The Earth and the Moon - Content

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  1. Introduction to the Earth
  2. Day and Night & Seasons
  3. Northern Lights
  4. Introduction to the Moon
  5. Lunar Surface
  6. Phases of the Moon
  7. Solar and Lunar Eclipses
  8. Tides and Tidal Interaction
  9. Summary

1. Introduction to the Earth

Terre

The Earth is our home, and is the only planet in the solar system with the exact 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, but since its beginnings the Earth has been a unique planet. About 71% of the Earth’s surface is covered by water, and is the only planet in the solar system with water in liquid form. Unlike other planets in our solar system, the amount of energy we receive from the Sun generates a climate ideal for life. We experience such diverse weather patterns on Earth because of our atmosphere and the constant circulation of air due to the Earth’s rotation. Our atmosphere is unlike that of any other planets, and has played a role in the Earth’s ability to sustain life. The air we breathe is rich in nitrogen (77%) and oxygen (21%), unlike the toxic carbon dioxide found on Venus and Mars. Our upper atmosphere blocks harmful radiation from the Sun while still allowing heat to escape, and the weight of the air above us is not heavy enough to crush us as the atmosphere of Venus would. 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

Because we live on the Earth and can study it directly, we know more about our home planet than any other. But before understanding our place in the universe and the solar system, ancient civilizations had difficulty explaining events such as the day/night cycle, tides, eclipses, and seasons. It is impossible to explain scientifically the causes for the seasons on Earth without the understanding that our planet is in orbit around the Sun. This heliocentric (Sun-centred) model of our solar system helps to explain the seasons on Earth. It is often thought that it is warmer in summer because the Earth is simply closer to the Sun, but our proximity to the Sun does not affect our seasons. The seasons are caused because the Earth 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 Sun travels higher in our sky, and the northern hemisphere receives more direct sunlight (and therefore, more intense energy and heat). Our days are also longer during the summer, and the direct sunlight combined with longer days result in warmer temperatures. The point along the ecliptic when the Sun is furthest north of the celestial equator is called the summer solstice and is the longest day of the year in the northern hemisphere. Winter occurs for the exact opposite reason: the Sun is south of the celestial equator and as a result is far lower in the sky, which results in our receiving less energy from the Sun combined with short days. There is also a winter solstice, which is the day (around December 21) when the Sun is furthest south of the ecliptic. It should be noted that when it is winter in the northern hemisphere, the southern hemisphere is getting the longer days with more direct sunlight, so it is summer there. 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 is when the Sun crosses 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.

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3. Northern Lights

Aurore boréale

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 our 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 glow 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 magnetic field. The Earth is like a big magnet and generates a large magnetic field around the planet that reverses polarity every few thousand years. The charged particles from the Sun become trapped by this magnetic field and then collect near the Earth's magnetic poles, which 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 sunspot maximums we can expect to see magnificent auroral displays here on Earth.

4. Introduction to the Moon

Lune
*Image © Dave Vandervelde, 2002

The Earth is vibrant and full of colour, but our closest neighbour in space is a dark and desolate world. Despite its lifeless appearance, the Moon is a significant object in our night sky. A full moon high in the sky will dominate the night with its brilliance, and a crescent Moon visible just after sunset is a beautiful sight. But what exactly is the Moon? How did it originate, what is it made of, what causes its brilliance and its monthly progression through its phases, and what effect does the Moon have on our lives here on Earth?

There are a few theories about the origin of the Moon, but the collision ejection theory is the most probable. It 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.

The Moon's diameter is about a quarter the size of the Earth's, and its mass is about 80 times less. The Moon completes one orbit of the Earth every 27.3 days (a sidereal month) at an average distance of about 384,400 kilometres. The orbit of the Moon is not perfectly circular, and its distance from the Earth will vary through the sidereal month. When the Moon is at its closest (~ 360,000 km) it is said the Moon is at perigee, and when at its furthest (~ 410,000 km) it is at apogee. 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. How did 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.

5. Lunar Surface

When looking at a photograph of the Moon one can distinguish between the darker, apparently smooth regions and the lighter coloured rugged regions. The dark regions are called Maria (MAR-ee-uh; "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 we find on Earth. When rocks and particles hit our atmosphere, the friction with the air causes the particles to slow down, giving off energy in the form of heat and light (also known as meteors, to be discussed in Module 5). Because of the heat generated, these particles actually burn up and disintegrate. The Moon has no appreciable atmosphere to protect it, so for millions of years rocks 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 the Maria were, and the extent to which craters pocketed the surface was unknown. Telescopes revealed only a limited amount of information about the Moon, and it wasn't until space probes traveled to the Moon to take high-resolution images of its entire surface that we began to learn more about our satellite. The culmination of our knowledge of the Moon came with the Apollo program. The United States had six successful manned Apollo missions to the Moon, and their study of the surface and collection of sample lunar rocks greatly improved our understanding of the Moon.

6. Phases of the Moon

Les phases de la lune

By observing the Moon over a period of several weeks, one would notice that the Moon rises at different times each night and that there is a regular progression through lunar phases. In one synodic month, the Moon progresses through one lunar cycle and will vary between being completely dark (new moon) to 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. Les phases de la luneBecause 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 growing crescent phases until we see the right half illuminated, known as a first quarter moon. After continuing through gibbous phases, the Moon becomes full about 15 days after new moon. The Moon is now in the opposite of the sky from the Sun and is fully lit. The Moon rises at sunset and sets at sunrise during full moon, so the moon is high in the sky while full. The Moon then begins to wane until it reaches the next phase called last quarter, before returning to new moon to begin another synodic month. The saying "Once in a blue moon" is a referral to when two full moons occur in the same calendar month. Because full moons occur more than 29 days apart, a "blue moon" could only occur on the 30th or 31st day of a month.

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7. Solar and Lunar Eclipses

Solar Eclipses

Les phases de la lune
Solar Eclipse animation

During the Moon's orbit around the Earth, it will occasionally pass through our shadow, or 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 a full moon. 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. In a 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, but because the Moon is relatively small, the shadow during totality never exceeds 270 kilometres in width. During a partial eclipse, the observer is located in a portion of the shadow (the penumbra) and the Moon only partially covers the Sun. Despite being partially obscured, the Sun is still so bright it appears no different to the unaided eye (never look directly at the Sun, even during an eclipse). Éclipse totalIt is not until totality when the observer is located within the central region of the Moon's shadow (the umbra) that the Sun becomes blackened. Solar eclipses last at most about seven minutes, and during that time only the corona of the Sun is visible, and stars will appear 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 new moon.If the Moon is near apogee during a solar eclipse, it appears smaller than normal and is not large enough to fully cover the Sun. This is called an annular eclipse, and a ring of the Sun is visible around the Moon. It would seem that an eclipse should occur at every new and full moon, but this does not happen because the Moon's orbit is tilted slightly from the orbital plane of the Earth around the Sun. For this reason, the shadows are usually cast above or below the other body and an eclipse will not occur.

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Lunar Eclipse

8. Tides and Tidal Interaction

Eclipses do not affect our lives on Earth, other than the beauty of witnessing such an event. But for the millions of people in the world living along the ocean, the daily fluctuations of the water level are an important fact of life. Tides occur because of the gravitational attraction between the water in the oceans and the Sun and Moon. The Sun and Moon actually pull at the oceans and cause a tidal bulge (the tidal influence of the Moon is about twice that of the Sun). There are approximately two high tides and two low tides each day, and when it is high tide at one coastal location, it is low tide along a different coast a quarter of the way around the Earth. Because tides occur due to both the Sun and Moon, there are two kinds of tides which depend on the orientation of the Sun and Moon. A spring tide occurs near full or new moon, and causes the greatest tidal differences because the Sun and Moon act together to create one large tidal bulge. A neap tide occurs near a quarter moon when the Sun and Moon are at right angles from each other, causing two smaller tidal bulges. In addition to the effect of the orientation of the Sun and Moon, the distance to the Moon will also affect the tide levels. During perigee, the gravitational pull of the Moon is about 40% greater than if it were at apogee. The world's greatest tides occur in Canada, in the Bay of Fundy in Nova Scotia. If the Moon is near perigee during a spring tide, the water level at high tide can be as much as 16 metres higher than at low tide at a place called Minas Basin. The location and shape of the shoreline combined with the depth of the water are the key reasons that the Bay experiences such dramatic tidal variations.

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9. Summary

Our home planet is unique in its ability to support life. The Earth's surface is covered with water and the crust is constantly changing by plate tectonics. Our planet's atmosphere is unique in its composition of nitrogen and oxygen; it supports life and causes dramatic weather patterns. The Earth and the Moon are close neighbours in space, but appear as very different worlds. The Earth is full of life and is under constant change, while the Moon is a dark and barren world with little activity.

The Moon's surface is composed entirely of rock, and lacks both an atmosphere and magnetic field. Even though the Moon is lifeless and unchanging, it nevertheless affects our lives on Earth. The Moon brightens our night sky with its regular progression through its phases and its beauty has influenced civilizations for thousands of years. The Moon is also the main cause of the tides we see along the ocean coastline and we still look up in wonder during an eclipse. It is also believed that the Moon was born of the Earth, so we may be more closely related than it would appear. While the Earth and its moon seem to be very different and unrelated, our world would be a very different place without the Moon.