My Science School

Planets and their moons are not the only objects in our Solar System. Billions of small rocky bodies, called asteroids, also orbit the Sun. An asteroid, often called a minor planet, is a small body made up of rock and metal left over from the formation of the Solar System. Asteroids can range in size from almost 1000km (610 miles) in diameter, to the size of a small car.

4.6 billion Years ago, our solar system formed from a collection of gas and dust surrounding our nascent sun. While much of the gas and dust in this protoplanetary disk coalesced to form the planets, some of the debris was left over.

Some of debris was shattered remnants of planetesimals – bodies within the young sun's solar nebula that never grew large enough to become planets, and scientists theorize that large collisions in the early, chaotic solar system pulverized these planetesimals into smaller pieces. Other debris never came together due to the massive gravitational pull from Jupiter.

These rocky remnants are now the asteroids that travel about our solar system. Since these "leftovers" contain clues about the early days of our solar system, scientists are eager to study them.

Asteroids are rocky, metallic bodies that orbit the sun. They are made from different kinds of rock and metals, with the metals being mostly nickel and iron. They are sometimes called "minor planets" but they are much, much smaller than the planets or moons. They don't have atmospheres, but about 150 asteroids are known to have small "moons" orbiting them, and some even have two moons. There are also binary (double) asteroids, where two rocky bodies of roughly equal size orbit each other, as well as triple asteroid systems.

At least one asteroid has rings. This surprise discovery was made in 2013 when scientist watched Asteroid Chariklo pass in front of a star. The asteroid made the background star "blink" several times, which led to the discovery that two rings are surrounding the asteroid.

Mars has one of the most dramatic surfaces of any planet in the Solar System. Enormous volcanoes dominate the landscape, the largest of which —Olympus Mons — is over 25 kilometres (15.5 miles) tall. This is three times larger than Mount Everest on Earth! The giant canyon Vales Mariners is long enough to stretch across the entire United States of America.

Mar’s surface is a dry, barren wasteland marked by old volcanoes and impact craters. The entire surface can be scoured by a single sand storm that hides it from observation for days at a time. Despite the formidable conditions, Mar’s surface is better understood by scientists than any other part of the Solar System, except our own planet, of course.

Mars is a small world. Its radius is half of the Earth’s and it has a mass that is less than one tenth. The Red Planet’s total surface area is about 28% of Earth. While that does not sound like a large world at all, it is nearly equivalent to all of the dry land on Earth. The surface is thought to be mostly basalt, covered by a fine layer of iron oxide dust that has the consistency of talcum powder. Iron oxide(rust as it is commonly called) gives the planet its characteristic red hue.

In the ancient past of the planet volcanoes were able to erupt for millions of years unabated. A single hotspot could dump molten rock on the surface for millenia because Mars lacks plate tectonics. The lack of tectonics means that the same rupture in the surface stayed open until there was no more pressure to force magma to the surface. Olympus Mons formed in this manner and is the largest mountain in the Solar System. It is three time taller than Mt. Everest. These runaway volcanic actions could also partially explain the deepest valley in the Solar System. Valles Marineris is thought to be the result of a collapse of the material between two hotspots and is also on Mars.

As it exists today, Mars is a planet hostile to life. Unlike Earth, Mars has no ozone layer to protect life from the Sun’s lethal ultraviolet radiation. There is no breathable oxygen in the air, and giant dust storms are common around the planet. The first astronauts to live on Mars will probably do so in large domes that can contain an artificial, Earth-like atmosphere.

Earth is the only place that we know for certain supports life. Many claims have been made by observers who thought they saw evidence of life on Mars, but we now know they were tricked by the very difficult measurements. From Earth, even with our most powerful telescopes, we just cannot see enough detail on Mars to answer this question. We need a close-up look at the planet.

While robotic spacecraft have given us wonderful views, no humans have ever tried to journey to Mars, and no such missions will be attempted for many years. In fact, whoever will turn out to be the first people on Mars may be your age today, and when you are an adult, perhaps you will watch -- or even participate!-- as people make the first voyage to that planet.

In the meantime, NASA is working hard now to discover whether there is life on Mars. The United States and other countries have been sending spacecraft to orbit or land there since the 1960s, and each mission teaches us more about this fascinating planet. We have learned that even though Mars is more similar to Earth than anywhere else in the solar system, and therefore is a good place to look for life, it is still different from Earth in many ways.

A compass point to the North Pole on Earth because our whole planet acts like a giant magnet, but Mars does not act this way. Besides turning a compass needle, Earth's magnetic field turns away dangerous particles of space radiation. Without a magnetic field on Mars and with much, much less air than on Earth, more harmful space radiation reaches its surface. Although some measurements tell us there probably is water on Mars, there is far less than on Earth. And it is so cold there that most of the water is probably not liquid but rather is ice. Overall, Mars would be a pretty uncomfortable place to try to live!

Even if there were no life on Mars, it would be exciting to know whether there used to be life there. So in addition to looking for living bacteria, NASA will be searching for tiny fossils that might indicate life got a start early in Mars' history but, unlike on our home planet, it did not survive and evolve into larger life forms.

Many of the studies of Mars will involve robots, like the ones that have gone there before, but getting more advanced with each flight. Someday a spacecraft may pick up samples from Mars and bring them back to Earth where they can be studied in our best laboratories. Eventually, humans may make the daring journey, but many important problems have to be solved before trying such an expensive, difficult, and exciting voyage.

When mars first formed it had a much thicker atmosphere than it does today. Because the planet’s gravity is not very strong, this atmosphere gradually escaped into space. The climate became increasingly cold, and all the water on Mars froze. Today, the water on Mars exists only as an icy, permafrost layer deep in the soil. Temperatures in Mars’ Polar Regions are so low that carbon dioxide in the atmosphere freezes, covering sheets of water ice with a layer of frosty crystals of dry ice.

Almost all water on Mars today exists as ice, though it also exists in small quantities as vapor in the atmosphere. What was thought to be low-volume liquid brines in shallow Martian soil, also called recurrent slope lineae, may be grains of flowing sand and dust slipping downhill to make dark streaks. The only place where water ice is visible at the surface is at the north polar ice cap. Abundant water ice is also present beneath the permanent carbon dioxide ice cap at the Martian South Pole and in the shallow subsurface at more temperate conditions. More than 21 million km3 of ice have been detected at or near the surface of Mars, enough to cover the whole planet to a depth of 35 meters (115 ft). Even more ice is likely to be locked away in the deep subsurface.

Some liquid water may occur transiently on the Martian surface today, but limited to traces of dissolved moisture from the atmosphere and thin films, which are challenging environments for known life. No large standing bodies of liquid water exist on the planet's surface, because the atmospheric pressure there averages just 600 pascals (0.087 psi), a figure slightly below the vapor pressure of water at its melting point; under average Martian conditions, pure water on the Martian surface would freeze or, if heated to above the melting point, would sublime to vapor. Before about 3.8 billion years ago, Mars may have had a denser atmosphere and higher surface temperatures, allowing vast amounts of liquid water on the surface, possibly including a large ocean that may have covered one-third of the planet. Water has also apparently flowed across the surface for short periods at various intervals more recently in Mars' history. On December 9, 2013, NASA reported that, based on evidence from the Curiosity rover studying Aeolis Palus, Gale Crater, contained an ancient freshwater lake that could have been a hospitable environment for microbial life.

Many lines of evidence indicate that water ice is abundant on Mars and it has played a significant role in the planet's geologic history. The present-day inventory of water on Mars can be estimated from spacecraft imagery, remote sensing techniques (spectroscopic measurements, radar, etc.), and surface investigations from landers and rovers. Geologic evidence of past water includes enormous outflow channels carved by floods, ancient river valley networks, deltas, and lakebeds; and the detection of rocks and minerals on the surface that could only have formed in liquid water. Numerous geomorphic features suggest the presence of ground ice (permafrost) and the movement of ice in glaciers, both in the recent past and present. Gullies and slope lineae along cliffs and crater walls suggest that flowing water continues to shape the surface of Mars, although to a far lesser degree than in the ancient past.

Although the surface of Mars was periodically wet and could have been hospitable to microbial life billions of years ago, the current environment at the surface is dry and subfreezing, probably presenting an insurmountable obstacle for living organisms. In addition, Mars lacks a thick atmosphere, ozone layer, and magnetic field, allowing solar and cosmic radiation to strike the surface unimpeded. The damaging effect of ionizing radiation on cellular structure is another one of the prime limiting factors on the survival of life on the surface. Therefore, the best potential locations for discovering life on Mars may be in subsurface environments. On November 22, 2016, NASA reported finding a large amount of underground ice on Mars; the volume of water detected is equivalent to the volume of water in Lake Superior. In July 2018, Italian scientists reported the discovery of a sub glacial lake on Mars, 1.5 km (0.93 mi) below the southern polar ice cap, and extending sideways about 20 km (12 mi), the first known stable body of water on the planet.

Terraforming is the process of changing the environment of a planet to make it more like Earth. Many scientists have proposed terraforming Mars as a way of dealing with over-crowding on Earth. Nobody knows exactly how terraforming would work, and whether it would have a damaging effect on Mars’ natural environment, but in theory, Mars could be transformed into a second Earth, where many forms of life could live naturally. The diagrams to the right show how it could be done.

Terraforming or terraformation (literally, “Earth-shaping”) of a planet, moon, or other body is the hypothetical process of deliberately modifying its atmosphere, temperature, surface topography or ecology to be similar to the environment of Earth to make it habitable by Earth-like life.

The concept of terraforming developed from both science fiction and actual science. The term was coined by Jack Williamson in a science-fiction short story (“Collision Orbit”) published during 1942 in Astounding Science Fiction, but the concept may pre-date this work.

Even if the environment of a planet could be altered deliberately, the feasibility of creating an unconstrained planetary environment that mimics Earth on another planet has yet to be verified. Mars is usually considered to be the most likely candidate for terraforming. Much study has been done concerning the possibility of heating the planet and altering its atmosphere, and NASA has even hosted debates on the subject. Several potential methods of altering the climate of Mars may fall within humanity's technological capabilities, but at present the economic resources required to do so are far beyond that which any government or society is willing to allocate to it. The long timescales and practicality of terraforming is the subject of debate. Other unanswered questions relate to the ethics, logistics, economics, politics, and methodology of altering the environment of an extraterrestrial world.

The most convincing evidence for life on the red planet comes from a Martian meteorite that landed on Earth around 13,000 years ago. This meteorite contained microscopic structures that could have been formed by living organisms.

The general consensus now is that the original rock formed 4 billion years ago on Mars. It was eventually catapulted into space by an impact and wandered the solar system for millions of years before landing on Earth 13,000 years ago.

Over 50 other meteorites have been identified as coming from Mars, but ALH84001 is by far the oldest, with the next in age being just 1.3 billion years old. "That alone makes ALH84001 a very important sample," says Allan Treiman of the Lunar and Planetary Institute. "It’s our only hope to understand what Mars was like at this time period."

The first thing that struck researchers examining the meteorite was the presence of 300-micron-wide carbonate globules that make up 1% of the rock. Dave McKay from NASA’s Johnson Space Center and his colleagues determined that the carbonate most likely formed in the presence of water.

Although evidence for a wet ancient Mars has accumulated in the subsequent years, the claim that ALH84001 once sat in water was pretty revolutionary at the time, says Kathie Thomas-Keprta, also from the Johnson Space Center.

Inside the ALH84001 carbonates, McKay spotted odd features that resembled very small worm-like fossils, so he asked Thomas-Keprta to look at them more closely with electron microscopy. "I kind of thought he was crazy," she says. "I thought I would join the group and straighten them out."

In the end, she helped the team characterize the biomorphic features, as well as unusual grains of the mineral magnetite found in the meteorite. In a 1996 Science paper, these two phenomena – along with the chemical distribution in the globules and the detection of large organic molecules – were taken collectively as signatures of biological activity occurring long ago on Mars.

Mars has been known as the red planet for thousands of years. The Ancient Romans named the planet Mars because it reminded them of their God of anger and war. Mars gets its striking colour from large amounts of iron oxide (rust) in its soil.

Even photos from spacecraft show that it’s a rusty red color. The hue comes from the fact that the surface is actually rusty, as in; it’s rich in iron oxide. Iron left out in the rain and will get covered with rust as the oxygen in the air and water reacts with the iron in the metal to create a film of iron oxide.

Mars’ iron oxide would have formed a long time ago, when the planet had more liquid water. This rusty material was transported around the planet in dust clouds, covering everything in a layer of rust. In fact, there are dust storms on Mars today that can rise up and consume the entire planet, obscuring the entire surface from our view. That dust really gets around.

But if you look closely at the surface of Mars, you’ll see that it can actually be many different colours. Some regions appear bright orange, while others look browner or even black. But if you average everything out, you get Mars’ familiar red colour.

If you dig down, like NASA’s Phoenix Lander did in 2008, you get below this oxidized layer to the rock and dirt beneath. You can see how the tracks from the Curiosity Rover get at this fresh material, just a few centimeters below the surface. It’s brown, not red.

And if you could stand on the surface of Mars and look around, what colour would the sky be? Fortunately, NASA’s Curiosity Rover is equipped with a full colour camera, and so we can see roughly what the human eye would see. The sky here is blue because of Raleigh scattering, where blue photons of light are scattered around by the atmosphere, so they appear to come from all directions. But on Mars, the opposite thing happens. The dust in the atmosphere scatters the red photons makes the sky appear red. We have something similar when there’s pollution or smoke in the air.

But here’s the strange part. On Mars, the sunsets appear blue. The dust absorbs and deflects the red light, so you see more of the blue photons streaming from the Sun. A sunset on Mars would be an amazing event to see with your own eyes. Let’s hope someone gets the chance to see it in the future.

When mars first formed it had a very thick atmosphere. However, the gases have long since disappeared into space due to the planet’s weak gravity. Mars’ atmosphere is now very thin, and made mainly of carbon dioxide.

The atmosphere of Mars is the layer of gas surrounding Mars. It is primarily composed of carbon dioxide (95.32%), molecular nitrogen (2.6%) and argon (1.9%). It also contains trace levels of water vapor, oxygen, carbon monoxide, hydrogen and other noble gases. The atmosphere of Mars is much thinner than Earth’s. The surface pressure is only about 610 Pascal’s (0.088 psi) which is less than 1% of the Earth’s value. The currently thin Martian atmosphere prohibits the existence of liquid water at the surface of Mars, but many studies suggest that the Martian atmosphere was much thicker in the past. The highest atmospheric density on Mars is equal to the density found 35 km above the Earth’s surface. The atmosphere of Mars has been losing mass to space throughout history, and the leakage of gases still continues today.

The atmosphere of Mars is colder than Earth’s. Owing to the larger distance from Sun, Mars receives less solar energy and has a lower effective temperature (about 210 K). The average surface emission temperature of Mars is just 215 K, which is comparable to inland Antarctica. The weaker greenhouse effect in the Martian atmosphere (5 °C, versus 33 °C on Earth) can be explained by the low abundance of other greenhouse gases. The daily range of temperature in the lower atmosphere is huge (can exceed 100 °C near the surface in some regions) due to the low thermal inertia. The temperature of the upper part of the Martian atmosphere is also significantly lower than Earth’s because of the absence of stratospheric ozone and the radiative cooling effect of carbon dioxide at higher altitudes.

Dust devils and dust storms are prevalent on Mars, which are sometimes observable by telescopes from Earth. Planet-encircling dust storms (global dust storms) occur on average every 5.5 earth years on Mars and can threaten the operation of Mars rovers. However, the mechanism responsible for the development of large dust storms is still not well understood.

The Martian atmosphere is an oxidizing atmosphere. The photochemical reactions in the atmosphere tend to oxidize the organic species and turn them into carbon dioxide or carbon monoxide. Although the most sensitive methane probe on the recently launched ExoMars Trace Gas Orbiter failed to find methane in the atmosphere over the whole Mars, several previous missions and ground-based telescope detected unexpected levels of methane in the Martian atmosphere, which may even be a bio signature for life on Mars. However, the interpretation of the measurements is still highly controversial and lacks a scientific consensus.

Of all the planets in the Solar System, Mars most resembles Earth. Its day is only slightly over 24 hours, and it is tilted at the same angle as our planet, meaning that seasons are very similar to ours. Early on in its history, Mars had water on its surface. Oceans formed, kept warm by volcanic activity, and primitive life may have started here. Today, freezing conditions on Mars, and the planet’s thin atmosphere, mean that life can no longer exist on the planet’s surface.

The search for life on Mars shouldn’t focus exclusively on the distant past, some researchers say. Four billion years ago, the Martian surface was apparently quite habitable, featuring rivers, lakes and even a deep ocean. Indeed, some astrobiologists view ancient Mars as an even better cradle for life than Earth was, and they suspect that life on our planet may have come here long ago aboard Mars rocks blasted into space by a powerful impact.

Things changed when Mars lost its global magnetic field. Charged particles streaming from the sun were then free to strip away the once-thick Martian atmosphere, and strip it they did. This process had transformed Mars into the cold, dry world we know today by about 3.7 billion years ago, observations by NASA's MAVEN orbiter suggest. (Earth still has its global magnetic field, explaining how our planet remains so livable.)

One of the most promising hiding places is the Martian underground. Though the Red Planet's surface has no liquid water these days — apart, possibly, from temporary flows on warm slopes now and again — there’s a likely lot of the wet stuff in buried aquifers. For example, observations by Europe’s Mars Express orbiter suggest that a big lake may lurk beneath the Red Planet’s South Pole.

Earth’s diverse residents advertise their presence in dramatic and obvious ways; an advanced alien civilization could probably figure out pretty quickly, just by scanning our atmosphere, that our planet is inhabited. 

We don’t see any such clear-cut evidence in the Martian air, but scientists have spotted some intriguing hints recently. For example, NASA's Curiosity rover has rolled through two plumes of methane inside the 96-mile-wide (154 kilometers) Gale Crater, which the six-wheeled robot has been exploring since its 2012 touchdown. The rover mission also determined that baseline methane concentrations in Gale's air go through cycles seasonally.

More than 90% of Earth's atmospheric methane is produced by microbes and other organisms, so it's possible the gas is a signature of modern Martian life.

But the jury is most definitely still out on that. Abiotic processes can generate methane, too; the reaction of hot water with certain types of rock is one example. And even if the Mars methane is biogenic, the creatures that created it could be long dead. Scientists think the Red Planet methane plumes leaked out from underground, and there's no telling how long the gas lay trapped down there before making its way to the surface.

In the 19th century, the astronomer Giovanni Schiaparelli claimed that Mars was covered by a network of channels. Many people believed that these were canals created by an intelligent civilization to help carry water from the Polar Regions to drier areas around the Equator. Recent photographs of Mars have shown that there are many channels on the planet, but scientists believe these were created naturally by running water billions of years ago.

The space-heads among you have undoubtedly heard about the Curiosity rover's first significant discovery: the remnants of an ancient streambed on Mars, which would seem to indicate the presence of water in the planet's history. This jagged pile of alluvial rock and dust my not took like much, but it brings to mind one of my favorite pieces of Martian historical arcana.

For a time in the late 19th century, it was believed that there were canals on Mars. The Italian astronomer Giovanni Schiaparelli, who observed Mars in 1877, was the first to describe, name, and lovingly illustrate mysterious straight lines along its equatorial regions, which he called canal. Viewed with the telescopes of the day, in brief instances of still air amidst the optical strangeness of atmosphere, Mars was tough to figure. There are areas which appear darker or lighter (these are called Albedo features); to an enthusiastic observer, it was easy to speculate of continents, oceans, or even straight-line canals.

Beset by the same optical illusions, many astronomers seconded Schiaparelli's observations. The maps of the day show a Mars riven with peculiar webs and lines–lines which successive high-resolution mapping of the planet have definitively shown do not exist. The mechanism that caused this illusion appears to be internal: faced with a shifting landscape of foggy forms, glimpsed at through simple lenses of glass through the refractive index of Earth’s atmosphere, the human brain tends to impose order.

The persistence of belief in Martian canals is often attributed to a linguistic fluke, that the Italian canal, meaning "channel" (or watercourse, and not necessarily of unnatural origin), was mistranslated to the English "canal." I really love this narrative of language shaping reality, but unfortunately it's the astronomical equivalent of an urban legend. "Canal," in fact, was used in the earliest English accounts, and Schiaparelli made no move to correct the misunderstanding, if he was aware of it.

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.