar Photography (#litres_trial_promo)

Lunar Photography with a Telescope (#litres_trial_promo)

Understanding Signal-to-Noise Ratio (SNR) (#litres_trial_promo)

Processing Videos and Images (#litres_trial_promo)

Creating Videos from DSLR Images (#litres_trial_promo)

Revealing the Moon’s True Colours (#litres_trial_promo)

Glossary of Terms (#litres_trial_promo)

Software References (#litres_trial_promo)

Observing Log (#litres_trial_promo)

Index to Lunar Atlas Features (#litres_trial_promo)

Acknowledgements (#litres_trial_promo)

About the Publisher (#litres_trial_promo)

Introduction (#ulink_28a948f3-8274-5603-a6a5-e324be3473f0)

The Moon is our celestial companion; a source of light; a comfort to many; an icon. It is older than history, and has accompanied our species, hanging silently above, since we emerged from the oceans, where its presence is still felt today. It is the master of the tides – perhaps the key to life itself – and it has inspired stories, poetry, music, and visual artworks of great beauty.

Today we live in an age where the entirety of the Moon’s surface has been mapped in astonishing detail from orbit, and human beings have left imprints in its soil. It may seem like there is nothing left to discover there, and yet the Moon keeps calling to us – a hypnotic siren song, urging us to revisit it. Our longing to explore has never been stronger.

A modern amateur telescope – even an inexpensive one – can take you on your own personal voyage to the Moon, where you’ll find a timeless landscape whose deep shadows and brilliant highlights are ever changing. Its serene character betrays its true nature as a world of incredible extremes. You can spend a lifetime enjoying these views, and placing yourself there in your mind, just as every great lunar observer before you has, since the invention of the telescope four centuries ago.

It is my hope that this guide will get you better acquainted with the Moon, enabling you to begin making your own observations, and producing your own images. There is no better destination for new space travellers, and the advice ahead will help you take one small step to reach it. Good luck!

Dedicated to Patrick Moore, my mentor and friend. I miss you.

Tom Kerss

The Moon setting in the morning sky over Icelandic mountains.

The Solar System’s largest moons. Left to right: Ganymede, Titan, Callisto, Io, Moon, Europa, Triton.

The Moon and its Origin (#ulink_7b58e632-9224-5670-b552-e03f35210a3e)

The Moon is another world, our nearest neighbour in space, and due to its close proximity and gravitational bond, a natural satellite of the Earth. To date, it is the only other world to have been visited by human beings, but its familiar face has been pondered since a time long forgotten. It was once considered a mysterious and divine signaller, but our understanding of the Moon advanced suddenly with the development of the space age, which delivered the epic and unprecedented Apollo programme.

The Moon wasn’t always the way we see it today. Indeed, it wasn’t always there at all. Our unmistakable natural satellite coalesced from a ring of material ejected from the Earth’s crust in a catastrophic collision of worlds about four billion years ago. Despite being one of hundreds of moons in the Solar System, it is unusually large for its relatively small parent world. It ranks fifth largest, after Jupiter’s Ganymede, Callisto and Io, and Saturn’s Titan, with an average diameter of 3,475 km. This makes it just a few hundred kilometres larger than the smallest of Jupiter’s four large satellites, Europa.

Due to it having formed much closer to the Earth than it is today, the Moon would have once loomed much larger in our skies, glowing from the intense heat of great seas of lava all over its surface. Over the aeons, it has cooled and solidified, and moved much farther away. This recession continues today, but at such a slow rate – approximately 4 cm per year – that it was all but undetectable until very precise measurements were made in the latter part of the twentieth century.

Like the Earth, the Moon is a differentiated body, meaning its internal structure is layered. Moonquakes have been detected using seismometers on the surface of the Moon, allowing scientists to map its density. It has a small (less than 700 km wide) core of solid and partially molten hot material, likely to be mostly iron, with a maximum temperature around 1,600°C. Above this, the Moon’s mantle is partially molten and largely solid, with a crust of igneous material. Despite having cooled long ago, the Moon’s surface has been frequently reheated by large impacts, and the violent history of collisions is almost perfectly preserved on its surface today.

It has been shown – using magma samples returned by Apollo astronauts – that at one point in its early history, the Moon had a thin, noxious atmosphere released by volcanic activity on its primordial surface, but this was stripped away long ago by solar wind. With almost no atmosphere to speak of today, the Moon’s surface is not subjected to weather erosion. Impact craters, created by extremely high energy events, have been untouched for hundreds of millions or billions of years, allowing us to look back deep into time by exploring the surface.

The internal structure of the Moon.

Without an atmosphere, the Moon does not distribute heat across its surface, resulting in incredible extremes. The day side has been recorded to reach 120 °C, whereas the night side can plummet to a chilling -170 °C. This enormous variation in temperature presents a unique challenge for both human and robotic explorers.

The Moon’s makeup is consistent with the lighter terrestrial material found in the Earth’s crust. As such, it has a low density and very low mass for its size. Despite the Moon being just over one quarter the width of the Earth, our planet is about 81 times heavier. We’ve evolved under terrestrial gravity, and would feel superhuman on the Moon, where the gravitational force felt at the surface is just 16.5 per cent of that on Earth. Everything feels about six times lighter there, and with no atmospheric drag, it is possible to throw things extremely far. Even the powdery regolith on the lunar surface travelled surprisingly far when kicked up by the Apollo astronauts.

Large impacts on the Moon have thrown material over hundreds of kilometres across its surface. The 93-km-wide crater Copernicus was formed roughly 800 million years ago, by an object similar in size (a few kilometres across) to the one responsible for the K-T extinction impact on Earth 66 million years ago. Enormous rays of ejected material can be seen stretching away from it in all directions.

Sunrise in Copernicus Crater. Dramatic ejecta rays are visible stretching away from the crater in all directions. This view comes from one of the Royal Observatory’s Victorian telescopes.

The Moon continues to influence us here on Earth, as it has for the entire history of life. Its gentle gravitational tug, not felt by us individually, generates a measurable attraction with the surface water of our planet, creating the tides in our oceans and seas. It is thought that without the tides, there might be no life on our planet at all, and almost certainly no modern land-borne species, including humans. The Moon is the master of the oceans, which collectively form the largest habitat on Earth, and just as we have left a stamp on it, it too has made its mark on us, woven deep into our collective history.

The Waxing Crescent Moon sets from the south coast of England. The Moon is the master of the tides.

The Moon’s Orbit and Rotation (#ulink_31d723e8-2710-527b-9f54-57e50648108b)

The appearance of the Moon in the sky depends on where it is in its orbit. There are many factors to consider regarding the orbit of the Moon. Fortunately, these factors do not greatly affect our view of the Moon, but they are important to understand in order to predict special moongazing events.

Today, the Moon completes one sidereal orbit of the Earth every 27.3 days, which means it reaches the same right ascension in the sky after this period. A common misconception is that the Moon takes 28 days to orbit the Earth, but this has never been the case! Due to the motion of both the Earth and Moon around the Sun, there is a discrepancy between the length of one lunar orbit and the period of time between one New Moon and the next, which is 29.5 days. During this period, known as a lunar synodic month, the familiar phases of the Moon take their turn to appear. We see different amounts of the lit and unlit side of the Moon because the apparent angle between the Moon and Sun changes continuously.

The Moon rotates on its own axis once every 27.3 days, in the same direction (anticlockwise as seen from above the North Pole) as its orbit around the Earth. This results in the misleading illusion that it doesn’t appear to rotate at all, as we always see one side from Earth. In fact, the Moon keeps its familiar side facing the Earth throughout its orbit, and the far side is never seen. Only the Apollo astronauts have seen the far side with their own eyes, but robotic orbiters have mapped the entire lunar surface in incredible detail. This synchronised orbit and rotation of the Moon is no accident, but speaks to its ancient relationship with the Earth. The two became tidally locked, carefully curating the Moon’s rotation period. Tidal locking occurs elsewhere in the Solar System, for example between Pluto and its large companion Charon.

The distance between the Moon and the Earth also changes, as the Moon’s orbit is not perfectly circular, varying between 356,500 km (221,500 miles) at lunar perigee and 406,700 km (252,700 miles) at lunar apogee, if measured between the centres of both bodies. When Full Moon occurs around lunar perigee, it appears slightly larger and somewhat brighter than average – an event colloquially known as a supermoon. These events are not very rare, but perigee and Full Moon do not always align, because lunar perigees are separated by a period of approximately 27.5 days. This is known as an anomalistic month. The difference between this period and a sidereal month means the perigee point of the Moon’s orbit undergoes gradual precession, taking nearly nine years to move all the way around the Earth once. This is known as precession of the line of apsides.

Because the Moon’s orbit is elliptical, its speed changes as it moves. However, the rotation rate of the Moon remains constant. This discrepancy produces an effect called libration – an apparent ‘wobbling’ of the Moon – which allows us to see slightly more of its eastern or western sides (known as its limbs) as it moves ahead, or falls behind in its orbit relative to its rotation. Accounting for both extremes, we can see a total of 59 per cent of the surface of the Moon, but how much we see of the east and west depends on how favourable the libration is with respect to that limb. Because this effect is small, and features on the extreme limbs are less well-known, maps of the Moon in this guide do not show any overall libration bias.

The far side of the Moon captured by NASA’s Lunar Reconnaissance Orbiter, heavily cratered but with far fewer maria than the near side. Mare Orientale (Oriental Sea) is shown in the top left.

The Moon’s orbit around the Earth.

When the Moon crosses the ecliptic, it is said to be crossing a node. The ascending node is the point where the Moon moves northward from the southern celestial hemisphere into the northern celestial hemisphere. The opposite point in the Moon’s orbit is called the descending node. Eclipses are possible, and indeed inevitable, only when the Moon is full or new at one of the nodes. When the Moon is not crossing a node, it cannot coincide with the Sun in the sky (a solar eclipse) or the Earth’s shadow (a lunar eclipse.)

A diagram of lunar libration caused by the elliptical shape of the Moon’s orbit.

Lunar Cycle and Phases (#ulink_3bd32925-2d9b-5979-a197-fdb52b7de28a)

The Moon is the second brightest object in the sky after the Sun. Unlike the Sun, it is safe to observe directly without filters. If you own a telescope, the Moon alone can provide a lifetime of mesmerising views, ever changing as the shadows play across its surface. In the observing section, you can find advice on choosing and using a telescope to explore the Moon.

Around New Moon, there is an approximate period of 1.5 days when the Moon is an extremely thin crescent near the Sun and too faint to be seen, even around sunset or sunrise. For the remainder of the Moon’s 29.5-day synodic month, the phases are said to wax on to Full Moon and wane off to the next New Moon. The phases are illustrated on page (#ulink_96934c4f-a73e-5eed-9d05-0717452d0fa8) and explained below, with the approximate number of days that separate them. Note that there is no half-moon, as this is not an astronomical term, but rather two quarter-moon phases. The number of days given are averages, and vary slightly due to the Moon’s elliptical orbit.