OBJECTIVES - Mrs. Yu's Website



eARTH sCIENCE uNIT 7 lESSON 1 THE EARTH'S PLACE IN THE SOLAR SYSTEMIt seems as if the earth supplies all the things we need. However, the idea that everything comes from the earth is misleading. Our ability to survive actually depends upon forces beyond the earth.Beyond the earth? Sounds crazy, but it's true. To fully understand our lives, we must also understand our solar system, our galaxy, and the greater universe. Let's start with our solar system.OBJECTIVESDistinguish planets from other types of solar system objects.Explain the relative distances of the planets from the pare the size of the earth to the other planets.Kepler's laws of planetary motion can be used to make calculations.VOCABULARYastronomical unita unit of measurement based on the average distance of the earth from the sun; one unit equals 149.6 million kilometersdwarf planetan object that orbits the sun and is large enough for its gravity to make it spherical but that is too small to have cleared its orbit of all other large objectsgas giantsthe four outer planets of the solar system; named for their high concentrations of hydrogen and heliumplanetan object that orbits the Sun, is spherical, and has cleared its orbit of other large objectsterrestrial planetsthe four inner planets; named for their rocky crustsLesson OutlineAstronomers define planets as objects that 1) orbit a star, 2) are large enough so their gravity makes them round, and 3) have cleared other objects from their orbital paths.There are eight planets in our solar system. Pluto is now classified a dwarf planet.The solar system is very large—approximately 60 trillion kilometers across. The planets are all relatively near the sun. The furthest planet, Neptune, is about 4.5 billion kilometers from the Sun or less than 1/10,000th of the distance from the Sun to the edge of the solar system.The earth is almost 150 million kilometers from the Sun, the third closest of the eight planets.Astronomers also measure distances in the solar system using astronomical units or A.U.s. One A.U. is equal to the distance between Earth and the Sun or almost 150 million kilometers. Therefore, Earth is 1.0 A.U. from the Sun. Neptune, the furthest planet, is 30.1 A.U. from the Sun. This means that Neptune is 30.1 times further from the Sun than Earth is.The planets vary greatly in size. Jupiter is the largest planet and Mercury is smallest. The earth is the fifth largest planet, in both diameter and mass.Although only fifth largest, Earth is the densest planet.Definition of a Planet Before 2006, there were nine planets in our solar system, Mercury, Venus, Earth, Mars, Jupiter, Saturn, Neptune, Uranus, and Pluto, but now, there are only eight. In 2005, astronomers discovered an object beyond Pluto, which is actually larger than Pluto. If Pluto is a planet, then this larger object, now called Eris, should also qualify as a planet.Many astronomers had begun to question whether Pluto was large enough to be a planet. With the discovery of Eris, the debate over the definition of a planet became more important. Members of the International Astronomical Union decided to rethink their definition of a planet. They decided planets must have similar specific characteristics.There are three characteristics that an object must exhibit in order to be considered as a planet.First the object must orbit a star,?and secondly, it must be large enough that its gravity makes them spherical.?Finally, a planet must clear their orbits of other objects.Pluto, which had been the smallest planet, meets the first two criteria in that it orbits the Sun and is spherical.However, Pluto has not cleared its orbit of all other objects. Therefore, astronomers have reclassified Pluto as a dwarf planet. Eris is also a dwarf planet, and astronomers are considering additional objects for dwarf planet status.Distance in the Solar System[Model of Earth’s solar system] Our solar system is the region of space affected by the gravity of the Sun. It contains the Sun, planets, dwarf planets, asteroids, comets, rocks, dust, and gases.All regular matter in the solar system revolves around the Sun at various distances.How big is the solar system? [Model of Oort Cloud] The outermost region of the solar system is called the Oort Cloud. This "cloud" contains billions of icy objects orbiting the Sun. Astronomers estimate that the Oort Cloud extends 30 trillion kilometers from the Sun.[Neptune] The region of the solar system containing the planets is relatively small. The furthest planet, Neptune, is almost 4.5 billion kilometers from the Sun. That is less than one ten-thousandth of the distance to the outer edge of the Oort Cloud. The Oort Cloud defines the boundary of our solar system.[Model of Earth and Sun] Earth, which is the third planet from the Sun, is about 150 million kilometers from the Sun. This is still a very great distance compared to distances we measure on the earth. To avoid the use of huge numbers to describe distances in the solar system, astronomers developed a new measure of distance called astronomical unit or A.U.An A.U. is equal to the distance from Earth to the Sun or 150 million kilometers. [1 A.U. = 150 million km] Mercury and Venus are closer to the Sun than the Earth. Therefore, in A.U. measurements, they are less than 1.0 A.U. from the Sun.So, you may ask, "How can scientists even measure such massive distances?" A distance measured in astronomical units compares the distance of objects in the solar system to the distance between the Sun and Earth. In other words, a distance of 2 A.U. is twice the distance of the Earth from the Sun.[Chart of planet distance from the Sun:First column is labeled Planet, second column is labeled Distance from the Sun (in A.U.’s)?Mercury: 0.4Venus: 0.7Earth: 1.0Mars: 1.5Jupiter: 5.2Saturn: 9.5Uranus: 19.2Neptune: 30.1]Looking at this table, you can see that Jupiter is 5.2 times the distance from the earth to the Sun, while Mercury is less than half the distance from the Sun to the earth.We can also measure the distance from the Sun to the edge of the solar system in A.U.s. We just need to divide the distance from the Sun to the edge (30 trillion kilometers) by the distance of one A.U. (150 million kilometers). The equation looks like this:30,000,000,000,000 km / 150,000,000 km/A.U. = 200,000 A.U.This means that the edge of the solar system is 200,000 times the distance from the Sun to the earth. Or, in other words, the earth is only 1/200,000th of the way from the Sun to the edge of the solar system! Our solar system may be small compared to the vast distances of the universe. However, the solar system is huge compared to distances on Earth.Kepler’s LawsUntil the 1600’s it was widely accepted that earth and the other planets move around the sun in a circular path. After much scientific investigation, Johannes Kepler discovered that the planets do not move about the sun in a circle but instead they follow the path of an ellipse, an elongated circle. An ellipse is defined by points whose sum of the distances to the two foci are a constant. The major axis goes thru the two foci; the minor axis bisects and is perpendicular to the major axis.The perihelion is the point where the planet is closest to the sun, and the aphelion is the point where the planet is furthest away from the sun. The eccentricity, a measure of elongation, is calculated by dividing the distance between the foci by the length of the major axis. A circle has an eccentricity of zero because the distance between the two foci is zero, they are the center of the circle.Kepler’s 1st Law of planetary motion states that planets move around the sun in an ellipse with the sun being one of the foci.Kepler’s 2nd Law says that a planets orbit sweeps over equal areas of space in equal amounts of time. As a planet moves further away from the sun towards the aphelion, the gravitational force between the sun and the planet decreases and the planet slows down. As the planet moves closer to the perihelion, the gravitational force between the planet and the sun increases and the planets speed increases.Kepler’s 3rd Law states that the square of the orbital period, P, is proportional to the cube of the semi-major axis, a, of its orbit. The semi-major axis is measured in astronomical units (AU). One AU is the distance from the sun to the earth and equals ninety-three million miles.P2?= a3Sizes of the PlanetsOur solar system is composed of objects that vary tremendously in size, from the enormous Sun down to microscopic grains of minerals. After the Sun, the planets are the next largest objects in the solar system. The size of the planets can be described both by their diameter, which is how far they are across, and by their mass, which is the amount of material they contain.PlanetDiameter (km)Diameter compared to Earth'sMass (kg)Mass compared to Earth'sDensity (g/cm3)Density compared to Earth'sMercury4,8790.383.3 x 10230.065.430.98Venus12,1040.954.9 x 10240.825.240.95Earth12,7561.006.0 x 10241.005.521.00Mars6,7940.536.4 x 10230.113.940.71Jupiter142,98411.211.9 x 1027316.71.330.24Saturn120,5269.455.7 x 102695.00.700.13Uranus51,1184.018.7 x 102514.51.300.24Neptune49,5283.881.0 x 102616.71.760.32As you can see from the table above, the planets differ greatly in size. The smallest planet, Mercury, is less than 4/100th the diameter of Jupiter, the largest planet. The Earth is the fifth largest planet, both in diameter and in mass. The Earth is the largest of the four inner planets but smaller than the four outer planets. Jupiter, the largest planet, has a diameter more than 11 times greater and a mass more than 316 times greater than Earth.Although Earth is only fifth largest, it is the densest of the planets. Density refers to how much mass is contained in a unit of space. So even though the earth occupies less space than the four outer planets, it has more mass for each unit of space it occupies.Why are the largest planets less dense than Earth? The four outer planets are less dense because they are primarily composed of hydrogen and helium, the lightest of all the elements. That is why astronomers call the outer planets?gas giants. By contrast, the earth is mostly composed of iron, oxygen, silicon, and magnesium, which are heavier elements. Most astronomers think that, initially, all the planets had similar compositions but that particles from the Sun blew away most of the lighter gases from the four inner planets. We call the four inner planets?terrestrial planets?because of their rocky crusts. ................
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