Grade 8 Astronomy Notes PDF

Grade 8 Astronomy (Earth & Solar System) Summary Notes Astronomy Knowledge from Weeks 1-2 : Organization of the Solar System / The Inner Planets 1.1) Our solar system is 4.5 billion years old and is composed of one star (the sun) and eight planets. In addition to these main objects, there are also...

Grade 8 Astronomy (Earth & Solar System) Summary Notes Astronomy Knowledge from Weeks 1-2 : Organization of the Solar System / The Inner Planets 1.1) Our solar system is 4.5 billion years old and is composed of one star (the sun) and eight planets. In addition to these main objects, there are also countless moons, asteroids, comets, dwarf planets, and man-made objects (satellites, probes, rovers). Distances are often given in astronomical units, based on the average distance between the sun and the Earth (1 au = 150 million km). Uranus is 20 astronomical units from the sun. This number indicates that its distance is 20 times farther than Earth is away from the sun (or about 3 billion kilometers). 1.2) There are three areas of the solar system where large planets were not able to form. The asteroid belt (2 - 3 AU), lying between the orbits of Mars and Jupiter, is the remnants of a failed planet due to the gravitational tug-of-war between the sun and Jupiter. The largest object in the asteroid belt is Ceres, which is classified as a dwarf planet. Beyond the orbit of Neptune lies the Kuiper belt (30 - 50 AU), a region of comets, dwarf planets (including Pluto), and icy bodies. Thousands of AU beyond the Kuiper belt and at the edge of the solar system lies the Oort cloud, which is a hypothesized spherical region of small icy bodies. 1.3) One of the areas of focus in this part of the unit will be related to time on other worlds. Our concepts of time such as days, months, seasons, and years are very much Earth-specific. Other planets, inside or outside of our solar system, have completely different ways of perceiving the passage of time. We can learn quite a lot about a planet’s days and years by looking at its revolution (time to orbit the sun) and rotation (time to spin once around the axis). Earth, for example, takes about 365 days to orbit the sun (one revolution) and about 24 hours to spin once around its axis. Some planets rotate much faster than Earth (such as Jupiter) and some spin much more slowly than the Earth (such as Mercury and Venus). 1.3) Mercury is the smallest and the closest planet to the sun, about 0.4 AU away. Due to its lack of an atmosphere, it’s extremely hot (430oC) on the day side and it’s very cold (-160oC) on the night side. And because it’s so close, it’s the fastest moving planet and it has the shortest year, about 88 Earth-days. Mercury spins once about its axis in approximately 58 days; this is one definition of a “day”. If we consider the time from sunrise to sunrise (or noon to noon), we get another definition (and length) of a day. For Mercury, its solar day (noon to noon) lasts 176 days, or two years on Mercury. If you were to live on Mercury, you could celebrate two birthdays every day. 1.4) Venus is known as Earth’s sister planet or Earth’s twin, due to their similar size. In most other ways however, Venus is very different from Earth. It has a thick atmosphere of the greenhouse gases CO2 (97%), which makes the planet the hottest in the solar system (460oC), regardless of location on the planet. Venus also spins backwards (also known as retrograde) extremely slowly. A revolution around the sun (a year) takes about 225 days. It takes 243 days to spin once around its axis. The time from noon to noon is, however, only 117 days, and you get almost two full solar days per year. 1.5) I’m going to briefly mention Earth to further clarify how a rotation can be different from a solar day. Earth spins once in 23 hours and 56 minutes. But while spinning once, the Earth has also moved along its orbit. Therefore, your place on the planet is not going to be at the same time of the day when you first began. The planet needs to keep turning for another few minutes to reach 24 hours (noon to noon OR sunrise to sunrise, for example). The time for one rotation is known as a sidereal day, and the time from noon to noon is called a solar day (solar days would be far more important for astronauts or aliens on other celestial bodies). Mercury and Earth have longer solar days than sidereal days. Venus has shorter solar days. We’re going to skip the other qualities of Earth for now, with the goal of applying what we’ve learned about the other planets to Earth. 1.6) The motion of the Moon, the closest moon to the sun, is very interesting and worth mentioning. Ignoring the sun, the moon orbits the Earth in 27 days. Interestingly, it also spins around its axis once in 27 days. And even though the moon spins, we always see the same face. This phenomenon is an example of tidal locking, where the body rotates at the same rate as it revolves. Most moons in the solar system are tidally locked and always show the same face to their host planets. 1.7) Mars is the 4th planet from the sun and the farthest rocky planet from the sun (excluding asteroids and dwarf planets). Due to its distance, it’s usually very cold on Mars; its warmest temperatures are similar to Earth’s (35oC) and only happen near the equator during the summer. The planet is smaller than Venus and Earth, but larger than Mercury, meaning that gravity is quite weak and the planet has a very thin atmosphere (mostly made of CO2). Also due its low gravity, it becomes possible for the planet to have very tall volcanoes (Mons Olympus is the tallest in the solar system…2.5 times taller than Everest). Mars’ rotational motion  is very similar to Earth’s with 25-hour solar days. The year is much longer on Mars, however, due to its distance - 687 days. Mars has two small moons : Phobos & Deimos. 1.8) Watch this video to learn more about Ceres & the Asteroid Belt. Astronomy Knowledge from Weeks 3-4 : The Outer Planets & Kuiper Belt 2.1) The four outer planets include the planets Jupiter, Saturn, Uranus, and Neptune, and they’re found between the Asteroid Belt and the Kuiper Belt. They are sometimes referred to as the Gas Giants. That term is also sometimes used to refer to just Jupiter and Saturn, as they’re primarily made of hydrogen and helium. The final two planets, Uranus and Neptune, are often called the Ice Giants since they include several frozen gases (water, ammonia, methane, carbon dioxide). 2.2) Jupiter is the fifth planet from the sun and by far the largest. The planet rotates in just 10 hours, giving it the shortest day of any planet. It is orbited by approximately 100 moons, including four large moons discovered by Galileo and Simon Marius. These Galilean moons are, from closest to farthest, Io, Europa, Ganymede, and Callisto. Io’s proximity to Jupiter means that it experiences huge tidal forces. In fact Jupiter’s gravity is able to pull the crust up 100m once per Io day. The tidal pulling of the crust leads to numerous volcanoes, which shoot lava hundreds of kilometers into space. Europa, Ganymede, and Callisto likely have large subsurface oceans of liquid water. Ganymede is the largest moon in the solar system and is even larger than the planet Mercury. All of these moons have very thin atmospheres and are tidally locked. 2.2) Saturn, famous for its rings and low density, is next. It rotates almost as fast as Jupiter (11 hours). It has a tilt of 27°, meaning it and its moons would have seasons slightly more extreme than Earth does (though it’s always cold that far out). Thanks to the tilt, we can see Saturn’s rings for much of the year. If the tilt was 0°, we’d barely see them at all, right ? It currently has the most discovered moons of any planet (146 of them ! …all tidally locked, of course). Its largest moon, Titan, is the second largest moon in the solar system and is also larger than Mercury. It has the thickest atmosphere of any moon, mostly composed of nitrogen (like Earth !) and methane (not like Earth…). Due to its atmosphere and the molecular composition, Titan’s surface and weather is surprisingly Earth-like. It has lakes, rivers, oceans, clouds, rain, seasons, wind, erosion, and volcanoes. The temperature, however, is -180°C. 2.3) Uranus orbits the sun every 84 years and spins once on its axis every 16 - 17 hours. Though the planet spins faster than Earth, its 98 degree tilt means that a day means very different things for different parts of the planet. Because of the tilt, Uranus has extreme seasons. The poles would experience up to half the revolution (=42 years) in perpetual light, and the other half of the orbit in total darkness. The temperate regions would have a shorter period with permanent sun, a shorter period with permanent darkness, and many days with variable day & night cycles. The equator would experience the sun on the horizon during summer and winter, and proper day/night cycles in spring and fall. Uranus’s atmosphere includes water, ammonia, and methane among other things. Its blue color is attributed to the methane, and its fart smell is caused by its hydrogen sulfide clouds. Uranus was the first planet to be discovered (by the Englishman William Herschel and was originally named after King George III). Since it was discovered by the English, they did not name the moons after Greek or Roman gods, they named them after characters in Shakespeare’s plays mostly. These include Titania, Umbriel, Miranda, Puck, Oberon, etc (and they’re all tidally locked). Titania is the largest of them and is the 8th largest in the solar system. 2.4) Neptune spins at a similar rate as Uranus, but takes 165 years to orbit the sun once. Neptune is similar in composition and color to Uranus, and has the strongest winds in the solar system (> 2000 km/hr). Neptune was discovered due to unexpected motion by Uranus, later revealed to be caused by Neptune. Neptune has 16 moons, the largest of which was probably a captured dwarf planet, now known as Triton. Triton was likely in the Kuiper Belt when it was captured. Though Triton, like the other moons, is tidally locked, it orbits counterclockwise, and is the only known large moon to do so (there are several small moons that do so). Triton is the 7th largest moon.  2.5) The Kuiper Belt extends beyond the orbit of Neptune (30 AU) and includes comets (Halley’s is the most easily visible), dwarf planets, and lots of small rocky and icy bodies.The dwarf planets include Pluto, Makemake (originally called “Easterbunny”), Haumea (originally called “Santa”), Gonggong, Sedna, Eris, Quaoar, and Orcus. Most of the dwarf planets have moons. Pluto has five moons, and its biggest is called Charon. Charon and Pluto are mutually tidally locked to each other, the two faces forever staring at each other. Astronomy Knowledge from Weeks 5 & 7 : The Earth (Time & Calendars); Seasons 5.1) Though we take days, weeks, months, and years for granted as a natural way to tell time on Earth, they are all derived from astronomy related to Earth motion and the movement of the moon. As discussed previously, our day of 24 hours is based on the concept of a solar day, and our year is based on our orbit around the sun. The original calendars on Earth were created based on the phases of the moon, one month per lunar cycle (about 29.5 days). Because there aren’t an even number of lunar months in a year, they aren’t very good at helping predict seasons. Gradually, humans discovered the true number of days in a year (about 2000 years ago) and began using a solar calendar. 5.2) After many changes to the calendar (including a year with 455 days called the Year of Confusion)) the Romans eventually created a 365-day calendar under Julius Caesar (with help from Cleopatra of Egypt). The following year, the month Quintilis was renamed after Caesar (July), and a few decades later Sextilis was renamed August after the first emperor of Rome, Augustus. Most importantly Caesar introduced a leap day every four years to account for the drift in the seasons. Earth takes approximately 365.25 days to orbit the sun afterall. The table to the right shows how the Roman calendar changed over time. 5.3) Despite these clear improvements, the Julian Calendar was not good enough. Many religions’ festivals required the people to celebrate them at the same time every year. And the seasons began to drift. In 1582 Pope Gregory (and his advisors) made slight changes to remove 10 days to realign the seasons and they also reduced the number of leap days. At first only Catholic countries made the changes. A couple hundred years later, the British Isles and its colonies made the change, but they needed to delete 11 days. The Russians only changed in 1918, and needed to remove 13 days from the calendar. Today, most of the world has adopted this Gregorian Calendar. 7.1) A planet with no tilt has just one special line of latitude: the equator (#1), the midline separating north and south. Every day on that planet (no matter how slow or fast it rotates) the star’s most direct rays will be above the equator. A planet with a tilt like ours means that the direct rays vary throughout the year. A planet with a tilt has five special lines of latitude. On Earth, the sun’s rays only hit the equator directly on two days of the year: 21 March and 21 September. These days are known as equinoxes and everywhere on the planet has 12 hours of daytime and 12 hours of nighttime. These days also mark the beginning of Spring or Fall, depending on your hemisphere. 7.2) The second and third lines of latitude are the limits to the sun’s direct rays, north and south of the equator. Since Earth’s tilt is 23.5 degrees, the limit of the sun’s direct rays is uncoincidentally also 23.5 degrees, north and south of the equator. In the north, we call that line the Tropic of Cancer (#2). The sun’s direct rays reach this line of Tropic only on 21 June (plus or minus a day). This day is a solstice, when the northern hemisphere has its longest day and the southern hemisphere has its shortest day. In the south, the line is called the Tropic of Capricorn (#3). The sun’s rays reach Capricorn on 22 December (plus or minus a day). This day is also a solstice, but now it’s the south’s turn to have the longest day. The region between these two lines of latitude is called the Tropics. The word comes from the Greek language and means “to turn” referring to the place where the sun’s rays turn around. 7.3) Nowhere above 23.5 N or below 23.5 S gets direct sunshine, only indirect sunshine. Most of this area is known as the Subtropical and Temperate Zones (also known as the middle latitudes) and includes (mostly) the US, Europe, northern Africa, Japan and the Koreas, New Zealand, Iran, and Turkey, among other places. Subtropical and Temperate zones don’t receive direct sunlight, but they do get some sunlight every single day of the year. North and South of the temperate zones lie the Polar regions. 7.4) Due to our tilt, some areas may receive 24 hours of sunshine in a day or vice versa 24 hours of darkness in a day. These are the polar regions, and they begin at the Arctic Circle (#4) (and go up to the North Pole) and at the Antarctic Circle (#5) (and go down to the South Pole). These lines are found at 66.5 degrees North and South (note that 90 - 23.5 = 66.5). Every location in these zones will have at least a day of sunshine and at least a day of darkness. The closer you get to the poles, the more dramatic their longest and shortest days become. Longyearbyen in Svalbard is the farthest north town with a population of over 1,000 people. It has over 100 straight days of sunshine in summer and over 100 days of nonstop darkness in winter. The Poles get six months of daylight, followed by 6 months of darkness. They flip seasons at the equinoxes on 21 March and 21 September. 7.5) Eclipses happen when the sun, the Earth, and the moon all line up. When the moon is in the middle, it can block the light from the sun, creating a solar eclipse. The area covered by any solar eclipse is very small, and the odds are low that the eclipse will cross your path. Zimbabwe’s next total solar eclipse, for example, will be in 2095. When the Earth is in the middle, the Earth should block the light to the moon. However, some light gets filtered and bent by Earth’s atmosphere and shines dimly over the moon, turning it a reddish grey. This is called a lunar eclipse, and it happens twice a year on average, during 0, 1, 2, or 3 of the year’s full moons. When a lunar eclipse happens, about half of the globe should be able to see it (ignoring cloudy / rainy conditions). The next total lunar eclipse visible from Zimbabwe will be in September, 2025. 8.1) While compasses were invented more than 2000 years ago (by the Chinese), Earth’s magnetic nature was only discovered about 400 years ago, recounted by William Gilbert in his 1600 book called De Magnete. Since the Earth is a magnet, it has poles, like a bar magnet. The magnetic poles are near to the geographic poles, but wander from year to year. Currently the magnetic north pole is heading across the Arctic Ocean toward Siberia. Compasses do not detect the magnetic poles. Instead, they detect the magnetic field lines (see image to the left) that pass through us as they move from pole to pole. This magnetic field is not only detected by compasses, it can also be used by migratory birds, sea turtles, and salmon for navigation. 8.2). Earth’s magnetic field extends far out into space. Generally known as the magnetosphere, it offers protection for the Earth from the sun’s solar wind. Solar wind is composed of charged particles (electrons, protons, and alpha particles (2P2N)) and has the capacity to strip away the atmosphere of planets without a magnetosphere. Other than Earth, the four giant planets have magnetospheres, as do Mercury and Ganymede. Venus, Mars, and all dwarf planets and moons (except Ganymede) lack magnetospheres. 8.3) Though the solar wind is deflected far out in space, there are two regions where the wind can get quite close to Earth : near the poles. The magnetosphere is much thinner in these areas. The solar wind, at times, can interact with nitrogen and oxygen in the atmosphere creating beautiful green, purple, and blue plasmas, called aurorae (Aurora borealis in the north and Aurora australis in the south). 8.4) When planets form, they are very hot and consist of liquid rock and metal. The heavier metals like iron sink toward the planet’s core. Lighter rocks rise to the surface. Thus Earth’s core is primarily iron and nickel. Though the inner core is solid, the outer core remains liquid. Liquid iron and nickel swirl around in convection currents in thick blobs. That swirling action is what creates the magnetic field. If the outer core were to cool down, that swirling motion could stop as would the magnetosphere. There is strong evidence that Mars’s magnetosphere stopped 3.8 billion years ago, which led to the thinning of its atmosphere and loss of most of its water. 8.5) Because the orientation of the magnetic poles depend on the movement of the blobs of iron and nickel in the core, it sometimes happens that the poles reverse. There is evidence that this occurs, on average, every 700,000 years. This is known as a geomagnetic reversal. Currently, the pole that we refer to as the magnetic north pole is actually the South Pole of the Earth magnet. It just so happens to be in the north. And in the near future, the poles will likely flip again. 8.6) The evidence for geomagnetic reversals is found at the bottom of the Atlantic ocean, along an underwater mountain range called the Mid-Atlantic Ridge. This mountain range forms the boundary of several tectonic plates. Magma from the mantle pushes these plates east (toward Europe and Africa) and west (toward the Americas). Thus the Atlantic ocean is spreading at a rate of approximately 2.5cm per year. The volcanic rock (from the magma / lava) is known to be magnetized in the direction of Earth’s magnetic field, at the time it solidifies. So the rock closest to the ridge is aligned to Earth’s current magnetic orientation. Several hundred meters away, east and west, there are bands of rock in which the magnetic orientation is opposite of Earth’s current situation. Another few hundred meters away, it switches again, showing that the reversal is periodic.