Ancient Astronomy in Grade 11 physical science.pptx
- 3. The Greeks are very much noted for their major contributions in different fields. They were not only great philosophers. They were great scientists and mathematicians as well. It was in Greece that the Golden Age of early astronomy was centered. Being philosophers, the Greeks used philosophical arguments to explain the natural events happening around them including the movements of the stars and other heavenly bodies. But they were also observers. They made use of their observational data to explain certain events. They were the ones who measured the sizes and the distances of the sun and the moon using the basics of geometry and trigonometry which they also developed. The early Greeks had a geocentric view of the earth. For them, it was the center of the universe; hence, a motionless sphere. The sun, moon, Mercury, Venus, Mars, Jupiter, and Saturn orbited the Earth. They also believed that stars traveled daily around the earth. However, they all stayed in a transparent, hollow sphere located beyond the planets. They called this sphere as the celestial sphere.
- 4. Key Terms • Oblate spheroid: the shape of the Earth. It has bulging equator and squeezed poles. • Solstice: either of the two times in the year, the summer solstice and the winter solstice, when the sun reaches its highest or lowest point in the sky at noon, marked by the longest and shortest days. • Eclipse: an obscuring of the light from one celestial body by the passage of another between it and the observer or between it and its source of illumination. • Heliocentrism: the astronomical model in which the Earth and planets revolve around the Sun. • Geocentrism: any theory of the structure of the solar system (or the universe) in which Earth is assumed to be at the center of it all. Around 500 B.C., most Greeks believed that the Earth was round, not flat. It was Pythagoras and his pupils who were first to proposed a spherical Earth. In 500 to 430 B.C., Anaxagoras further supported Pythagoras' proposal through his observations of the shadows that the Earth cast on the Moon during a lunar eclipse. He observed that during a lunar eclipse, the Earth's shadow was reflected on the Moon's surface. The shadow reflected was circular. Around 340 B.C., Aristotle listed several arguments for a spherical Earth which included the positions of the North Star, the shape of the Moon and the Sun, and the disappearance of the ships when they sail over the horizon.
- 5. North Star The North Star was believed to be at a fixed position in the sky. However, when the Greeks traveled to places nearer the equator, like Egypt, they noticed that the North Star is closer to the horizon. The Shape of the Sun and the Moon Aristotle argued that if the Moon and the Sun were both spherical, then perhaps, the Earth was also spherical. Disappearing Ships If the Earth was flat, then a ship traveling away from an observer should become smaller and smaller until it disappeared. However, the Greeks observed that the ship became smaller and then its hull disappeared first before the sail as if it was being enveloped by the water until it completely disappeared.
- 6. The Size of the Spherical Earth Ancient scholars tried to provide proof of a spherical Earth and its circumference through calculations. It was Eratosthenes who gave the most accurate size during their time. While he was working at the Library of Alexandria in Northern Egypt, he received correspondence from Syene in Southern Egypt which stated that a vertical object did not cast any shadow at noontime during the summer solstice. But this was not the case in Alexandria where, at noon time during the summer solstice, a vertical object still casts a shadow. These observations could only mean that the Sun, during this time in Alexandria, was not directly overhead. Figure 1: Shows how Eratosthenes measured the circumference of the Earth. Eratosthenes then determined the angle the Sun made with the vertical direction by measuring the shadow that a vertical stick cast. He found out that in Alexandria, the Sun makes an angle of 7.2° from the vertical while 0° in Syene. To explain the difference, he hypothesized that the light rays coming from the sun are parallel, and the Earth is curved. From his measurements, he computed the circumference of the Earth to be approximately 250 000 stadia (a stadium is a unit of measurement used to describe the size of a typical stadium at the time), about 40 000 kilometers.
- 7. Our understanding about the different heavenly bodies can be credited to the important findings of the following Greek astronomers: a. Anaxagoras Anaxagoras was able to explain what causes the phases of the moon. According to him, the moon shone only by reflected sunlight. Since it is a sphere, only half of it illuminated at a time. This illuminated part that is visible from the earth changes periodically. b. Eudoxus Eudoxus proposed a system of fixed spheres. He believed that the Sun, the moon, the five known planets and the stars were attached to these spheres which carried the heavenly bodies while they revolved around the stationary Earth. c. Aristotle He was a student of Plato. For him, the earth is spherical in shape since it always casts a curved shadow when it eclipses the moon. He also believed that the earth was the center of the universe. The planets and stars were concentric, crystalline spheres centered on the earth.
- 8. d. Aristarchus He was the very first Greek to profess the heliocentric view. The word helios means sun; centric means centered. This heliocentric view considered the sun as the center of the universe. He learned that the sun was many times farther than the moon and that it was much larger than the earth. He also made an attempt to calculate the distance of the sun and the moon by using geometric principles. He based his calculations on his estimated diameters of the earth and moon, and expressed distance in terms of diameter. However, the measurements he got were very small and there were a lot of observational errors. e. Eratosthenes The first successful attempt to determine the size of the earth was made by him. He did this by applying geometric principles. He observed the angles of the noonday sun in two Egyptian cities that were almost opposite each other- Syene (now Aswan) in the south and Alexandria in the north. He assumed they were in the same longitude. f. Hipparchus He was considered as the greatest of the early Greek astronomers. He observed and compared the brightness of 850 stars and arranged them into order of brightness or magnitude. He developed a method for predicting the times of lunar eclipses to within a few hours. Aside from this, he also measured the length of the year to within minutes of the modern value. g. Claudius Ptolemy He believed that the earth was the center of the universe. His Ptolemic Model claimed that the planets moved in a complicated system of circles. This geocentric model also became known as the Ptolemic System.
- 9. The Ptolemic Model Claudius Ptolemy developed a model that was able to explain the observable motions of the planets. Figure 2: Ptolemic Model showing Geocentrism. According to the Ptolemic Model, the sun, the moon, and the other planets move in circular orbits around the earth. However, if observed night after night, these planets move slightly eastward among the stars. At a certain point, the planet appears to stop then moves in the opposite direction for some time; after which it will resume its eastward motion. This westward drift of the planets is called retrograde motion. To justify his earth-centered model using retrograde motion, he further explained that the planets orbited on small circles, called epicycles, revolving around large circles called deferents.
- 10. Examples of Astronomical Phenomena Before the Advent of Telescope The roots of astronomy reach back to prehistoric times when humans first noted stars in the night sky. The earliest astronomers divided the night sky into groups of stars called constellations. The names of the constellations are mainly a carryover from the names assigned by early Greek, Babylonian and Egyptian astronomers. The grouping of stars and the significance given to them varied from culture to culture. In some cultures, the constellations stimulated story-telling and the creation of great myths. In some cases, the constellations honored great heroes like Hercules and Orion or served as navigational aids for travelers and sailors. On the other hand, some people believed that constellations provided a guide for planting and harvesting crops for they were seen to move periodically in the sky, in concert with the seasons. Charts of these periodic movements became some of the first calendars. Stars were thought to be points of light on great revolving celestial sphere having the earth as its center. Positions of the sphere were believed to affect earthly events and so were carefully measured. Keen observations and logical reasoning gave birth to both Astrology and later, to Science.
- 11. Even before the advent of the telescopes, ancient astronomers were able to observe the following: 1. rising and setting of the Sun in the east and the west, respectively, 2. point where the Sun rises and sets in the horizon varies in a year, 3. phases of the moon, 4. lunar eclipse, 5. solar eclipse, 6. daily and annual motion of the stars, and 7. planets Mercury, Venus, Mars, Jupiter, and Saturn. Rising and Setting of the Sun Babylonian and Egyptian civilizations used a primitive version of a sundial, called gnomon, in systematically observing the motion of the sun. By looking at the shadows that the gnomon casts, they were able to observe that the sun rises in the eastern part of the sky, reaches its highest point in midday, and sets in the western part of the sky. Figure 1: The figure shows the ancient stele used as a gnomon, a primitive version of sundial
- 12. Also, they recorded that the points where the sun rises and sets on the horizon varies over a year and these variations happen periodically. They observed that these variations are related to weather and so concluded that seasonal changes in climate happen during a course of one year. The stars continue to circle during the day, but the brilliance of the Sun makes them difficult to see. (The Moon can often be seen in the daylight, however.) On any given day, we can think of the Sun as being located at some position on the hypothetical celestial sphere. When the Sun rises—that is, when the rotation of Earth carries the Sun above the horizon—sunlight is scattered by the molecules of our atmosphere, filling our sky with light and hiding the stars above the horizon. For thousands of years, astronomers have been aware that the Sun does more than just rise and set. Earth's orbit around the Sun is slightly elliptical. This means that the Sun travels across the sky at slightly different speeds from day to day depending upon where Earth is in its orbit. Earth's axis is also not perpendicular to the plane of its orbit. Instead, Earth is tilted on its axis approximately 23.4°. This what gives us our seasons here on Earth. When the North Pole is tilted toward the Sun, the Northern Hemisphere experiences summer, and the Sun is high in the sky at noon. During the winter, the North Pole is tilted away from the Sun, and at noon the Sun doesn't get nearly as high in the sky.
- 13. Earth's tilt also explains why the longest day of the year occurs on the summer solstice (usually around June 21). Likewise, the shortest day of the year occurs on the winter solstice (usually around December 21). The combination of Earth's elliptical orbit and the tilt of its axis results in the Sun taking different paths across the sky at slightly different speeds each day. This gives us different sunrise and sunset times each day. Once the summer solstice passes, you'll notice the days begin to get shorter each day. This trend continues until the winter solstice, the shortest day of the year. After the winter solstice, days get slightly longer each day up until the summer solstice, and the process repeats year after year. The path of the Sun appears to take around the celestial sphere each year is called the ecliptic. Because of its motion on the ecliptic, the Sun rises about 4 minutes later each day with respect to the stars. Earth must make just a bit more than one complete rotation (with respect to the stars) to bring the Sun up again.
- 14. Phases of the Moon A moon, also called a satellite, is a relatively small object that is orbiting around a planet. Earth’s moon is the fifth biggest moon in the solar system. On average, the distance between the Earth and the moon is 384,000 kilometers. The moon is about four times smaller than the width of the Earth. The gravity of the Earth pulls on the moon such that one face of the moon is always facing us, and we can never see the other side. Just like the Earth, half of the moon is always lit by sunlight and the other half is in shadow. As the moon orbits the Earth, we see a different phase of the moon. It takes 27 days, 7 hours, and 43 minutes for our Moon to complete one full orbit around Earth. This is called the sidereal month, and is measured by our Moon's position relative to distant “fixed” stars. It takes our Moon about 29.5 days to complete one cycle of phases (from full Moon to full Moon). There are eight phases within about a month. The time interval between a full (or new) moon and the next repetition of the same phase, a synodic month, averages about 29.53 days. Therefore, in those lunar calendars in which each month begins on the day of the new moon, the full moon falls on either the 14th or 15th day of the lunar month.
- 15. Figure 2: The figure shows the eight (8) phases of the moon At any given moment, rays of sunlight illuminate one-half of the moon’s surface. Because the moon both rotates on an axis and revolves around the earth, we have only the moon’s phase, changes in its visible shape that occur in monthly cycles. The first half of the moon cycle begins with the new moon (totally dark; we see nothing) and climaxes with the full moon. The new moon phase occurs when the sun, moon and earth are lined up, with the moon in the middle.
- 16. Eclipses There are two types of eclipses, lunar eclipse and solar eclipse. Lunar Eclipse Figure 3: The figure shows the lunar eclipse The lining up of the earth, moon, and sun produces a lunar eclipse when the moon passes into the shadow of the earth. Usually, a lunar eclipse either precedes or follows a solar eclipse by two weeks. Just as all solar eclipses involve a new moon, all lunar eclipses involve a full moon. A lunar eclipse may be partial or total. All observers on the dark side of the earth see a lunar eclipse at the same time. Interestingly, when the moon is fully eclipsed, it is still visible and reddish.
- 17. Solar Eclipse Figure 4: The figure shows the solar eclipse Sometimes, the moon comes between the sun and the Earth. Then, it hides briefly from our sight. We call this an eclipse of the sun. Ancient people feared an eclipse, because it was supposed to show that the gods were angry, or that there would be floods, wars and other disasters. A solar eclipse occurs when the moon’s shadow falls on the earth. Because of the large size of the sun, rays of sunlight taper to provide an umbra and a surrounding penumbra. An observer in the umbra part of the shadow experiences darkness during the day a total eclipse, totality. Totality begins when the sun disappears behind the moon and ends when the sun appears on the other edge of the moon. The average time of totality is 2 to 3 minutes, and a maximum of 7.5 minutes.
- 18. Diurnal Motion Diurnal motion is the apparent daily revolution of the celestial sphere around the celestial poles as a direct effect of the Earth’s rotation on its axis. In other words, it refers to the apparent movement of stars and other celestial bodies around Earth. The circular path that the celestial bodies take to complete the diurnal motion is called diurnal circle. The apparent motion of celestial bodies viewed from Earth is east to west. If you observe the night sky, the stars seem to move in a counter- clockwise direction (from east to west) with respect to Polaris or North Star. Similarly, the apparent daily motion of the sun, which is the closest star to Earth, is counter-clockwise. You can observe that the sun rises in the east and sets in the west.
- 19. Annual Motion Annual motion is the apparent yearly movement of the stars as observed from Earth as a direct effect of the Earth’s revolution around the sun. The sun revolves 360 degrees a year around a path on the celestial sphere called the ecliptic. The sun moves eastward with respect to the stars on the celestial sphere. It can be observed that the sun’s altitude changes with season. Its altitude is at maximum during summer solstice and at minimum during winter solstice. Also, sunrise and sunset points in the horizon changes with season. The sun rises north of east in the summer, and south of east in the winter. As the sun revolves around the ecliptic, different stars and constellations appear on the horizon throughout the year. These are known as the constellations of the Zodiac.
- 20. Precession of the Equinoxes As the sun revolves around the ecliptic, it intersects the celestial equator twice during a year at two points. These points are called the equinoxes: vernal and autumnal. During an equinox, the length of daytime is almost equal to the length of nighttime. Vernal or spring equinox happens every March 20 while autumnal equinox occurs every September 22. The gravitational force of the sun and the moon on Earth causes the cyclic precession or “wobbling” of the Earth’s axis of rotation. Precession of the equinoxes is the apparent motion of the equinoxes along the ecliptic as Earth ‘wobbles,’ and this motion happens about every 26 000 years. At present, Earth’s North Pole points to Polaris. However, it will eventually point to another star, Vega, because of precession.
- 21. Planets Discovered Before the Invention of Telescope Mercury, Venus, Mars, Jupiter, and Saturn are the planets discovered before the invention of the telescope.
- 22. The first telescopes were created in the Netherlands in 1608. Spectacle makers Hans Lippershey & Zacharias Janssen and Jacob Metius independently created telescopes.
- 23. Named in honor of the trailblazing astronomer Edwin Hubble, the Hubble Space Telescope is a large, space-based observatory that has changed our understanding of the cosmos since its launch and deployment by the space shuttle Discovery in 1990.