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Module 3: The Earth and the MoonBackground Information Contents:
Data sheet The Earth is our home, and is the only planet in the solar system with exactly the conditions required to support life. The Earth is thought to have formed about 4.6 billion years ago along with the rest of the solar system, and since its beginnings the Earth has been a unique planet. The Earth is the only planet in the solar system with water in a liquid form, covering about 71% of its surface. Unlike the other planets in our solar system, the amount of energy Earth receives from the Sun generates a climate ideal for life. The Earth experiences very diverse weather patterns because of its atmosphere and the constant circulation of air due to its rotation. Earth’s atmosphere is unlike any other planet’s, and has played an instrumental role in Earth’s ability to sustain life. The air we breathe is rich in nitrogen (77%) and oxygen (21%), quite unlike the high levels of toxic carbon dioxide found on Venus and Mars. The upper atmosphere blocks harmful radiation from the Sun while still allowing heat to escape, and the weight of the air above is not heavy enough to crush us, unlike the atmosphere of Venus. The Earth is dynamic and has been under constant change since its birth; the thick crust of our planet is constantly shifting (plate tectonics), causing earthquakes and volcanoes to continually reshape the surface. The biological diversity on our planet is incredible, and makes our planet a wonderful place to live.
2. Day and Night & the Seasons
As we live on the Earth and can study it directly, we know more about our home planet than any other. Because ancient civilizations had a very limited understanding of Earth’s place in the universe and the solar system, they experienced great difficulty explaining events such as the day and night cycle, tides, eclipses and seasons. . These events are impossible to explain scientifically without first understanding that the Earth is in constant orbit around the Sun, as described in the heliocentric (Sun-centred) model of the solar system. Although it is often thought that it is warmer in summer because the Earth is simply closer to the Sun, our proximity to the Sun has little affect on the seasons. Rather, the seasons are caused because the Earth’s axis of rotation is tilted at an angle of 23.5 degrees from the orbital plane while we circle the Sun. Because of this tilt, the ecliptic and the celestial equator are inclined 23.5 degrees from each other. The progression of the Sun along the ecliptic will cause it to be located north of the celestial equator for half the year and south for the other half of the year. The northern hemisphere experiences the warm temperatures of summer while the Sun is north of the celestial equator. This is because the northern hemisphere is tilted towards the Sun at this time, and as a result is exposed to the Sun’s light for a longer period of time over one complete axial rotation (one day) during the summer. This leads to earlier sunrises and later sunsets, and because the Sun is in the sky for a longer period of time, we receive more heat from the Sun over the course of a day in the summer. The 23.5 degree tilt also causes the Sun to follow a higher path in the sky, resulting in the northern hemisphere receiving more direct sunlight and therefore providing the warmer temperatures. The point along the ecliptic when the Sun is furthest north of the celestial equator (at its highest point in the sky) is called the summer solstice (usually 21 June) and is the longest day of the year in the northern hemisphere. Winter in the northern hemisphere occurs for the exact opposite reason: the Sun is south of the celestial equator and as a result the days are shorter in duration and less energy is received because the Sun is far lower in the sky. The winter solstice (usually December 21) occurs when the Sun is furthest south of the ecliptic. It should be noted that when it is winter in the northern hemisphere, it is summer in the southern hemisphere with longer days of more direct sunlight. In addition to the solstices, which officially mark the first days of summer and winter, there are also two equinoxes. These are the two points where the ecliptic crosses the celestial equator. The vernal equinox officially marks the first day of spring in the northern hemisphere (autumn in the south) and is when the Sun crosses the celestial equator moving north. The autumnal equinox occurs when the Sun crosses the same plane traveling south, and is officially the first day of autumn (spring in the south). On these two days, the Sun is in the sky for 12 hours; these are the only days of the year where day and night are of equal duration. Load Flash Applet
Living in Canada, we are fortunate to have occasional views of the Northern Lights. Also known as the aurora borealis, Northern Lights are caused by charged particles from the Sun interacting with the Earth’s upper atmosphere. The solar particles collide with air molecules and boost their energy, which is then released as light when the molecules return to their original energy state. The gases in the upper atmosphere then fluoresce like a fluorescent light. Depending on the amount of energy given to the air molecules, auroral displays can glow with different colours, although green is the most common. The particles from the Sun are charged (positive protons and negative electrons), and are therefore influenced by the Earth’s magnetosphere. The Earth is like a big magnet and generates a large magnetic field around the planet that reverses polarity every few thousand years. Geologists believe that this magnetic field is caused by the flow of electric currents through the liquid regions of the Earth’s interior as the planet rotates. The Earth’s magnetic field extends into space and surrounds the planet, extending out from the magnetic poles and forming semi-circular loops between them. The Van Allen Belts are two zones of the magnetosphere in which charged particles from the Sun become trapped. The particles travel along the magnetic field lines within these belts and will then interact with the Earth’s atmosphere where the field lines cross the atmosphere, near the magnetic poles (the magnetic north pole is currently located northwest of the Canadian arctic islands, in the Arctic Ocean). This is why the aurora are seen more regularly at northern latitudes. There is also a south magnetic pole, and so there are also Southern Lights, known as aurora australis. The charged particles from the Sun are often released in huge doses by solar flares, which originate in complex groups of sun spots (see Module 2). As a result, the 11-year sunspot cycle will also affect the aurora; during sun spot maximums we can expect to see magnificent auroral displays here on Earth.
Although the Earth is vibrant and full of colour, the Moon, our closest neighbour in space, is a dark and desolate world. Despite its lifeless appearance, however, the Moon remains a significant object in our night sky. But what exactly is the Moon? Where did it come from, what is it made of, what causes its brilliance, its monthly phase progression, and what effect does the Moon have on our lives here on Earth? Although there are a few theories about the origin of the Moon, the collision ejection theory is the most probable. This theory suggests that the Moon formed when a large asteroid collided with the Earth about 4.6 billion years ago, ejecting molten debris into space, which eventually cooled and formed the Moon. Although we will never know for certain the exact origin of the Moon, supercomputer simulations of a collision and the density and composition of lunar rock support this theory, making it the most widely accepted.
When compared with the Earth, the Moon’s diameter is about a quarter the size and has a mass about 80 times less. The Moon completes one orbit of the Earth every 27.3 days (a sideral month) at an average distance of about 384,400 kilometres. Because the Earth is moving in its own orbit around the Sun during the sidereal month, it actually takes the Moon an extra 2 days and 5 hours to return to the same spot in the sky with respect to the Sun (a synodic month). For this reason, the lunar cycle (time for the Moon to go through a complete set of phases), is 29.5 days. Interestingly enough, the Moon has a synchronous orbit, revolving once on its own axis in the same amount of time it takes to orbit the Earth so that we always see the same “face” of the Moon. Why does this happen? This occurs because the Earth exerts tidal forces on the Moon, causing its near side to be held in place facing the Earth. We had never seen the far side of the Moon until space probes photographed it for the first time in 1959. It is a common misconception that the far side of the Moon is actually the “dark side of the Moon”. In fact, the far side of the Moon gets sunlight just as the near side does; we just can’t see it from the Earth. It is important to remember that the orbit of the Moon is not perfectly circular, and thus its distance from the Earth will vary through the sidereal month from perigee, its closest point (~ 360,000 km), to apogee, its furthest point (~ 410,000 km). The variation, while relatively small, plays an important role in the eclipse process and also affects the strength of the tides.
When looking at the surface of the Moon one can distinguish between the darker, apparently smooth regions and the lighter coloured rugged regions. The dark regions are called Maria ("Mare" singular), and were formed by recent lava flows which filled low-lying regions and have since dried and hardened. (Keep in mind that recent in terms of the Moon is 3 billion years!) The Maria are covered with rocks called mare basalt, which are very similar to the dark rocks found in volcanic regions on the Earth and are composed mainly of iron, titanium and magnesium. The light coloured areas of the Moon are called terrae, or highlands, and are much more rugged and are pocketed with thousands of craters. The rocks found in the terrae are rich in calcium and aluminium and are similar to rocks found in old mountain ranges on the Earth. Within the terrae are other features, such as cliffs and mountain ranges, although not to the scale as on Earth. The Moon has no appreciable atmosphere to protect it, so for millions of years meteoroids have been slamming into the surface, creating the thousands of impact craters we see today. Even though the Moon is our closest neighbour in space, our knowledge of it was still rather limited until the space race began in the 1950’s. Before telescopes, it was unclear what exactly the Maria were, and the extent to which craters pocketed the surface was unknown. Although telescopes were able to reveal important information about the Moon, high-resolution images of the entire surface were not available until space probes traveled to the Moon. With these detailed images, scientists began to gain knowledge of our satellite in greater detail. The culmination of our knowledge of the Moon came with the Apollo space program, which saw the United States successfully land six manned missions on the Moon. These missions enabled astronauts and scientists to study the surface first-hand and to collect and return samples of lunar rocks to the Earth, thus greatly improving our understanding of our only natural satellite.
By observing the Moon over a period of several weeks, one will notice that the Moon rises and sets at different times each night, and that there is a regular progression through lunar phases. In a synodic month, the Moon progresses through one lunar cycle and will vary between being a completely dark new moon and a fully illuminated full moon . The lunar phases are caused because the orbit of the Moon around the Earth will vary the Moon’s position in relation to the Sun. Half of the Moon is always lit by the Sun, but the portion that we see will change depending on where the Moon is in its orbit. The synodic month “begins” at new moon. Because the Moon is in the same part of the sky as the Sun, the illuminated half of the Moon is not facing us and is not visible. During new moon, the Moon rises and sets at the same time as the Sun, and is therefore in the sky during the day. There is then a progression through the growing crescent phase until we see the right half illuminated; this is known as a first quarter moon. After the first quarter phase, there is another progression, this time through the waxing gibbous phases. The Moon becomes full about 15 days after new moon. During a full moon, the Moon is opposite the Sun and is fully lit. The Moon rises at sunset and sets at sunrise when it is full, so the Moon is always visible in the night sky while full. The Moon then begins to wane through another gibbous phase until it reaches its next phase called last or third quarter moon, and again proceeds through another crescent phase, ultimately returning to the new moon almost 30 days later. The saying “Once in a blue moon” is a referral to when two full moons occur in the same calendar month. Load Flash Applet
Solar Eclipses:
Lunar Eclipse:  courtesy Brian Martin Watch
a Solar Eclipse animation During the Moon’s orbit around the Earth, it will occasionally pass through the Earth’s shadow, or will cast its shadow on the Earth. These events are known as lunar eclipses and solar eclipses, respectively. Lunar eclipses occur when the Moon passes through the Earth’s shadow. Because the Moon has to be on the opposite side of the sky from the Sun for this to occur, a lunar eclipse can only take place during the full moon phase. During a lunar eclipse, the Earth’s shadow will travel across the face of the Moon, appearing as though a bite has been taken from it. At total eclipse, the Moon will not darken completely but instead glow deep red because the Earth’s gravity will refract (bend) a small amount of light from the Sun onto the lunar surface. Because the Earth casts a relatively large shadow, lunar eclipses occur a couple of times a year and are visible to large regions on the Earth, lasting up to 100 minutes. Although the frequency of solar eclipses is not considerably different from that of lunar eclipses, they are rarely seen because they are visible only along an extremely narrow path of the Earth. During a solar eclipse, the Moon obstructs the Sun and casts its shadow on the Earth. However, because the Moon is relatively small, the shadow cast during totality never exceeds 270 kilometres in width. If the observer is located in only a portion of the shadow (the penumbra), they would observe a partial solar eclipse where the Moon would only partially cover the Sun. Despite being partially obscured, the Sun is still so bright it would appear no different to the unaided eye (never look directly at the Sun, even during a solar eclipse). It is not until the observer is located within the central region of the Moon’s shadow (the umbra) that the Sun becomes completely covered. Totality of a solar eclipse lasts at most about seven and a half minutes, at which time only the Sun’s corona is visible and several stars will be visible in the daytime sky. Because a solar eclipse can only occur when the Sun and Moon are in the same region of the sky, it can only take place during the new moon phase. In addition to the partial and total eclipses, there is a third type of solar eclipse called the annular eclipse. The annular eclipse can only occur when the Moon is near apogee (its furthest distance from the Earth). At this time, the Moon appears fractionally smaller in size than usual and is therefore not large enough to fully cover the Sun. During an annular eclipse, the observer would see a bright golden arc or a full ring of light around the darkened Moon. Load Flash Applet
8. Tides and Tidal Interaction
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