The Moon is Earth’s constant companion. It is the first object in the sky that is pointed out to us as young children, along with stories of astronauts walking on the surface.
It is illuminated by reflected sunlight as the Moon moves around the Earth. As the month progresses, its appearance changes through different phases from new moon to full moon and back again. As the illuminating light changes angle, this causes shadows that give the surface a three-dimensional effect, enhancing the lunar features.
Even with just your eyes, detail of light and dark regions can be seen. The large darker regions are plains of solidified lava flows. Very early in the evolution of the Solar System there was a period called the “late heavy bombardment”, caused by changes in the orbits of the giant planets – Jupiter, Saturn, Uranus and Neptune. As they swept inwards, they scattered many of the asteroids, hurtling them towards the inner Solar System. The Moon is a permanent history of this violent time when large impacts occurred, creating huge basins. As the Moon was still quite molten just beneath the crust, the basins would fill with erupting lava.
Ancient astronomers had no understanding of these processes and initially thought that these regions were seas and oceans not dissimilar to those on Earth, and so named them “mare”, which is Latin for “sea”. They have some wonderful names, such as Mare Tranquillitatis or Sea of Tranquillity, where in 1969, Apollo 11 landed and Neil Armstrong and Buzz Aldrin became the first humans to set foot on the lunar surface. With the aid of the map, see how many regions you can find.
The moon is also peppered with craters, recording billions of years of impacts. To see them you will need a pair of binoculars, ideally 10 or 15 times magnification, but even less magnification will still reveal some detail. Alternatively, a telescope, even a small one, will show a fair amount of detail.
One of the most prominent craters is Copernicus, named after the 16th century Polish astronomer Nicolaus Copernicus. He is famous for proposing the comprehensive heliocentric theory of placing the Sun at the centre of a celestial sphere that included the orbits of Mercury, Venus, Earth, Mars, Jupiter, Saturn and an outermost sphere of fixed stars surrounding all the planets. The crater Copernicus is 96km in diameter and some 38km deep. It is particularly striking because of the rays radiating from it, formed from fine material thrown upward and outwards from the impact; a feature which is called an ejecta blanket. The crater is about 800 million years old, so the ejecta blanket, like the lunar surface, has weathered to a darker colour.
Another notable crater is Tycho, named after the 16th century Danish nobleman, astronomer, astrologer and alchemist Tycho Brahe. He made many comprehensive and unprecedentedly accurate astronomical observations of the position and brightness of over 1000 stars. This was the first really comprehensive star catalogue of the night sky; all recorded with the naked eye. He is also well-known for losing part of his nose in a sword duel with a fellow Danish nobleman, when they drunkenly quarrelled over who was the superior mathematician! The crater Tycho is 85 km wide and 4.7km deep. It is one of the brightest craters and can just be seen with the naked eye if you have good eyesight. It is only about 108 million years ago, so the crater and its huge ray system have not yet weathered too much and thus, it stands out against the darker lunar surface.
The brightest crater, Aristarchus, albeit smaller at in 40km diameter, can also be seen with the naked eye as a bright spot on the lunar surface. Again, it is bright because it is relatively young in age and the impact exposed much underlying material that has not yet weathered. It is named after the ancient Greek astronomer, Aristarchus of Samos. He was likely the first person to propose the idea that the Sun was at the centre of the Universe. At the time, in 270BC, “The Universe” was thought to be a sphere that encompassed the Earth and Sun. What would Aristarchus think today of a Universe that is some 93 billion light-years across with over 200 billion trillion stars and their planets?
Also look for the rayed crater Kepler, which is slightly smaller at 32km diameter and 2.6km depth. It is again named after an astronomer, this time the early 17th century German astronomer Johannes Kepler. Building on the observations and theories of others, including Tycho Brahe’s highly accurate observations and the refined heliocentric model by Nicolaus Copernicus, Kepler devised his famous three laws of planetary motions which described the orbits of the known planets as ellipses. His laws, albeit slightly refined subsequently, are still used today.
There is one final crater that deserves a mention, which is De la Rue crater. It is quite large in diameter at 136km and is likely the remnant of several impact craters merged together, possibly some 3.9 billion years ago. It is named after Guernseyman, Warren De La Rue, a 19th Century astronomer, chemist and inventor. He pioneered the use of early photography techniques for astronomy, allowing more detail to be accurately recorded. He is also famous for inventing the photoheliograph, the first instrument of its kind to photograph the Sun. The use of this instrument confirmed that prominences were a solar phenomenon. Originally, when prominences were observed by the naked eye during a total solar eclipse, they were thought to be a property of light passing through Earth’s atmosphere and not a feature of the Sun at all.
De La Rue Crater sits on the edge of the north-eastern limb of the Moon and the best time to view it is four days or so after a new moon, when the Moon will be low on the horizon just after sunset, through to just after the first quarter moon. So, between 25-30 October and 24-29 November. To find it look for the more prominent craters of Hercules, Atlas and Endymion, then look very slightly towards the north.
If you want to explore the lunar features in more detail, then look online at LROC QuickMap. There are also many Apps and guides to observing the Moon. With a modest pair of binoculars, a lot of detail will be visible and looking on the nights either side of a full moon, will give the best three-dimensional effect.
Also, look towards the south and you will see the outer planet and gas giant, Saturn, with its icy rings just starting to appear after a ring plane crossing – when the rings appear to disappear as they are seen directly edge-on from our viewpoint on Earth.
There is also the Orionid meteor shower between 2 October to the 7 November, peaking on 21 October. There will be a new moon, so it will be nice and dark. The best time to view it will be from about 11pm onwards. Look toward the east and find the Orion Constellation, which will be rising above the horizon, and find the unmistakably bright red giant star, Betelgeuse, on Orion’s shoulder. The radiant – where the paths of the meteors appear to originate – is just above this. The Orionid meteor shower is the debris left behind from Halley’s comet, which orbits the Sun about every 76 years. It last visited the inner Solar System in 1986, and will visit again in 2061.
Finally, rising behind Orion, you will see the other gas giant, Jupiter.
Dr Jean M Dean, FRAS
