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Earth & Moon
- Equatorial Diameter: 12,756 km
- Polar Diameter: 12,714 km
- Mass: 5.97 x 10^24 kg
- Moons: 1
- Orbit Distance: 149,598,262 km (1 AU)
- Orbit Period: 365.24 days
- Surface Temperature: -88 to 58°C
- The Earth’s rotation is gradually slowing. This deceleration is happening almost imperceptibly, at approximately 17 milliseconds per hundred years, although the rate at which it occurs is not perfectly uniform. This has the effect of lengthening our days, but it happens so slowly that it could be as much as 140 million years before the length of a day will have increased to 25 hours.
- The Earth was once believed to be the center of the universe. Due to the apparent movements of the Sun and planets in relation to their viewpoint, ancient scientists insisted that the Earth remained static, whilst other celestial bodies travelled in circular orbits around it. Eventually, the view that the Sun was at the center of the universe was postulated by Copernicus, though this is also not the case.
- Earth has a powerful magnetic field. This phenomenon is caused by the nickel-iron core of the planet, coupled with its rapid rotation. This field protects the Earth from the effects of solar wind.
- There is only one natural satellite of the planet Earth. As a percentage of the size of the body it orbits, the Moon is the largest satellite of any planet in our solar system. In real terms, however, it is only the fifth largest natural satellite.
- Earth is the only planet not named after a god. The other seven planets in our solar system are all named after Roman gods or goddesses. Although only Mercury, Venus, Mars, Jupiter, and Saturn were named during ancient times, because they were visible to the naked eye, the Roman method of naming planets was retained after the discovery of Uranus and Neptune.
- The Earth is the densest planet in the Solar System. This varies according to the part of the planet; for example, the metallic core is denser than the crust. The average density of the Earth is approximately 5.52 grams per cubic centimeter. © Space Facts. com
Throughout human history we have sought to understand our home planet. However, the learning curve has been steep, with many errors having been made along the way. For example, it was not until the time of the ancient Romans that the world was understood to be spherical rather than flat. A second example is the belief that the Sun revolved around the Earth. Only in the sixteenth-century, through the work of Copernicus, did we accept that, in fact, the Earth was merely a planet orbiting the Sun. Perhaps most importantly, it is during the last two centuries that science has allowed us to see that the Earth is both an ordinary and unique place in the Solar System. On one hand, many of its characteristics are rather unexceptional. Take, for example, its size, interior, and geological processes—being the fifth largest out of the eight planets, it is close to the median in terms of size; its interior structure is almost identical to the three other terrestrial planets; and the same geological processes that shape its surface can be found not only on other planets, but also on planetary moons. However, the Earth is special in one very important regard—in all of the solar system, the Earth is the only world known to foster life.
Atmosphere The ability for Earth to possess life is dependent in many ways on its atmosphere. The composition of the atmosphere is roughly 78% nitrogen (N2), 21% oxygen (O2), 1% argon, with trace amounts of carbon dioxide (CO2) and other gases. Nitrogen and oxygen are essential to DNA and biological energy production, respectively, without which life could not be sustained. Additionally, the oxygen found in what is known as the ozone layer of the atmosphere protects the surface of the planet by absorbing harmful solar radiation. Remarkably, the significant amount of oxygen present in the atmosphere is due to the life found on Earth. As a byproduct of making sugars, plants convert the carbon dioxide in the atmosphere into oxygen. Essentially, this means that without plants the amount of carbon dioxide in the atmosphere would be much greater and the oxygen levels much lower. On one hand, if carbon dioxide levels were much higher, it is likely the Earth would suffer from a runaway greenhouse effect like that on Venus. On the other hand, if the percentage of carbon dioxide were any lower there would not be a greenhouse effect at all, thus making temperatures far colder. Therefore, the carbon dioxide levels are just right to maintain hospitable temperatures ranging from -88° C to 58° C.
Oceans When viewing Earth from space, there is one overwhelming feature--the oceans of liquid water. In terms of surface area, these oceans cover approximately 70% of the Earth. What is even more amazing than this percentage is that a single drop of liquid water is yet to be found on any other planet in the Solar System. In this regard, the Earth is truly unique. Like the Earth’s atmosphere, the presence of liquid water is vital for life. In fact, life is believed to have first developed 3.8 billion years ago in the oceans, only later evolving the ability to survive on land. The existence of the oceans is attributed to two sources. The first of these is the Earth itself. It is conjectured that large amounts of water vapor were trapped within the Earth during its formation. Over time, the planet’s geological mechanisms, primarily its volcanic activity, released this water vapor into the atmosphere. Once in the atmosphere, this vapor condensed and fell to the planet’s surface as liquid water. The second source is theorized to have originated from the ancient comets that struck the Earth. Upon impact, they deposited substantial amounts of water ice on the planet.
Surface Although most of the Earth’s surface lies beneath its oceans, the remaining “dry” surface is quite remarkable. When comparing the Earth to other solid bodies in the Solar System, its surface stands out due to its lacking impact craters. It is not that the Earth has been spared the numerous impacts by small bodies; rather, it is because the evidence of these impacts has been erased. Although there are many geological processes responsible for this, the two most important are weathering and erosion. In many ways these two mechanisms can be thought of as working in tandem. Weathering is the breaking down of surface structures into smaller pieces by the atmosphere. Moreover, there are two types of weathering: chemical and physical. An example of chemical weathering is acid rain. An example of physical weathering is abrasion of river beds caused by rocks suspended in flowing water. The second mechanism, erosion, is simply the movement of weathered particles by water, ice, wind or gravity. Thus, impact craters have been “smoothed out” through weathering and erosion by being broken apart and redistributed to other areas on the Earth’s surface. Two other geological mechanisms have helped to shape the Earth’s surface. The first is volcanic activity. This process consists of the releasing of magma (molten rock) from the Earth’s interior through a rupture in the its crust. Some effects of volcanic activity can be the resurfacing of Earth’s crust or formation of islands (think of the Hawaiian Islands). The second mechanism is orogeny, or the formation of mountains through the compression of tectonic plates. An example of mountains created through this process is the Rocky Mountains.
Interior Similar to the other terrestrial planets, Earth’s interior is believed to consist of three components: a core, a mantle, and a crust. At present, the core is thought to be comprised of two separate layers--an inner core composed of solid nickel and iron and an outer core composed of molten nickel and iron. The mantle is very dense and almost entirely solid silicate rock; its thickness is roughly 2,850 km. Finally, the crust is also composed of silicate rock and varies in thickness. While the continental crust ranges from 30 to 40 km in thickness, the oceanic crust is much thinner at only 6 to 11 km. Yet another distinguishing feature of the Earth when compared to the other terrestrial planets is that its crust is divided into cool, rigid plates that rest upon the hotter mantle below. Furthermore, these plates are in constant motion. Along the boundaries of these plates two processes, known as subduction and spreading, can occur. During subduction two plates come into contact (sometimes violently, producing earthquakes) and one plate is forced under the other. Separation, on the other hand, is when two plates are moving away from each other.
Orbit & Rotation At roughly 365 days, the Earth’s orbit around the Sun is familiar to us. The length of our year is due in large part to the Earth’s average orbital distance of 1.50 x 108 km. What many people are not familiar with is that at this orbital distance it takes sunlight, on average, about eight minutes and twenty seconds to reach the Earth. With an orbital eccentricity of .0167, the Earth’s orbit is one of the most circular in all the Solar System. This means that the difference between Earth’s perihelion and aphelion is relatively small. As a result of this small difference, the intensity of the sunlight Earth receives remains almost constant year-round. However, the Earth’s position in its orbit is responsible, in part, for the varying seasons it experiences. The Earth’s axial tilt is approximately 23.45°. That is, the axis the Earth rotates about is tilted slightly with respect to the plane in which the Earth orbits the Sun. The effect of this tilt, along with position of the Earth in its orbit, means that at certain times the amount of sunlight the northern hemisphere receives is greater than that of the southern hemisphere, and vice versa. This variation in intensity is what produces the warmer temperatures during the summer and colder temperatures during the winter. A second commonly known characteristic is that the Earth takes approximately twenty-four hours to complete one rotation. This is fastest among the terrestrial planets, but easily slower than that of all the gas giants. © The Planets.org
Atmosphere The ability for Earth to possess life is dependent in many ways on its atmosphere. The composition of the atmosphere is roughly 78% nitrogen (N2), 21% oxygen (O2), 1% argon, with trace amounts of carbon dioxide (CO2) and other gases. Nitrogen and oxygen are essential to DNA and biological energy production, respectively, without which life could not be sustained. Additionally, the oxygen found in what is known as the ozone layer of the atmosphere protects the surface of the planet by absorbing harmful solar radiation. Remarkably, the significant amount of oxygen present in the atmosphere is due to the life found on Earth. As a byproduct of making sugars, plants convert the carbon dioxide in the atmosphere into oxygen. Essentially, this means that without plants the amount of carbon dioxide in the atmosphere would be much greater and the oxygen levels much lower. On one hand, if carbon dioxide levels were much higher, it is likely the Earth would suffer from a runaway greenhouse effect like that on Venus. On the other hand, if the percentage of carbon dioxide were any lower there would not be a greenhouse effect at all, thus making temperatures far colder. Therefore, the carbon dioxide levels are just right to maintain hospitable temperatures ranging from -88° C to 58° C.
Oceans When viewing Earth from space, there is one overwhelming feature--the oceans of liquid water. In terms of surface area, these oceans cover approximately 70% of the Earth. What is even more amazing than this percentage is that a single drop of liquid water is yet to be found on any other planet in the Solar System. In this regard, the Earth is truly unique. Like the Earth’s atmosphere, the presence of liquid water is vital for life. In fact, life is believed to have first developed 3.8 billion years ago in the oceans, only later evolving the ability to survive on land. The existence of the oceans is attributed to two sources. The first of these is the Earth itself. It is conjectured that large amounts of water vapor were trapped within the Earth during its formation. Over time, the planet’s geological mechanisms, primarily its volcanic activity, released this water vapor into the atmosphere. Once in the atmosphere, this vapor condensed and fell to the planet’s surface as liquid water. The second source is theorized to have originated from the ancient comets that struck the Earth. Upon impact, they deposited substantial amounts of water ice on the planet.
Surface Although most of the Earth’s surface lies beneath its oceans, the remaining “dry” surface is quite remarkable. When comparing the Earth to other solid bodies in the Solar System, its surface stands out due to its lacking impact craters. It is not that the Earth has been spared the numerous impacts by small bodies; rather, it is because the evidence of these impacts has been erased. Although there are many geological processes responsible for this, the two most important are weathering and erosion. In many ways these two mechanisms can be thought of as working in tandem. Weathering is the breaking down of surface structures into smaller pieces by the atmosphere. Moreover, there are two types of weathering: chemical and physical. An example of chemical weathering is acid rain. An example of physical weathering is abrasion of river beds caused by rocks suspended in flowing water. The second mechanism, erosion, is simply the movement of weathered particles by water, ice, wind or gravity. Thus, impact craters have been “smoothed out” through weathering and erosion by being broken apart and redistributed to other areas on the Earth’s surface. Two other geological mechanisms have helped to shape the Earth’s surface. The first is volcanic activity. This process consists of the releasing of magma (molten rock) from the Earth’s interior through a rupture in the its crust. Some effects of volcanic activity can be the resurfacing of Earth’s crust or formation of islands (think of the Hawaiian Islands). The second mechanism is orogeny, or the formation of mountains through the compression of tectonic plates. An example of mountains created through this process is the Rocky Mountains.
Interior Similar to the other terrestrial planets, Earth’s interior is believed to consist of three components: a core, a mantle, and a crust. At present, the core is thought to be comprised of two separate layers--an inner core composed of solid nickel and iron and an outer core composed of molten nickel and iron. The mantle is very dense and almost entirely solid silicate rock; its thickness is roughly 2,850 km. Finally, the crust is also composed of silicate rock and varies in thickness. While the continental crust ranges from 30 to 40 km in thickness, the oceanic crust is much thinner at only 6 to 11 km. Yet another distinguishing feature of the Earth when compared to the other terrestrial planets is that its crust is divided into cool, rigid plates that rest upon the hotter mantle below. Furthermore, these plates are in constant motion. Along the boundaries of these plates two processes, known as subduction and spreading, can occur. During subduction two plates come into contact (sometimes violently, producing earthquakes) and one plate is forced under the other. Separation, on the other hand, is when two plates are moving away from each other.
Orbit & Rotation At roughly 365 days, the Earth’s orbit around the Sun is familiar to us. The length of our year is due in large part to the Earth’s average orbital distance of 1.50 x 108 km. What many people are not familiar with is that at this orbital distance it takes sunlight, on average, about eight minutes and twenty seconds to reach the Earth. With an orbital eccentricity of .0167, the Earth’s orbit is one of the most circular in all the Solar System. This means that the difference between Earth’s perihelion and aphelion is relatively small. As a result of this small difference, the intensity of the sunlight Earth receives remains almost constant year-round. However, the Earth’s position in its orbit is responsible, in part, for the varying seasons it experiences. The Earth’s axial tilt is approximately 23.45°. That is, the axis the Earth rotates about is tilted slightly with respect to the plane in which the Earth orbits the Sun. The effect of this tilt, along with position of the Earth in its orbit, means that at certain times the amount of sunlight the northern hemisphere receives is greater than that of the southern hemisphere, and vice versa. This variation in intensity is what produces the warmer temperatures during the summer and colder temperatures during the winter. A second commonly known characteristic is that the Earth takes approximately twenty-four hours to complete one rotation. This is fastest among the terrestrial planets, but easily slower than that of all the gas giants. © The Planets.org
- Diameter: 3,475 km
- Mass: 7.35 × 10^22 kg (0.01 Earths)
- Orbits: The Earth
- Orbit Distance: 384,400 km
- Orbit Period: 27.3 days
- Surface Temperature: -233 to 123 °C
The dark side of the moon is a myth. In reality both sides of the Moon see the same amount of sunlight however only one face of the Moon is ever seen from Earth. This is because the Moon rotates around on its own axis in exactly the same time it takes to orbit the Earth, meaning the same side is always facing the Earth. The side facing away from Earth has only been seen by the human eye from spacecraft.The rise and fall of the tides on Earth is caused by the Moon. There are two bulges in the Earth due to the gravitational pull that the Moon exerts; one on the side facing the Moon, and the other on the opposite side that faces away from the Moon. The bulges move around the oceans as the Earth rotates, causing high and low tides around the globe.The Moon is drifting away from the Earth. The Moon is moving approximately 3.8 cm away from our planet every year. It is estimated that it will continue to do so for around 50 billion years. By the time that happens, the Moon will be taking around 47 days to orbit the Earth instead of the current 27.3 days.A person would weigh much less on the Moon. The Moon has much weaker gravity than Earth, due to its smaller mass, so you would weigh about one sixth (16.5%) of your weight on Earth. This is why the lunar astronauts could leap and bound so high in the air.The Moon has only been walked on by 12 people; all American males. The first man to set foot on the Moon in 1969 was Neil Armstrong on the Apollo 11 mission, while the last man to walk on the Moon in 1972 was Gene Cernan on the Apollo 17 mission. Since then the Moon has only been visited by unmanned vehicles.The Moon has no atmosphere. This means that the surface of the Moon is unprotected from cosmic rays, meteorites and solar winds, and has huge temperature variations. The lack of atmosphere means no sound can be heard on the Moon, and the sky always appears black.The Moon has quakes. These are caused by the gravitational pull of the Earth. Lunar astronauts used seismographs on their visits to the Moon, and found that small moonquakes occurred several kilometers beneath the surface, causing ruptures and cracks. Scientists think the Moon has a molten core, just like Earth.The first spacecraft to reach the Moon was Luna 1 in 1959. This was a Soviet craft, which was launched from the USSR. It passed within 5995 km of the surface of the Moon before going into orbit around the Sun.The Moon is the fifth largest natural satellite in the Solar System. At 3,475 km in diameter, the Moon is much smaller than the major moons of Jupiter and Saturn. Earth is about 80 times the volume than the Moon, but both are about the same age. A prevailing theory is that the Moon was once part of the Earth, and was formed from a chunk that broke away due to a huge object colliding with Earth when it was relatively young.During the 1950’s the USA considered detonating a nuclear bomb on the Moon.The secret project was during the height cold war was known as “A Study of Lunar Research Flights” or “Project A119” and meant as a show of strength at a time they were lagging behind in the space race. © Space Facts. com
10 Things About the Moon
by Joe Rao, Space.com March 18, 2011
1) There are actually four kinds of lunar months Our months correspond approximately to the length of time it takes our natural satellite to go through a full cycle of phases. From excavated tally sticks, researchers have deduced that people from as early as the Paleolithic period counted days in relation to the moon's phases. But there are actually four different kinds of lunar months. The durations listed here are averages. 1. Anomalistic – the length of time it takes the moon to circle the Earth, measured from one perigee (the closest point in its orbit to Earth) to the next: 27 days, 13 hours, 18 minutes, 37.4 seconds. 2. Nodical – the length of time it takes the moon to pass through one of its nodes (where it crosses the plane of the Earth's orbit) and return to it: 27 days, 5 hours, 5 minutes, 35.9 seconds. 3. Sidereal – the length of time it takes the moon to circle the Earth, using the stars as a reference point: 27 days, 7 hours, 43 minutes, 11.5 seconds 4. Synodical – the length of time it takes the moon to circle the Earth, using the sun as the reference point (that is, the time lapse between two successive conjunctions with the sun – going from new moon to new moon): 29 days, 12 hours, 44 minutes, 2.7 seconds. It is the synodic month that is the basis of many calendars today and is used to divide the year.
2) We see slightly more than half of the moon from Earth Most reference books will note that because the moon rotates only once during each revolution about the Earth, we never see more than half of its total surface. The truth, however, is that we actually get to see more of it over the course of its elliptical orbit: 59 percent (almost three-fifths). The moon's rate of rotation is uniform but its rate of revolution is not, so we're able to see just around the edge of each limb from time to time. Put another way, the two motions do not keep perfectly in step, even though they come out together at the end of the month. We call this effect libration of longitude. So the moon "rocks" in the east and west direction, allowing us to see farther around in longitude at each edge than we otherwise could. The remaining 41 percent can never be seen from our vantage point; and if anyone were on that region of the moon, they would never see the Earth.
3) It would take hundreds of thousands of moons to equal the brightness of the sun The full moon shines with a magnitude of -12.7, but the sun is 14 magnitudes brighter, at -26.7. The ratio of brightness of the sun versus the moon amounts to a difference of 398,110 to 1. So that's how many full moons you would need to equal the brightness of the sun. But this all a moot point, because there is no way that you could fit that many full moons in the sky. The sky is 360 degrees around (including the half we can't see, below the horizon), so there are over 41,200 square degrees in the sky. The moon measures only a half degree across, which gives it an area of only 0.2 square degrees. So you could fill up the entire sky, including the half that lies below our feet, with 206,264 full moons — and still come up short by 191,836 in the effort to match the brightness of the sun.
4) The first- or last-quarter moon is not one half as bright as a full moon If the moon's surface were like a perfectly smooth billiard ball, its surface brightness would be the same all over. In such a case, it would indeed appear half as bright. But the moon has a very rough topography. Especially near and along the day/night line (known as the terminator), the lunar landscape appears riddled with innumerable shadows cast by mountains, boulders and even tiny grains of lunar dust. Also, the moon's face is splotched with dark regions. The end result is that at first quarter, the moon appears only one eleventh as bright as when it's full. The moon is actually a little brighter at first quarter than at last quarter, since at that phase some parts of the moon reflect sunlight better than others.
5) A 95-percent illuminated moon appears half as bright as a full moon Believe it or not, the moon is half as bright as a full moon about 2.4 days before and after a full moon. Even though about 95 percent of the moon is illuminated at this time, and to most casual observers it might still look like a "full" moon, its brightness is roughly 0.7 magnitudes less than at full phase, making it appear one-half as bright.
6) The Earth, seen from the moon, also goes through phases However, they are opposite to the lunar phases that we see from the Earth. It's a full Earth when it's new moon for us; last-quarter Earth when we're seeing a first-quarter moon; a crescent Earth when we're seeing a gibbous moon, and when the Earth is at new phase we're seeing a full moon. From any spot on the moon (except on the far side, where you cannot see the Earth), the Earth would always be in the same place in the sky. From the moon, our Earth appears nearly four times larger than a full moon appears to us, and – depending on the state of our atmosphere – shines anywhere from 45 to 100 times brighter than a full moon. So when a full (or nearly full) Earth appears in the lunar sky, it illuminates the surrounding lunar landscape with a bluish-gray glow. From here on the Earth, we can see that glow when the moon appears to us as a crescent; sunlight illuminates but a sliver of the moon, while the rest of its outline is dimly visible by virtue of earthlight. Leonardo da Vinci was the first to figure out what that eerie glow appearing on the moon really was.
7) Eclipses are reversed when viewing from the moon Phases aren't the only things that are seen in reverse from the moon. An eclipse of a moon for us is an eclipse of the sun from the moon. In this case, the disk of the Earth appears to block out the sun. If it completely blocks the sun, a narrow ring of light surrounds the dark disk of the Earth; our atmosphere backlighted by the sun. The ring appears to have a ruddy hue, since it's the combined light of all the sunrises and sunsets occurring at that particular moment. That's why during a total lunar eclipse, the moon takes on a ruddy or coppery glow. When a total eclipse of the sun is taking place here on Earth, an observer on the moon can watch over the course of two or three hours as a small, distinct patch of darkness works its way slowly across the surface of the Earth. It's the moon's dark shadow, called the umbra, that falls on the Earth, but unlike in a lunar eclipse, where the moon can be completely engulfed by the Earth's shadow, the moon's shadow is less than a couple of hundred miles wide when it touches the Earth, appearing only as a dark blotch.
8) There are rules for how the moon's craters are named The lunar craters were formed by asteroids and comets that collided with the moon. Roughly 300,000 craters wider than 1 km (0.6 miles) are thought to be on the moon's near side alone. These are named for scholars, scientists, artists and explorers. For example, Copernicus Crater is named for Nicolaus Copernicus, a Polish astronomer who realized in the 1500s that the planets move about the sun. Archimedes Crater is named for the Greek mathematician Archimedes, who made many mathematical discoveries in the third century B.C. The custom of applying personal names for lunar formations began in 1645 with Michael van Langren, an engineer in Brussels who named the moon's principal features after kings and great people on the Earth. On his lunar map he named the largest lunar plain (now known as Oceanus Procellarum) after his patron, Phillip IV of Spain. But just six years later, Giovanni Battista Riccioli of Bologna completed his own great lunar map, which removed the names bestowed by Van Langren and instead derived names chiefly from those of famous astronomers — the basis of the system which continues to this day. In 1939, the British Astronomical Association issued a catalog of officially named lunar formations, "Who's Who on the Moon," listing the names of all formations adopted by the International Astronomical Union. Today the IAU continues to decide the names for craters on our moon, along with names for all astronomical objects. The IAU organizes the naming of each particular celestial feature around a particular theme. The names of craters now tend to fall into two groups. Typically, moon craters have been named for deceased scientists, scholars, explorers, and artists who've become known for their contributions to their respective fields. The craters around the Apollo crater and the Mare Moscoviense are to be named after deceased American astronauts and Russian cosmonauts.
9) The moon encompasses a huge temperature range If you survey the Internet for temperature data on the moon, you're going to run into quite a bit of confusion. There's little consistency even within a given website in which temperature scale is quoted: Celsius, Fahrenheit, even Kelvin. We have opted to use the figures that are quoted by NASA on its Website: The temperature at the lunar equator ranges from an extremely low minus 280 degrees F (minus 173 degrees C) at night to a very high 260 degrees F (127 degrees C) in the daytime. In some deep craters near the moon's poles, the temperature is always near minus 400 degrees F (minus 240 degrees C). During a lunar eclipse, as the moon moves into the Earth's shadow, the surface temperature can plunge about 500 degrees F (300 degrees C) in less than 90 minutes.
10) The moon has its own time zone It is possible to tell time on the moon. In fact, back in 1970, Helbros Watches asked Kenneth L. Franklin, who for many years was the chief astronomer at New York's Hayden Planetarium, to design a watch for moon walkers that measures time in what he called "lunations," the period it takes the moon to rotate and revolve around the Earth; each lunation is exactly 29.530589 Earth days. For the moon, Franklin developed a system he called "lunar mean solar time," or Lunar Time (LT). He envisioned local lunar time zones similar to the standard time zones of Earth, but based on meridians that are 12-degrees wide (analogous to the 15-degree intervals on Earth). "They will be named unambiguously as '36-degree East Zone time,' etc., although 'Copernican time,' 'West Tranquility time' and others may be adopted as convenient." A lunar hour was defined as a "lunour," and decilunours, centilunours and millilunours were also introduced. Interestingly, one moon watch was sent to the president of the United States at the time, Richard M. Nixon, who sent a thank you note to Franklin. The note and another moon watch were kept in a display case at the Hayden Planetarium for several years. Quite a few visitors would openly wonder why Nixon was presented with a wristwatch that could be used only on the moon. Forty years have come and gone without the watch becoming a big seller.
© Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y.
1) There are actually four kinds of lunar months Our months correspond approximately to the length of time it takes our natural satellite to go through a full cycle of phases. From excavated tally sticks, researchers have deduced that people from as early as the Paleolithic period counted days in relation to the moon's phases. But there are actually four different kinds of lunar months. The durations listed here are averages. 1. Anomalistic – the length of time it takes the moon to circle the Earth, measured from one perigee (the closest point in its orbit to Earth) to the next: 27 days, 13 hours, 18 minutes, 37.4 seconds. 2. Nodical – the length of time it takes the moon to pass through one of its nodes (where it crosses the plane of the Earth's orbit) and return to it: 27 days, 5 hours, 5 minutes, 35.9 seconds. 3. Sidereal – the length of time it takes the moon to circle the Earth, using the stars as a reference point: 27 days, 7 hours, 43 minutes, 11.5 seconds 4. Synodical – the length of time it takes the moon to circle the Earth, using the sun as the reference point (that is, the time lapse between two successive conjunctions with the sun – going from new moon to new moon): 29 days, 12 hours, 44 minutes, 2.7 seconds. It is the synodic month that is the basis of many calendars today and is used to divide the year.
2) We see slightly more than half of the moon from Earth Most reference books will note that because the moon rotates only once during each revolution about the Earth, we never see more than half of its total surface. The truth, however, is that we actually get to see more of it over the course of its elliptical orbit: 59 percent (almost three-fifths). The moon's rate of rotation is uniform but its rate of revolution is not, so we're able to see just around the edge of each limb from time to time. Put another way, the two motions do not keep perfectly in step, even though they come out together at the end of the month. We call this effect libration of longitude. So the moon "rocks" in the east and west direction, allowing us to see farther around in longitude at each edge than we otherwise could. The remaining 41 percent can never be seen from our vantage point; and if anyone were on that region of the moon, they would never see the Earth.
3) It would take hundreds of thousands of moons to equal the brightness of the sun The full moon shines with a magnitude of -12.7, but the sun is 14 magnitudes brighter, at -26.7. The ratio of brightness of the sun versus the moon amounts to a difference of 398,110 to 1. So that's how many full moons you would need to equal the brightness of the sun. But this all a moot point, because there is no way that you could fit that many full moons in the sky. The sky is 360 degrees around (including the half we can't see, below the horizon), so there are over 41,200 square degrees in the sky. The moon measures only a half degree across, which gives it an area of only 0.2 square degrees. So you could fill up the entire sky, including the half that lies below our feet, with 206,264 full moons — and still come up short by 191,836 in the effort to match the brightness of the sun.
4) The first- or last-quarter moon is not one half as bright as a full moon If the moon's surface were like a perfectly smooth billiard ball, its surface brightness would be the same all over. In such a case, it would indeed appear half as bright. But the moon has a very rough topography. Especially near and along the day/night line (known as the terminator), the lunar landscape appears riddled with innumerable shadows cast by mountains, boulders and even tiny grains of lunar dust. Also, the moon's face is splotched with dark regions. The end result is that at first quarter, the moon appears only one eleventh as bright as when it's full. The moon is actually a little brighter at first quarter than at last quarter, since at that phase some parts of the moon reflect sunlight better than others.
5) A 95-percent illuminated moon appears half as bright as a full moon Believe it or not, the moon is half as bright as a full moon about 2.4 days before and after a full moon. Even though about 95 percent of the moon is illuminated at this time, and to most casual observers it might still look like a "full" moon, its brightness is roughly 0.7 magnitudes less than at full phase, making it appear one-half as bright.
6) The Earth, seen from the moon, also goes through phases However, they are opposite to the lunar phases that we see from the Earth. It's a full Earth when it's new moon for us; last-quarter Earth when we're seeing a first-quarter moon; a crescent Earth when we're seeing a gibbous moon, and when the Earth is at new phase we're seeing a full moon. From any spot on the moon (except on the far side, where you cannot see the Earth), the Earth would always be in the same place in the sky. From the moon, our Earth appears nearly four times larger than a full moon appears to us, and – depending on the state of our atmosphere – shines anywhere from 45 to 100 times brighter than a full moon. So when a full (or nearly full) Earth appears in the lunar sky, it illuminates the surrounding lunar landscape with a bluish-gray glow. From here on the Earth, we can see that glow when the moon appears to us as a crescent; sunlight illuminates but a sliver of the moon, while the rest of its outline is dimly visible by virtue of earthlight. Leonardo da Vinci was the first to figure out what that eerie glow appearing on the moon really was.
7) Eclipses are reversed when viewing from the moon Phases aren't the only things that are seen in reverse from the moon. An eclipse of a moon for us is an eclipse of the sun from the moon. In this case, the disk of the Earth appears to block out the sun. If it completely blocks the sun, a narrow ring of light surrounds the dark disk of the Earth; our atmosphere backlighted by the sun. The ring appears to have a ruddy hue, since it's the combined light of all the sunrises and sunsets occurring at that particular moment. That's why during a total lunar eclipse, the moon takes on a ruddy or coppery glow. When a total eclipse of the sun is taking place here on Earth, an observer on the moon can watch over the course of two or three hours as a small, distinct patch of darkness works its way slowly across the surface of the Earth. It's the moon's dark shadow, called the umbra, that falls on the Earth, but unlike in a lunar eclipse, where the moon can be completely engulfed by the Earth's shadow, the moon's shadow is less than a couple of hundred miles wide when it touches the Earth, appearing only as a dark blotch.
8) There are rules for how the moon's craters are named The lunar craters were formed by asteroids and comets that collided with the moon. Roughly 300,000 craters wider than 1 km (0.6 miles) are thought to be on the moon's near side alone. These are named for scholars, scientists, artists and explorers. For example, Copernicus Crater is named for Nicolaus Copernicus, a Polish astronomer who realized in the 1500s that the planets move about the sun. Archimedes Crater is named for the Greek mathematician Archimedes, who made many mathematical discoveries in the third century B.C. The custom of applying personal names for lunar formations began in 1645 with Michael van Langren, an engineer in Brussels who named the moon's principal features after kings and great people on the Earth. On his lunar map he named the largest lunar plain (now known as Oceanus Procellarum) after his patron, Phillip IV of Spain. But just six years later, Giovanni Battista Riccioli of Bologna completed his own great lunar map, which removed the names bestowed by Van Langren and instead derived names chiefly from those of famous astronomers — the basis of the system which continues to this day. In 1939, the British Astronomical Association issued a catalog of officially named lunar formations, "Who's Who on the Moon," listing the names of all formations adopted by the International Astronomical Union. Today the IAU continues to decide the names for craters on our moon, along with names for all astronomical objects. The IAU organizes the naming of each particular celestial feature around a particular theme. The names of craters now tend to fall into two groups. Typically, moon craters have been named for deceased scientists, scholars, explorers, and artists who've become known for their contributions to their respective fields. The craters around the Apollo crater and the Mare Moscoviense are to be named after deceased American astronauts and Russian cosmonauts.
9) The moon encompasses a huge temperature range If you survey the Internet for temperature data on the moon, you're going to run into quite a bit of confusion. There's little consistency even within a given website in which temperature scale is quoted: Celsius, Fahrenheit, even Kelvin. We have opted to use the figures that are quoted by NASA on its Website: The temperature at the lunar equator ranges from an extremely low minus 280 degrees F (minus 173 degrees C) at night to a very high 260 degrees F (127 degrees C) in the daytime. In some deep craters near the moon's poles, the temperature is always near minus 400 degrees F (minus 240 degrees C). During a lunar eclipse, as the moon moves into the Earth's shadow, the surface temperature can plunge about 500 degrees F (300 degrees C) in less than 90 minutes.
10) The moon has its own time zone It is possible to tell time on the moon. In fact, back in 1970, Helbros Watches asked Kenneth L. Franklin, who for many years was the chief astronomer at New York's Hayden Planetarium, to design a watch for moon walkers that measures time in what he called "lunations," the period it takes the moon to rotate and revolve around the Earth; each lunation is exactly 29.530589 Earth days. For the moon, Franklin developed a system he called "lunar mean solar time," or Lunar Time (LT). He envisioned local lunar time zones similar to the standard time zones of Earth, but based on meridians that are 12-degrees wide (analogous to the 15-degree intervals on Earth). "They will be named unambiguously as '36-degree East Zone time,' etc., although 'Copernican time,' 'West Tranquility time' and others may be adopted as convenient." A lunar hour was defined as a "lunour," and decilunours, centilunours and millilunours were also introduced. Interestingly, one moon watch was sent to the president of the United States at the time, Richard M. Nixon, who sent a thank you note to Franklin. The note and another moon watch were kept in a display case at the Hayden Planetarium for several years. Quite a few visitors would openly wonder why Nixon was presented with a wristwatch that could be used only on the moon. Forty years have come and gone without the watch becoming a big seller.
© Joe Rao serves as an instructor and guest lecturer at New York's Hayden Planetarium. He writes about astronomy for The New York Times and other publications, and he is also an on-camera meteorologist for News 12 Westchester, N.Y.