My Science School

Despite being much smaller than the Earth, the Moon still has a great deal of influence on its parent planet. Its gravity is constantly pulling on Earth’s surface. This is not noticeable in relation to solid ground, but can clearly be seen in the movement of Earth’s tides. Twice a day, the oceans on Earth rise and fall. This is because the Moon’s gravitational pull is strongest on the side of Earth that is facing the Moon. Oceans on this side will be pulled into a bulge — high tide. Water on the opposite side is least affected by the Moon’s gravity, so it flows away from Earth in another bulge, resulting in another high tide. Areas of Earth at right angles to the Moon will have low tide.

A bigger instant effect would be on the ocean’s tides. But to understand the impact we need to know about how tides work. Tides are the result of the gravitational tug from the Moon and Sun that the Earth feels. If we disregard the Sun for now, the Earth’s oceans facing the Moon bulge up in response to the lunar gravitational force: a high tide. The difference in gravitational attraction on the near and far sides of the Earth means that, at the same time, there is also a high tide on the side furthest from the Moon. And because the ocean is liquid, between these two high tides there are two low tides. As the Earth is spinning, these high and low tides move across the globe over 24 hours, meaning each coastal location experiences two high tides and two low tides every day.

In reality, it is a little more complicated. The Moon’s 27-day orbit of the Earth means the times at which high and low tides occur change. You have to wait 12 hours plus 25 minutes between each high tide. And the Sun plays its part too. The Sun’s influence on tides is just under half as strong as the Moon’s.

When the Sun, Moon and Earth are all lined up, the Sun and Moon work together to produce ‘spring’ tides (though confusingly they don’t have to happen in spring). During spring tides, high tides are a little higher and low tides a little lower than normal. In contrast, when the Sun and Moon are at right angles to one another, the tides from the Sun partially cancel those from the Moon and we have the opposite: ‘neap’ tides. Here, high tides are a little lower and low tides a little higher than average.

          A lunar eclipse occurs when the Earth comes directly between the Sun and the Moon. As the Moon moves through Earth's shadow, the planet prevents direct sunlight from reaching the surface of the Moon. The Moon does not disappear but turns red because Earth's atmosphere bends the Sun’s rays. A lunar eclipse can occur only on the night of a full moon. The type and length of a lunar eclipse depend on the Moon's proximity to either node of its orbit.

          During a total lunar eclipse, Earth completely blocks direct sunlight from reaching the Moon. The only light reflected from the lunar surface has been refracted by Earth’s atmosphere. This light appears reddish for the same reason that a sunset or sunrise does: the Rayleigh scattering of bluer light. Due to this reddish color, a totally eclipsed Moon is sometimes called a blood moon.

          Unlike a solar eclipse, which can only be viewed from a relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of Earth. A total lunar eclipse can last up to nearly 2 hours, while a total solar eclipse lasts only up to a few minutes at any given place, due to the smaller size of the Moon's shadow. Also unlike solar eclipses, lunar eclipses are safe to view without any eye protection or special precautions, as they are dimmer than the full Moon.

          Earth’s shadow can be divided into two distinctive parts: the umbra and penumbra. Earth totally occludes direct solar radiation within the umbra, the central region of the shadow. However, since the Sun's diameter appears about one-quarter of Earth's in the lunar sky, the planet only partially blocks direct sunlight within the penumbra, the outer portion of the shadow.

          A penumbral lunar eclipse occurs when the Moon passes through Earth's penumbra. The penumbra causes a subtle dimming of the lunar surface. A special type of penumbral eclipse is a total penumbral lunar eclipse, during which the Moon lies exclusively within Earth's penumbra. Total penumbral eclipses are rare, and when these occur, the portion of the Moon closest to the umbra may appear slightly darker than the rest of the lunar disk.

          A partial lunar eclipse occurs when only a portion of the Moon enters Earth's umbra, while a total lunar eclipse occurs when the entire Moon enters the planet's umbra. The Moon's average orbital speed is about 1.03 km/s (2,300 mph), or a little more than its diameter per hour, so totality may last up to nearly 107 minutes. Nevertheless, the total time between the first and the last contacts of the Moon's limb with Earth's shadow is much longer and could last up to four hours.

          The relative distance of the Moon from Earth at the time of an eclipse can affect the eclipse's duration. In particular, when the Moon is near apogee, the farthest point from Earth in its orbit, its orbital speed is the slowest. The diameter of Earth's umbra does not decrease appreciably within the changes in the Moon's orbital distance. Thus, the concurrence of a totally eclipsed Moon near apogee will lengthen the duration of totality.

          A central lunar eclipse is a total lunar eclipse during which the Moon passes through the centre of Earth's shadow, contacting the anti-solar point. This type of lunar eclipse is relatively rare.

          If you are standing on the moon, the sky would always appear black, whether it was night or day. This is because there is no atmosphere to scatter sunlight. On Earth, atoms of oxygen and nitrogen in the atmosphere have an effect on sunlight passing through them. Light scatters when it passes through particles that are one tenth as large as the light’s wavelength. The atoms of oxygen and nitrogen are one tenth the size of the blue wavelength, so blue light is scattered more effectively than other colours.

          We see the sky as colored because our atmosphere interacts with the sunlight passing through it. This phenomenon is called "scattering." The type of scattering responsible for blue sky is known as Rayleigh scattering. Because this effect becomes sharply more pronounced as the energy of light increases, wavelengths at the blue end of the spectrum, where energy is the highest, are scattered preferentially. The sunlight reaching our eyes has a high ratio of short, bluish wavelengths compared to medium and long wavelengths, so we perceive the sky as being blue.

          Without an atmosphere the sky appears black, as evidenced by the lunar sky in pictures taken from the moon. But even a black sky has some lightness. At night, the sky always has a faint color, called “skyglow” by astronomers. Much of this skyglow is light pollution - sources of light prevalent in urban areas that reduce our ability to see stars, planets, and other celestial phenomena.

          In the absence of light from human sources, skyglow is present due to a faint airglow in the upper atmosphere (a permanent, low-grade aurora) and starlight scattered in the atmosphere. Even beyond our atmosphere, faint skyglow is caused by sunlight reflected off interplanetary dust (zodiacal light), and background light from faint, unresolved stars and nebulosity.

          Like the earth, half of the Moon is always lit by the Sun, while half remains in darkness. Its orbit around the Earth, and Earth’s orbit around the Sun, means that we see the Moon with different amounts of sunlight on its surface. Although it appears to be altering its shape, only the position of the Sun’s light on the Moon’s surface is changing. These phases follow a cycle from a new Moon, where the dark side is facing us and the Moon appears invisible, to a full Moon, where the entire sunlit part is visible.

          For millennia, humans have kept track of time by observing the changing face of the moon. In fact, you may have noticed that the word “moon” shares its first few letters with the word “month” — and that's no coincidence. 

          The phases of the moon — new moon, first quarter, full moon and last quarter — repeat themselves about once every month. But why does the moon have phases at all? To answer this question, it’s necessary to understand two important facts. First of all, the moon revolves around the Earth once every 29.5 days. And secondly, as the moon carries out its voyage around the planet, it's lit from varying angles by the sun. 

          One half of the moon is always illuminated by the sun. But here on Earth, we can’t always see the half of the moon that's lit up. What we call the phases of the moon represent the different fractions of the moon's lighted half that we can see as the moon circles the Earth.

          When the moon and the sun are on opposite sides of the Earth, we perceive the moon as full. However, when the sun and the moon are on the same side of the Earth, we say the moon is “new.” During a new moon, the side of the moon that we can see from Earth is not illuminated by direct sunlight at all.

          Between the new moon and the full moon, the moon is a crescent (less than half illuminated). It then waxes — grows bigger — into a half-moon (half-illuminated). The first half moon after the new moon is called the first quarter because at that point, the moon is one-quarter of the way through its monthly cycle of phases. After the first quarter come the gibbous moon (more than half illuminated) and finally a full moon.  This cycle of phases then repeats itself in reverse. After a full moon, the moon wanes — becomes smaller — into a gibbous moon, a half-moon (also called last quarter), a crescent and finally a new moon. 

          Just before and just after the new moon, when a slim crescent of the moon is lit, you can also see the rest of the moon lit dimly. This owes to sunlight that bounces off the Earth and illuminates the otherwise dark portion of the moon that’s facing us, an effect known as “earthshine.”

          The major phases of the moon — new moon, first quarter, full moon, last quarter and next new moon — occur, on average, about 7.4 days apart. If you need some help tracking these phases yourself (or if you want to see where the moon was on an important day in history), NASA provides an online calendar of the dates and times of all phases of the moon for the six thousand year period between 2000 BCE to 4000 CE. 

          Nobody knows exactly how the Moon formed. The most common theory is that shortly after Earth formed; it was hit by an object the size of Mars. The impact was so powerful that it sent billions of tonnes of molten material into space. This debris was held in orbit around Earth, and eventually solidified to form the Moon.

          Analysis of samples brought back from the NASA Apollo missions suggest that the Earth and Moon are a result of a giant impact between an early proto-planet and an astronomical body called Theia.

          ‘There used to be a number of theories about how the Moon was made and it was one of the aims of the Apollo program to figure out how we got to have our Moon,' says Sara. Prior to the Apollo mission research there were three theories about how the Moon formed. Capture theory suggests that the Moon was a wandering body (like an asteroid) that formed elsewhere in the solar system and was captured by Earth's gravity as it passed nearby. In contrast, accretion theory suggested that the Moon was created along with Earth at its formation. Finally, according to the fission scenario, Earth had been spinning so fast that some material broke away and began to orbit the planet.

          What is most widely accepted today is the giant-impact theory. It proposes that the Moon formed during a collision between the Earth and another small planet, about the size of Mars. The debris from this impact collected in an orbit around Earth to form the Moon.

          The moon experiences temperatures both hotter and colder than those on Earth. When the Sun is directly over-head, the temperature on the Moon’s surface is higher than the boiling point of water — 100°C (212°F). However, at night, the Moon becomes very cold, with temperatures dropping to —173°C (-280°F). Earth and the Moon are approximately the same distance from the Sun, and therefore receive the same amount of heat. But the lack of an atmosphere on the Moon means that its temperature range is much more extreme. The Sun’s radiation is not filtered out by gases in the atmosphere, and there are no clouds to stop heat escaping at night.

          The moon rotates on its axis in about 27 days. Daytime on one side of the moon lasts about 13 and half days, followed by 13 and a half nights of darkness. When sunlight hits the moon's surface, the temperature can reach 260 degrees Fahrenheit (127 degrees Celsius). When the sun goes down, temperatures can dip to minus 280 f (minus 173 c). Temperatures change all across the moon, as both the near and far side experience sunlight every lunar year, or terrestrial month, due to lunar rotation.

          The moon tilts on its axis about 1.54 degrees — much less than Earth's 23.44 degrees. This means the moon does not have seasons like Earth does. However, because of the tilt, there are places at the lunar poles that never see daylight.

          The Diviner instrument on NASA’s Lunar Reconnaissance Orbiter measured temperatures of minus 396 F (minus 238 C) in craters at the southern pole and minus 413 F (minus 247 C) in a crater at the northern pole. 

          “These super-cold brightness temperatures are, to our knowledge, among the lowest that have been measured anywhere in the solar system, including the surface of Pluto,” David Paige, Diviner's principal investigator and a UCLA professor of planetary science, said in a 2009 statement. Since then, NASA’s New Horizons mission set Pluto’s temperature range at a comparable minus 400 to minus 360 F (minus 240 to minus 217 C).

          Scientists suspected that water ice could exist in the moon's dark craters that are in permanent shadow. In 2010, a NASA radar aboard India's Chandrayaan-1 spacecraft detected water ice in more than 40 small craters at the moon's North Pole. They hypothesized that over 1.3 trillion lbs. of water ice hid among the permanently darkened craters.

          The Moon is Earth’s closest neighbour in space. Its orbit around Earth is elliptical, rather than circular, which means that its distance from us varies. At its closest point to Earth (its perigee), the Moon is 384,400km (240,000 miles) away. Incredibly, the Moon’s orbit is slowly carrying it away from Earth at a rate of around 5cm (2in) a year.

          The distance between London, where I live, and Oxford, where I used to live, is about 100 km (or 60 miles). It takes about 90 minutes by car and about 120 minutes by bus. I can easily make sense of that distance.

          Harder to consider: the distance between the Earth and the moon, which is 384,400 km (240,000  miles). It’s a fact we’ve likely all learned in high school. Unlike the distance between London and Oxford, however, it’s not easy to comprehend what 384,400 km means in real terms.

          Luckily, you don’t have to think too hard. A NASA spacecraft has solved that problem for us. In October, OSIRIS-REX, a spacecraft that’s bound to intersect an asteroid in August this year, took the photo above from about 5 million km (3 million miles) away from the Earth. NASA posted the picture on Jan. 2, providing the public with a unique view of our planet and its moon. The angle is great to get a grasp of what the distance between the two celestial bodies really looks like, but it’s not perfect.

          Here’s a back-of-the-envelope calculation to explain why. For ease, we’re going to use round figures. The Earth’s diameter is about 13,000 km (8,000 miles). That means, in the 390,000 km distance between the Earth and the moon.

          Although the moon’s interior structure is difficult to study, scientists believe that it has a small iron core. Surrounding this is a partially molten zone called the lower mantle. Above this lies the mantle, which is made up of dense rock, and the crust, which is also made of rock. Together, the mantle and the crust form the lithosphere, which can be up to 800km (500 miles) thick. There are only two basic regions on the Moon’s surface — dark plains called mania and lighter highlands. These heavily cratered highlands are the oldest parts of the Moon’s crust, dating back over four billion years. The darker plains are craters that were filled with lava.

          The composition of the Moon is a bit of a mystery. Although we know a lot about what the surface of the Moon is made of, scientists can only guess at what the internal composition of the Moon is. Here’s what we think the Moon is made of.

          Like the Earth, the Moon has layers. The innermost layer is the lunar core. It only accounts for about 20% of the diameter of the Moon. Scientists think that the lunar core is made of metallic iron, with small amounts of sulfur and nickel. Astronomers know that the core of the Moon is probably at least partly molten.

          Outside the core is the largest region of the Moon, called the mantle. The lunar mantle extends up to a distance of only 50 km below the surface of the Moon. Scientists believe that the mantle of the Moon is largely composed of the minerals olivine, orthopyroxene and clinopyroxene. It’s also believed to be more iron-rich than the Earth’s mantle.

          The outermost layer of the Moon is called the crust, which extends down to a depth of 50 km. This is the layer of the Moon that scientists have gathered the most information about. The crust of the Moon is composed mostly of oxygen, silicon, magnesium, iron, calcium, and aluminum. There are also trace elements like titanium, uranium, thorium, potassium and hydrogen.