Chapter-1 ast[1] (1)

© 2011 Education, Inc. Chapter 1 Astronomy Today Chapter 1 Charting the Heavens Units of Chapter 1 1.1 Our Place in Space 1.2 Scientific Theory and the Scientific Method 1.3 The “Obvious” View 1.4 Earth’s Orbital Motion 1.5 Motion of the Moon 1.1 Our Place in Space Earth is average—we don’t occupy any special place in the universe Universe: totality of all space, time, matter, and energy In what sense is our unique Earth just "ordinary" from a cosmic perspective? Answer: We live on a what seems to be an ordinary rocky planet called Earth one of the eight known planets orbiting the average star called the Sun. We are not in the center of our solar system, which is not in the center of the Milky Way, which is just one of billions of known galaxies in the universe. Our planet is made of common minerals that are abundant throughout the known universe, and are probably part of almost any planet forming around any other star now. We are connected to most distant realms of space and time not only by our imagination but also through a common cosmic heritage. Most of the elements that make up are body hydrogen,oxygen and carbon etc. were created Billions of year ago. 1.1 Our Place in Space Astronomy: study of the universe Scales are very large: measure in light-years, the distance light travels in a year—about 10 trillion miles Light year is the distance travelled by light in one year. Roughly 10^13km or 63,000 AU. Common misconception that light year is the unit of time. It is a unit of distance. Light year is a unit introduced by Astronomers' to help them describe immense distances. Another misconception is that stars looks like disks when seen through a telescope.The stars are roughly the same size as the sun,they are so far away that astronmers cannot see them as anything but points of light. Astronomers uses the metric system because it simplify calculations and they use scientific notation for very large or very small numbers Galaxy: is a great cloud of stars, gas and dust held together by the combined gravity of all its matter. Their shapes can be elliptical, spiral or irregular. here are probably more than 170 billion (1.7 × 1011) galaxies in the observable universe Galaxies ranges from 1500 to over 300,000 light year in diameter and some contain billion stars. In the night sky you see our galaxy as a great cloudy wheel of stars ringing the sky. This band of Stars is known as the Milky way. And our galaxy is called milky way galaxy. How does anyone know what our galaxy looks like if no one leave it and look back? Astronomer use evidence to guide their explanations as they imagine what the milky way looks like. Artists can then use those scientific description to create a painting. Many of the images in your book are artist renderings of objects. These images are not just guesses they are scientifically based illustration guided by the best information astronomer can gather. Milky way galaxy reproduce shows that our galaxy like many others has graceful spiral arms winding outward through its disk. Ours is fairly large galaxy. Only a century ago astronomers thought that it was the entire universe. Now they know that our galaxy is not unique it is only one of the many billions of galaxies scattered through out the universe. Conclusion; Our Galaxy contain our solar system plus billions of stars and whatever planets orbit around them. In other words billion of planetary systems. Whereas Universe include everything: all of the galaxies stars and planets.including our own galaxy and our solar system. New distance scale: 1 light year (ly) = Distance traveled by light in 1 year = 63,000 AU = 1013 km = 10,000,000,000,000 km (= 1 + 13 zeros) = 10 trillion km Nearest star to the Sun: Proxima Centauri, at a distance of 4.2 light years Diameter of the Milky Way: ~ 75,000 ly Distance to the nearest large galaxies: several million light years 1.2 Scientific Theory and the Scientific Method Scientific theories Important characteristics: Must be testable Must be continually tested Should be simple Should be elegant Scientific theories can be proven wrong, but they can never be proven right with 100 percent certainty. What is the scientific method, and how does science differ from religion? The scientific method is a process for discovering the best possible explanation as to why something occurs. The process begins when observations lead to the formulation of a hypothesis, a preliminary explanation that makes testable predictions. Investigators gather information and called data to test the predictions through observation and experimentation. The information is analyzed to find patterns in the data. If the patterns agree with the predictions, the hypothesis is considered a viable theory. If they do not agree, the hypothesis must be discarded or modified. Science therefore relies on measurable quantities and testable predictions to search for answers. By contrast, religion relies on decree by authority or personal revelation through faith, neither of which are testable. If a claim is not testable, it cannot be considered scientific. Scientific Theory and the Scientific Method Observation leads to theory explaining it. Theory leads to predictions consistent with previous observations. Predictions of new phenomena are observed. If the observations agree with the prediction, more predictions can be made. If not, a new theory should be made. 1.3 The “Obvious” View Simplest observation: Look at the night sky About 3000 stars visible at any one time; distributed randomly but human brain tends to find patterns 1.3 The “Obvious” View Group stars into constellations: Figures having meaning to those doing the grouping Useful: Polaris, which is almost due north Useless: Astrology, which makes predictions about individuals based on the star patterns at their birth What is a constellation? Why are constellations useful for mapping the sky? A constellation is a pattern of stars in the sky. Constellations are thus useful in naming and locating celestial objects. Constellation simply celebrate the most important mythical figures in each culture. The oldest constellations named by western culture originated in Assyria over 3000 years ago, and others were added by Babylonian and Greek astronomers during the classical age. Of these ancient constellation 48 are still in use. Different culture grouped stars and named constellatioin differently. The constellation you know as Orion was known as Al Jabar the giant to the ancient Syrians, as the white Tiger to the Chinese and Prajapati in the form of a stag in India. Many ancient cultures including the Greeks, northern Asians, and Native Americans associated the stars of the Big Dipper with a bear. To the ancients a constellation was a loose grouping of stars. Many of the fainter stars were not included in any constellation,and the stars of the southern sky not visible to the ancient astronomers of northern Lattitude were not grouped into Constellation. To correct the gaps and ambiguity astronomers added 40 modern constellations and in 1928 the International Astronomical Union Established 88 official constellation with clear defined boundaries. Consequently a constellation now represent not a group of stars but an area of the sky and any star within the region belongs to one and only one constellation. In addition to the 88 official constellation the sky contains a numbers of less formally defined grouping called Asterisms. The Big Dipper for example is a well known asterism that is part of the constellation Ursa Major. Another Asterism is the Great Square of Pegasus which includes three stars from Pegasus plus Alpheratz from Andromeda. The brightest star in a constellation is usually designated alpha and the second brightest Beta and so on… Circumpolar constellation: are those constellations that never rise or set. Also remember that although the constellation names came from Greek translated into Latin. Most of the stars names came from Arabic, though much altered by the passing centuries. The name of Betelgeuse , the bright orange star in Orion, for example came from the Arabic yad al jawaza meaning shoulder of Jawza. Naming individual stars is not very helpful because you can see 1000’s of them. How many names could you remember? 1.3 The “Obvious” View The celestial sphere: Stars seem to be on the inner surface of a sphere surrounding the Earth They aren’t, but can use two-dimensional spherical coordinates (similar to latitude and longitude) to locate sky objects The sky and its Motion; Ancient astronomers believed the sky was a great sphere surrounding Earth with the star stuck on the inside like thumbtacks in a ceiling. Modern astronomers know that the stars are scattered through space at different distances. There are three important principle's: 1) The sky appears to rotate westward around Earth each day, but that is consequence of the eastward rotation of Earth. That rotation produces day and night. 2) Astronomers measure angular distance across the sky as angles and express them as degrees, arc minutes and arc seconds. What you can see of the sky depends on where you are on Earth. If you lived in Australia, you would see many constellations and asterisms invisible from North America, but you would never see the Big Dipper. How many circumpolar constellation you can see depend on where you are. Remember your favorite star Alpha centauri is in the southern sky and isn’t visible from most of the United states. You can glimpse it above the southern horizon if you were in Miami, Florida. But you could easily see from Australia. The celestial sphere is an imaginary sphere of gigantic radius with the earth located at its center. The poles of the celestial sphere are aligned with the poles of the Earth. The celestial equator lies along the celestial sphere in the same plane that includes the Earth's equator. An astronomer can only see half the sky at a time, that is, only half the sky is above the horizon at any time. But the sky keeps moving as the earth rotates. The celestial sphere is a large sphere surrounding the earth and with it we can keep references to where celestial bodies lie in the sky. North Celestial Pole (NCP) and the South Celestial Pole (SCP) - these are just the north and south poles extended into space. Celestial Equator - The earth's equator, but at a much greater radius. If the earth's equator was a rubber band, then the celestial equator is the same rubber band just stretched away from the earth. Horizon - The horizon changes depending on your position on earth. Zenith- The point on the celestial sphere directly overhead. Meridian- The imaginary circle passing through the North and South points on our horizon and through the zenith is termed the celestial meridian. . The red "Ecliptic" is the sun's path. The sun is at the vernal equinox around March 21 and travels eastward. COMMON MISCONCEPTION: Lots of people believe that star are not there in the sky during day time. The star are actually there day and night. They are just invisible during the day because the sky is lit up by sunlight. Also many people insist that the favorite Polaris is the brightest star in the sky. It is actually the 51st visually brightest star. So Polaris is important because of its position not because of its brightness. The phenomenon we call "precession" was discovered by Greek astronomer Eratosthenes when he compared his own circa 200 BC records with older charts. What he saw was that the equinoxes in his day (where the sun's path crosses the celestial equator) were in a different position among the stars than the 150-year-old comparison charts showed. This is due to a gyroscopic wobble of earth's spin axis that takes 26000 years to complete. In this wobble motion, the tilt of the earth stays roughly constant at 23.4 degrees but the orientation is always changing. One consequence of precession is that the north star Polaris is drifting. It is only "north star" by coincidence today. Vega will be our north star for a time in the distant future. Another consequence is that precession introduces a difference between a sidereal (real) year and a tropical (by the sun) year because during the course of one year the position of the equinox changes slightly. The physical cause of the precession is a torque (twisting) of the earth, caused mostly by the sun's and the moon's gravity pulling on the equatorial bulges of the earth. If earth were NOT spinning, the sun and moon would pull the earth so that the bulges were flat in the sun-earth orbital plane. The earth rotation on its axis causes the cycle of day and night. Rotation is the turning of a body on its axis but revolution means motion of a body around a point outside the body. Earth rotates once a day on its axis and revolve once a year around the sun. The period from one noon to the next noon is our solar day. The stars position in the sky do not repeat themselves exactly from one night to the next. Each night the whole celestial sphere appears to be shifted a little relative to the horizon compared to the night before. The day measured by the stars called a sidereal day. Solar day and sidereal day are not same the reason for the difference between solar day and a sidereal day is that earth moves in two ways simultaneously. It rotates on the central axis while at the same time revolve around the sun. Each time earth rotates once on its axis it also moves small distance along its orbit about the sun. Earth therefore has to rotate through slightly more than 360 degree for the sun to return to the same apparent location in the sky. Thus the interval of time between noon one day and the noon next day is slightly greater than one sidereal day. Solar day is about 3.9 minute longer than the sidereal day. The motion of the Sun in the sky is an illusion caused by the Earth's spin. The Sun appears to rise in the east and move westward throughout the day because the Earth beneath us is spinning eastward, or counterclockwise, as viewed from the north. Therefore, all celestial objects have this apparent motion, including the Moon and stars. Seasons: There are seasons on Earth because the rotation axis of Earth is tilted with respect to the axis of the Sun. This tilt means that a given location on Earth receives different angles and intensities of sunlight over the course of Earth's orbit. When it is summer at your location, the Sun is almost directly overhead at noon. Therefore, the sunlight strikes the ground at almost a 90-degree angle and is very concentrated. In winter, however, the Sun never gets very high, even at noon, and the sunlight comes in at an extreme angle. This indirect sunlight is very diffuse and not efficient at heating. There is a popular misconception that the seasons on the Earth are caused by varying distances of the Earth from the Sun on its elliptical orbit. This is not correct. The primary cause of the seasons is the 23.5 degree of the Earth's rotation axis with respect to the plane of the ecliptic, This means that as the Earth goes around its orbit the Northern hemisphere is at various times oriented more toward and more away from the Sun, and likewise for the Southern hemisphere, as illustrated in the following figure. The Seasons in the Northern Hemisphere Thus, we experience Summer in the Northern Hemisphere when the Earth is on that part of its orbit where the N. Hemisphere is oriented more toward the Sun and therefore the Sun rises higher in the sky and is above the horizon longer, and the rays of the Sun strike the ground more directly. Likewise, in the N. Hemisphere Winter the hemisphere is oriented away from the Sun, the Sun only rises low in the sky, is above the horizon for a shorter period, and the rays of the Sun strike the ground more obliquely. Southern Hemisphere Seasons The seasons in the Southern Hemisphere are determined from the same reasoning, except that they are out of phase with the N. Hemisphere seasons because when the N. Hemisphere is oriented toward the Sun the S. Hemisphere is oriented away, and vice versa: Equinox A time at which the days and nights are the same length around the world. Occurs around March 21 and September 21 (but not necessarily on those dates). Occurs when the Sun is directly over the equator. The vernal equinox marks the beginning of the spring, when the Sun passes northward through the equatorial plane (Mar 21) The autumnal equinox marks the beginning of fall, when the Sun passes southward through the equatorial plane (Sept 21) Solstice A time at which either day or night is the longest it will be during the year. Occurs around June 21 and December 20 (but not necessarily on those dates). Called simply summer or winter solstice and mark the beginning of those seasons. Occurs when the sun is directly above 23.5 N latitude (Summer Solstice) or 23.5 S latitude (Winter Solstice). Will allow one pole to have 24 hours of daylight, while the other pole has a 24 hour night. Equinox Drawn for northern latitudes, these are the paths the sun takes across the sky on the equinoxes and solstices. Can you see that the summer path is longer (and therefore that the summer sun stays in the sky longer)? The diagram More Precisely 1-1: Angular Measure Full circle contains 360° (degrees) Each degree contains 60? (arc-minutes) Each arc-minute contains 60?? (arc-seconds) Angular size of an object depends on its actual size and distance from viewer 1.4 Earth’s Orbital Motion Daily cycle, noon to noon, is diurnal motion —solar day Stars aren’t in quite the same place 24 hours later, though, due to Earth’s rotation around Sun; when they are once again in the same place, one sidereal day has passed 1.4 Earth’s Orbital Motion Seasonal changes to night sky are due to Earth’s motion around Sun 1.4 Earth’s Orbital Motion Twelve constellations Sun moves through during the year are called the zodiac; path is ecliptic 1.4 Earth’s Orbital Motion Ecliptic is plane of Earth’s path around Sun; at 23.5° to celestial equator Northernmost point of path (above celestial equator) is summer solstice; southernmost is winter solstice; points where path crosses celestial equator are vernal and autumnal equinoxes Combination of day length and sunlight angle gives seasons Time from one vernal equinox to next is tropical year 1.4 Earth’s Orbital Motion Precession: rotation of Earth’s axis itself; makes one complete circle in about 26,000 years 1.4 Earth’s Orbital Motion Time for Earth to orbit once around Sun, relative to fixed stars, is sidereal year Tropical year follows seasons; sidereal year follows constellations—in 13,000 years July and August will still be summer, but Orion will be a summer constellation Astronomical Effects on Earth's Climate? The precession of Earth's spin axis, the slight ellipticity of the Earth's orbit. Is there any sense in which these effects are important? Possibly: CLIMATE describes the average weather over decades and centuries. (Weather is what happens on a day to day basis.) It is possible that these slight effects have an influence on the Earth's climate. Earth has gone through ice ages when worldwide climate was cooler and thick ice covered northern latitudes. Earliest was 5.7 x 10^8 years ago, then 2.8 x 10^8 years ago and then 3 x 10^6 years ago. The effect is somewhat periodic (2.5 x 10^8 year cycles). Cycles of glacial formation within ice ages have cycles of about 4 x 10^4 years. We are now living in a warm period which began 20,000 years ago. MILANKOVITCH's HYPOTHESIS (1920) suggests that the slight changes in the earth's shape, orbits, precession etc. affect the climate and trigger ice ages. For example the elliptical shape of the earth's orbit varies over 10^5 years. If the Earth's orbit became more elliptical and approached the sun more closely in the northern winters (winter solstice) then the northern winters would be warmer. Most of the land mass in where ice can accumulate is in the northern hemisphere. Thus we have a possible effect. Second Factor: The precession of Earth's spin axis also affects when the winters occur with respect to the Earth-sun distance. Again a small effect, but perhaps subtly relevant over 26,000 years. Third factor The inclination of Earth's spin axis to its orbital plane also varies over 40,000 years by about 1 degree. This would make the seasons more severe. Note that all this ignores the fact that the Sun's luminosity changes by a fraction of a percent over various cycles and periods of evolution so it is hard to say what is happening. Same applies for global warming and the greenhouse effect. This is how science works however. Many effects are considered, and some are ruled out eventually, but extensive testing and modeling is required. Note the hierarchy of time scales that are playing a role in this business, and their relation to our daily time scales, and that of the universe at large. 1.5 Motion of the Moon Moon takes about 29.5 days to go through whole cycle of phases—synodic month Phases are due to different amounts of sunlit portion being visible from Earth Time to make full 360° rotation around Earth, sidereal month, is about 2 days shorter 1.5 Motion of the Moon Eclipses occur when Earth, Moon, and Sun form a straight line 1.5 Motion of the Moon Lunar eclipse: Earth is between Moon and Sun Partial when only part of Moon is in shadow Total when it all is in shadow 1.5 Motion of the Moon Solar eclipse: Moon is between Earth and Sun Partial when only part of Sun is blocked Total when it all is blocked Annular when Moon is too far from Earth for total 1.5 Motion of the Moon Eclipses don’t occur every month because Earth’s and Moon’s orbits are not in the same plane 1.6 The Measurement of Distance Triangulation: Measure baseline and angles, can calculate distance Parallax: Similar to triangulation, but look at apparent motion of object against distant background from two vantage points 1.6 The Measurement of Distance 1.6 The Measurement of Distance Measuring Earth’s radius: Done by Eratosthenes about 2300 years ago; noticed that when Sun was directly overhead in one city, it was at an angle in another. Measuring that angle and the distance between the cities gives the radius. More Precisely 1-2: Measuring Distances with Geometry Converting baselines and parallaxes into distances More Precisely 1-2: Measuring Distances with Geometry Converting angular diameter and distance into size Summary of Chapter 1 Astronomy: Study of the universe Scientific method: Observation, theory, prediction, observation, … Stars can be imagined to be on inside of celestial sphere; useful for describing location Plane of Earth’s orbit around Sun is ecliptic; at 23.5° to celestial equator Angle of Earth’s axis causes seasons Moon shines by reflected light, has phases Summary of Chapter 1 (cont.) Solar day ? sidereal day, due to Earth’s rotation around Sun Synodic month ? sidereal month, also due to Earth’s rotation around Sun Tropical year ? sidereal year, due to precession of Earth’s axis Eclipses of Sun and Moon occur due to alignment; only occur occasionally as orbits are not in same plane Distances can be measured through triangulation and parallax