Читать книгу Moongazing: Beginner’s guide to exploring the Moon - Royal Greenwich Observatory - Страница 6
ОглавлениеThe 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 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 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.
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 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.
Waxing Crescent: 0–7.4 days. The Moon’s eastern limb emerges (east on the Moon is west in the sky). The crescent can first be sighted after sunset, once the Moon and Sun are separated by approximately seven degrees, centre to centre. Only one or two per cent of the Moon as seen from Earth appears to be illuminated at this time, as we are mostly viewing the night side of the Moon. The Waxing Crescent follows the Sun to the western horizon.
An illustration of the Moon’s inclined orbit. Eclipses will occur when the Full or New Moon crosses the ecliptic at the lunar nodes.
First Quarter: 7.4 days. Exactly half of the Moon’s near side appears to be illuminated. The terminator, where day meets night, runs vertically along its apparent meridian. Shadows are cast from the east to the west on the Moon at this time.
Waxing Gibbous: 7.4–14.8 days. When illumination is greater than 50 per cent, but less than 100 per cent, the Moon’s phase is said to be gibbous. Its western hemisphere is gradually revealed as the Sun begins to rise there.
Full Moon: 14.8 days. At 100 per cent phase illumination, the day side of the Moon is directly pointed at the night side of the Earth and the Moon is full. As the terminator is not visible, there is very little sense of relief on the lunar surface. Full Moon is the only time at which lunar eclipses can occur, when the Moon passes through the shadow of the Earth. The Moon must be close to its ascending or descending node for this to happen.
Waning Gibbous: 14.8–22.2 days. The Moon’s terminator emerges on the eastern limb and creeps towards the apparent meridian. Shadows now begin to grow eastward as the incoming sunlight favours the western hemisphere.
Last Quarter: 22.2 days. Only the western hemisphere of the Moon appears to be illuminated. As the Moon is now leading the Sun, it rises in the early morning. The brilliant crater Aristarchus is unmistakable in its dark surroundings near the western limb.
Waning Crescent: 22.2–29.53 days. The terminator moves westward towards the western limb (east in the sky) as the apparent angle between the Moon and Sun decreases. Eventually the Moon appears to become a very thin crescent impossible to see in the Sun’s glare. As it rises shortly before sunrise, the waning crescent is the least observed lunar phase.
New Moon: 29.52/0 days. At the moment of New Moon, a new lunar synodic month begins. The New Moon is the night side of the Moon, and too dark to see through the glare of the Sun. New Moon is only directly visible during a solar eclipse.
The location and time of moonrise and moonset depends on your latitude. For the British Isles, detailed information about the lunar calendar is available from the UK Hydrographical Office, a department of Her Majesty’s Nautical Almanac Office: http://astro.ukho.gov.uk.
Moon phases as seen from the Earth and space.
More general information can be retrieved from the US Naval Observatory’s Astronomical Applications Department: http://aa.usno.navy.mil.
Almost every feature on the surface of the Moon visible from the Earth has been given a name. These names were originally assigned on a somewhat informal basis by early telescopic observers to honour great philosophers and artists of antiquity. As larger telescopes have revealed smaller features, names have been assigned more formally, and the process of cataloguing features is now governed by the International Astronomical Union (IAU.) Craters are named for notable deceased scientists, mathematicians, artists, scholars and explorers. The lunar maria (seas) have older Latin names, which reflect abstract states of mind and weather phenomena considered important to mariners. Lunar montes (mountain ranges) are typically named after terrestrial mountain ranges or nearby craters. Other features, with few exceptions, are named after nearby craters, maria or montes.
Features on the Moon fall into one of several categories. Here each type of feature is described with a typical example shown as viewed through a large telescope.
Craters – Fairly circular depressions usually formed from impacts. Occasionally, chains of craters are grouped together and collectively termed catena. Craters feature sloped walls and, on many occasions, central peaks, left over from the crater formation, at which point the lunar surface was locally molten by the energy of the impactor.
Mare (plural: Maria) – Latin for ‘sea’. Large basins of solidified, ancient lava. The maria appear dark relative to the other terrain features. There is one large lunar ‘ocean’ in the Moon’s western hemisphere known as Oceanus Procellarum (Ocean of Storms).
Mons – An individual mountain on the Moon. Lunar mountains were formed by a variety of processes, and vary greatly in size and height. The tallest are approaching 5 km in height, comparable to Vinson Massif, the highest peak in Antarctica.
Montes – Large chains of mountain ranges formed by gigantic asteroid impacts billions of years ago. As with terrestrial ranges, prominent individual peaks often have their own names.
Vallis (Plural: Valles) – A valley or system of valleys formed by lava flows and collapsed lava tubes. These features snake across the surface, often near craters connected to volcanism.
Dorsum (Plural: Dorsa) – Derived from the Latin for the ‘back’, and connected to ridges or fins on the back of an animal, dorsa are subtle features resembling wrinkles in lunar maria. They are hard to see unless illuminated at a low angle, causing them to cast shadows.
Rima (Plural: Rimae) – Latin for ‘fissure’. Rimae, sometimes called Rilles (German for ‘grooves’) are fissures or cracks in the lunar surface. Not to be confused with valleys, they are often jagged in appearance, with straight sections permeated by kinks. They are seismological features in the Moon’s crust, and are sometimes found in crater floors.
Rupis (Plural: Rupes) – Latin for ‘rock’, rupes are escarpments in the lunar surface. They appear as large rifts, where a pronounced change in elevation can be seen. In fact, most rupes are very gentle slopes that are many kilometres wide.
Lacus – Derived from the Latin for an opening, a lacus is a lake, which as its name suggests is a very small lunar mare. These features appear as dark, often patchy regions of dark, smooth plains.
Sinus – Derived from the Latin for a gulf, a sinus on the Moon is a bay formed by a rugged ‘coastline’ of lunar highlands meeting a low elevation, smooth plain such as mare.
Palus (Plural: Paludes) – Though its original Latin name is closer in meaning to a pool, paludes are generally translated as marches. They are low lying, but relatively rugged regions. Whilst they are not as dark as mare, they do have a relatively low albedo when compared with other types of rugged terrain.
Images of the Apollo landing sites captured by NASA’s Lunar Reconnaissance Orbiter from low altitude. The remains of the Lunar Modules and various scientific instrument packages can be seen, as well as tracks left by the astronauts and rovers.
North-East
Due to atmospheric seeing, we are typically limited to observing features no smaller than one arcsecond in apparent size. On the Moon, this corresponds to just over a mile at best. This means that very small objects, such as those left on the Moon by the Apollo astronauts, cannot be resolved through the Earth’s atmosphere, but they have been imaged using the Lunar Reconnaissance Orbiter, which was only a few kilometres above the lunar surface.
It’s useful to memorise prominent features on the Moon and consider its near side being divided into four quadrants as shown here. A good knowledge of the locations of the largest lunar maria will help you when choosing areas to explore during your moongazing sessions.
Readers in the southern hemisphere should note that due to the convention of the coordinates on the Moon, it appears upside down in the sky compared with the charts in this guide. From Sydney, the Moon’s North Pole appears to be at the bottom. East and west are also reversed. From the northern hemisphere, the Moon’s eastern limb is on the right; from the southern hemisphere, it is on the left.
South-East
North-East
The north-eastern quadrant of the Moon is dominated by the dark Mare Serenitatis (Sea of Serenity), landing site of Apollo 17, and Mare Tranquillitatis (Sea of Tranquility) where Neil Armstrong and Buzz Aldrin became the first humans to step foot on the lunar surface during the Apollo 11 mission. Mare Crisium (Sea of Crises) lies near the eastern limb, and south of it Mare Fecunditatis (Sea of Fertility).
South-East
The south-eastern quadrant of the Moon is remarkable for having no maria, save for the southern region of Mare Nectaris (Sea of Nectar). It’s mostly composed of rugged highlands, pockmarked with high energy impacts ranging right across the history of the Moon. The prominent Tycho Crater is found in this region, with its bright rays of ejecta visible over 1,600 km (1,000 miles) from the impact site. Astronauts on the Apollo 16 mission collected material from these rays to be analysed on Earth.
South-West
South-West
The south-western quadrant of the Moon features several small maria and a portion of the giant Oceanus Procellarum (Ocean of Storms) as well as the landing sites of Apollo 12 and 14. Near the western limb, the dark crater Grimaldi can be seen, having the appearance of a small lunar sea in its own right. Byrgius crater, a popular sight, is to the south of Grimaldi.
North-West
North-West
The north-western quadrant of the Moon is home to prominent craters such as Kepler, Copernicus and Plato, as well as Aristarchus, the brightest crater on the entire surface. The bright craters and dark plains of Oceanus Procellarum and Mare Imbrium (Sea of Showers) make this one of the most striking, high-contrast regions of the Moon. Nestled in the western edge of Mare Imbrium is Sinus Iridum (Bay of Rainbows.)
The Moon has lingered in our skies as long as life has existed on Earth. It predates the earliest murmurings of consciousness by billions of years. When our ancestors began to take notice of the objects in the sky, the Moon was already a familiar sight, whose regular visits were probably a source of comfort, providing a source of natural light at night. But as the earliest humans attempted to explain the world around them, events in the sky became steeped in superstition. Then unpredictable, aberrant changes in the appearance of the Moon, such as eclipses, terrified ancient people as harbingers of doom. Today we flock to see them and enjoy the spectacle.
Eventually this perception of the Moon as a god of many moods began to fade, when as early as the fifth century BCE, astronomers in ancient Babylon learned to predict its eclipse cycle through careful record-keeping across generations. Meanwhile, in Asia, astronomers in what is now modern-day India worked out how to describe the apparent elongation between the Moon and Sun throughout the full synodic month.
The Mesopotamian god of the Moon, Sin, depicted on a tablet at the British Museum.
A great deal of progress was made when mathematical cosmologies – descriptions of what we now call the Solar System, but what was once considered to be the centre of the Universe – were developed in ancient Greece. Notably, Aristarchus in the second century BCE, considering the Moon as a spherical ball of earth or rock, computed its size and distance. His results weren’t precise, but they were certainly better than anything that had been achieved before. In the same century, Seleucus had correctly linked the Moon to the tides, long before Newton would describe the force responsible.
A few hundred years later, Ptolemy significantly improved on previous calculations about the size and distance of the Moon, but with the transition to the Middle Ages, this progress of understanding slowed down.
The next leap in lunar observation would occur with the invention of the telescope in the early seventeenth century. Skilled observers of the pre-telescopic era, such as Tycho Brahe, had attempted to discern the Moon’s surface, but even the best of their visual acuity was no match for the telescope. Many people incorrectly believe that Galileo Galilei pioneered the use of the telescope in astronomy, but he was actually beaten to it by Thomas Harriot, an English scientist then living in London. Harriot was credited with making the first sketch of the Moon with the aid of a telescope in July 1609, approximately four months before Galileo’s first use of such an instrument. Nevertheless, Galileo was the first to make a systematic study of the sky using the telescope, and in the following year published his ground-breaking Sidereus Nuncius (Starry Messenger), including sketches of the Moon, showing unprecedented detail.
Over the next 400 years, rapid improvements in telescope technology would greatly improve our perception of the lunar surface. Generations of skilled observers discovered and catalogued features large and small, attempting to explain their origin. The Moon became the subject of the very first successful astronomical photograph, which was a daguerreotype captured by John Draper in 1840.
