of Terms
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 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
