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The Earth our study of objects in the Solar System with the Earth because it is interesting in its own right, and it provides a test of many observing techniques that we wish to use for other objects in the Solar System. Applet requires Java-capable browser The Earth is, at least by human standards, a beautiful planet, as the following images indicate. Apollo 11 shot of Earth (Ref) Galileo shot of the Earth and Moon Galileo image of S. America (Ref) Here is an interactive viewer that displays either a map of the Earth showing the day and night regions at this moment, or a view of the Earth from the Sun, the Moon, the night side of the Earth, above any location on the planet specified by latitude, longitude and altitude, from a satellite in Earth orbit, or above various cities around the globe (Credit: John Walker). Here is the NASA Earth from Space image gallery, where you can find many images of the Earth taken from space. The Earth is the third planet from the Sun in our Solar System. It is the planet we evolved on and the only planet in our Solar System that is known to support life. SIZE The Earth is about 7,926 miles (12,756 km) in diameter. The Earth is the fifth-largest planet in our Solar System (after Jupiter, Saturn, Uranus, and Neptune). Eratosthenes (276-194 BC) was a Greek scholar who was the first person to determine the circumference of the Earth. He compared the midsummer's noon shadow in deep wells in Syene (now Aswan on the Nile in Egypt) and Alexandria. He properly assumed that the Sun's rays are virtually parallel (since the Sun is so far away). Knowing the distance between the two locations, he calculated the circumference of the Earth to be 250,000 stadia. Exactly how long a stadia is is unknown, so his accuracy is uncertain, but he was very close. He also accurately measured the tilt of the Earth's axis and the distance to the sun and moon. THE MOON The Earth has one moon. The diameter of the moon is about one quarter of the diameter of the Earth. The moon may have once been a part of the Earth; it may have been broken off the Earth during a catastrophic collision of a huge body with the Earth billions of years ago. The Earth and the moon. Photo taken by NASA's Galileo mission in 1990. MASS, DENSITY, AND ESCAPE VELOCITY The Earth's mass is about 5.98 x 1024 kg. The Earth has an average density of 5520 kg/m3 (water has a density of 1027 kg/m3). Earth is the densest planet in our Solar System. To escape the Earth's gravitational pull, an object must reach a velocity of 24,840 miles per hour (11,180 m/sec). LENGTH OF A DAY AND YEAR ON EARTH Each day on Earth takes 23.93 hours (that is, it takes the Earth 23.93 hours to rotate around its axis once - this is a sidereal day). Each year on Earth takes 365.26 Earth days (that is, it takes the Earth 365.26 days to orbit the Sun once). The Earth's rotation is slowing down very slightly over time, about one second every 10 Earth rising over the moon. Photo taken by NASA's Apollo 8 mission. years. THE EARTH'S ORBIT The Earth orbits, on average, 93 million miles (149,600,000 km) from the Sun. This distance is defined as one Astronomical Unit (AU). The Earth is closest to the Sun (this is called perihelion) around January 2 each year (91.4 million miles = 147.1 million km); it is farthest away from the Sun (this is called aphelion) around July 2 each year (94.8 million miles = 152.6 million km). Planet-Sun Orbital Diagram Label the aphelion (farthest point in orbit) and perihelion (closest point in orbit) of a planet in orbit. Answers Orbital Eccentricity The Earth' orbital eccentricity is 0.017; it has an orbit that is close to being circular. THE EARTH'S AXIS TILT AND THE SEASONS The Earth's axis is tilted from perpendicular to the plane of the ecliptic by 23.45°. This tilting is what gives us the four seasons of the year: Summer, Spring, Winter and Autumn. Since the axis is tilted, different parts of the globe are oriented towards the Sun at different times of the year. This affects the amount of sunlight each receives. For more information on the seasons, click here. SPEED At the equator, the Earth's surface moves 40,000 kilometers in 24 hours. That is a speed of about 1040 miles/hr (1670 km/hr). This is calculated by dividing the circumference of the Earth at the equator (about 24,900 miles or 40,070 km) by the number of hours in a day (24). As you move toward either pole, this speed decreases to almost zero (since the circumference at the extreme latitudes approaches zero). The Earth revolves around the Sun at a speed of about 30 km/sec. This compares with the Earth's rotational speed of approximately 0.5 km/sec (at middle latitudes - near the equator). For more information on the speed of the Earth, click here. TEMPERATURE ON EARTH The temperature on Earth ranges from between -127°F to 136°F (88°C to 58°C; 185 K to 311 K). The coldest recorded temperature was on the continent of Antarctica (Vostok in July, 1983). The hottest recorded temperature was on the continent of Africa (Libya in September, 1922). The size of the atmosphere in this The greenhouse effect traps heat in illustration is greatly exaggerated in order our atmosphere. The atmosphere lets to show the greenhouse effect. The Earth's some infrared radiation escape into atmosphere is about 300 miles (480 km) space; some is reflected back to the thick, but most of the Earth's atmosphere planet. is within 10 miles (16 km) of the Earth's surface. For more information on the greenhouse effect, click here. ATMOSPHERE The Earth's atmosphere is a thin layer of gases that surrounds the Earth. It is composed of 78% nitrogen, 21% oxygen, 0.9% argon, 0.03% carbon dioxide, and trace amounts of other gases. The atmosphere was formed by planetary degassing, a process in which gases like carbon dioxide, water vapor, sulphur dioxide and nitrogen were released from the interior of the Earth from volcanoes and other processes. Life forms on Earth have modified the composition of the atmosphere since their evolution. For more information on the atmosphere, click here. Earth Activities Earth coloring page The Earth and the Moon coloring page Find It!, a quiz on the Earth. Label the Planet-Sun Orbital Diagram. Answers. Continents Printout: Label the continents, the poles, and the equator. Ocean Map: Label the Oceans Interactive Earth Puzzle Make a globe from papier-mâché A simple-to-make earth pendant Earth day crafts Earth Cloze Printout: A fill-in-the-blanks activity on Earth. Answers How to write a report on a planet - plus a rubric. Equator The equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously The latitude of the equator is, by definition, 0°. The length of Earth's equator is about 40,075.0 km, or 24,901.5 miles. The equator is one of the five main circles of latitude that are based on the relationship between the Earth's axis of rotation and the plane of the Earth's orbit around the sun. It is the only line of latitude which is also a great circle. The imaginary circle obtained when the Earth's equator is projected onto the heavens is called the celestial equator. The Sun, in its seasonal movement through the sky, passes directly over the equator twice each year, on the March and September equinoxes. At the equator, the rays of the sun are perpendicular to the surface of the earth on these dates. Places on the equator experience the quickest rates of sunrise and sunset in the world. Such places also have a constant 12 hours of day and night throughout the year, while north or south of the equator day length increasingly varies with the seasons. The Earth bulges slightly at the equator. It has an average diameter of 12,750 km, but at the equator the diameter is approximately 43 km greater. Locations near the equator are good sites for spaceports (e.g., Guiana Space Centre in Kourou, French Guiana), as they are already moving faster than any other point on the Earth due to the Earth's rotation, and the added velocity reduces the amount of fuel needed to launch spacecraftTemperatures near the equator are high all year round (except at altitude). In many tropical regions people identify two seasons: wet and dry. However, most places close to the equator are wet throughout the year, and seasons can vary depending on a variety of factors including elevation and proximity to an ocean. The surface of the Earth at the equator is mainly ocean. The highest point on the equator is 4,690 m (15,387'), at 00°00′00″S, 77°59′31″W on the south slopes of Volcán Cayambe (summit 5,790 m, 18,996') in Ecuador. This is a short distance above the snow line, and is the only point on the equator where snow lies on the ground. Equatorial countries and territories , these are: São Tomé and Príncipe – passing through Ilhéu das Rolas, an islet in this archipelago Gabon Republic of the Congo Democratic Republic of the Congo Uganda – including some islets in Lake Victoria Kenya Somalia Maldives – misses every island, passing between Gaafu Dhaalu Atoll and Gnaviyani Atoll Indonesia – crosses many islands, most notably Sumatra, Borneo, Sulawesi, and Halmahera Kiribati – misses every island, passing between Aranuka and Nonouti Atolls in the Gilbert Islands Baker Island (unincorporated territory of the United States) – passes through territorial waters (NB the equator also passes through the exclusive economic zones around Howland Island and Jarvis Island, but not through their territorial waters) Ecuador (literal translation of its official name is "Republic of the Equator") – including Isabela Island in the Galápagos Islands Colombia Brazil – including some islands in the mouth of the Amazon River and passing through Macapá, capital of Amapá state Contrary to its name, no part of Equatorial Guinea's territory lies on the equator. However, its island of Annobón is about 100 miles (200 km) south of the equator, and the rest of the country lies to the north Exact length of the equator The equator is modeled exactly in two widely used standards as a circle of radius an integer number of meters. In 1976 the IAU standardized this radius as 6,378,140 m, subsequently refined by the IUGG to 6,378,137 m and adopted in WGS-84, though the yet more recent IAU-2000 has retained the old IAU-1976 value. In either case the length of the equator is by definition exactly 2π times the given standard, which to the nearest millimeter is 40,075,016.686 m in WGS-84 and 40,075,035.535 m in IAU1976 and IAU-2000. (Although millimeter precision can be important up to the scale of a mile, it has negligible physical significance at the scale of a geographic feature such as the equator. From a computational standpoint however millimeter precision or better can be valuable for maintaining consistent results when used in programs for surveying etc. As an overly simple example, if a program were to convert back and forth between the radius and the circumference of the earth sufficiently often while maintaining precision only to a meter each time, errors might accumulate until they became noticeable.) The geographical mile is defined as one arc minute of the equator, and therefore has different values depending on which standard equator is used, namely 1855.3248 m or 1855.3257 m for respectively WGS-84 and IAU-2000, a difference of nearly a millimeter. The earth is standardly modeled as a sphere flattened about .336% along its axis. This results in the equator being about .16% longer than a meridian (as a great circle passing through the two poles). The IUGG standard meridian is to the nearest millimeter 40,007,862.917 m, one arc minute of which is 1852.216 m, explaining the SI standardization of the nautical mile as 1852 m, more than 3 meters short of the geographical mile. Our Sun is a normal main-sequence G2 star, one of more than 100 billion stars in our galaxy. diameter: 1,390,000 km. mass: 1.989e30 kg temperature: 5800 K (surface) 15,600,000 K (core) The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System (Jupiter contains most of the rest). It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many more smaller stars than larger ones; the Sun is in the top 10% by mass. The median size of stars in our galaxy is probably less than half the mass of the Sun. The Sun is personified in many mythologies: the Greeks called it Helios and the Romans called it Sol. The Sun is, at present, about 70% hydrogen and 28% helium by mass everything else ("metals") amounts to less than 2%. This changes slowly over time as the Sun converts hydrogen to helium in its core. The outer layers of the Sun exhibit differential rotation: at the equator the surface rotates once every 25.4 days; near the poles it's as much as 36 days. This odd behavior is due to the fact that the Sun is not a solid body like the Earth. Similar effects are seen in the gas planets. The differential rotation extends considerably down into the interior of the Sun but the core of the Sun rotates as a solid body. Conditions at the Sun's core (approximately the inner 25% of its radius) are extreme. The temperature is 15.6 million Kelvin and the pressure is 250 billion atmospheres. At the center of the core the Sun's density is more than 150 times that of water. The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation. The surface of the Sun, called the photosphere, is at a temperature of about 5800 K. Sunspots are "cool" regions, only 3800 K (they look dark only by comparison with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field. A small region known as the chromosphere lies above the photosphere. The highly rarefied region above the chromosphere, called the corona, extends millions of kilometers into space but is visible only during a total solar eclipse (left). Temperatures in the corona are over 1,000,000 K. It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slighly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year. If you stay home, you're likely to see a partial eclipse several times per decade. But since the path of totality is so small it is very unlikely that it will cross you home. So people often travel half way around the world just to see a total solar eclipse. To stand in the shadow of the Moon is an awesome experience. For a few precious minutes it gets dark in the middle of the day. The stars come out. The animals and birds think it's time to sleep. And you can see the solar corona. It is well worth a major journey. The Sun's magnetic field is very strong (by terrestrial standards) and very complicated. Its magnetosphere (also known as the heliosphere) extends well beyond Pluto. In addition to heat and light, the Sun also emits a low density stream of charged particles (mostly electrons and protons) known as the solar wind which propagates throughout the solar system at about 450 km/sec. The solar wind and the much higher energy particles ejected by solar flares can have dramatic effects on the Earth ranging from power line surges to radio interference to the beautiful aurora borealis. Recent data from the spacecraft Ulysses show that during the minimum of the solar cycle the solar wind emanating from the polar regions flows at nearly double the rate, 750 kilometers per second, than it does at lower latitudes. The composition of the solar wind also appears to differ in the polar regions. During the solar maximum, however, the solar wind moves at an intermediate speed. Further study of the solar wind will be done by the recently launched Wind, ACE and SOHO spacecraft from the dynamically stable vantage point directly between the Earth and the Sun about 1.6 million km from Earth. The solar wind has large effects on the tails of comets and even has measurable effects on the trajectories of spacecraft. Spectacular loops and prominences are often visible on the Sun's limb (left). The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum. It coincides with an abnormally cold period in northern Europe sometimes known as the Little Ice Age. Since the formation of the solar system the Sun's output has increased by about 40%. The Sun is about 4.5 billion years old. Since its birth it has used up about half of the hydrogen in its core. It will continue to radiate "peacefully" for another 5 billion years or so (although its luminosity will approximately double in that time). But eventually it will run out of hydrogen fuel. It will then be forced into radical changes which, though commonplace by stellar standards, will result in the total destruction of the Earth (and probably the creation of a planetary nebula).