UNIT 3 The Study of the Universe Chapter 7: The Night Sky Chapter 8: Exploring Our Stellar Neighbourhood Chapter 9:The Mysterious Universe
CHAPTER 7 The Night Sky (Page 268) In this chapter, you will: describe different views of the night sky, as well as reasons why various cultures studied objects and events in the night sky explain the causes of: the seasons, the phases of the Moon, solar and lunar eclipses, the tides, and comets describe the major and minor components of the solar system discuss some Canadian contributions to the study of the solar system and the technology used to study space
Create Your Own Constellation (Page 269) Constellations are star patterns that represent different people and objects in the night sky. The stories about any particular constellation vary, depending on the culture observing the star pattern. Why would different cultures come up with different stories about the same constellation?
7.1 Ancient Astronomy (Page 271) Movement of objects through the night sky is very reliable. The rising and setting of the Sun, the phases of the Moon, and the position of the stars changing with the seasons are very consistent and measurable events. Mayan pyramids functioned as calendars Early peoples developed technologies based on observations of the sky. Structures that functioned as calendars helped people to mark the arrival and departure of the seasons. This information assisted people in activities such as the planting and harvesting of crops.
Early Calendars and Sky Observations (Page 272) Calendars represent a way of showing days, organized into a schedule of larger units of time such as weeks, months, seasons, or years. The calendar information is usually shown as a table or chart. Calendars such as the pyramid on the left allowed people to predict events such as the seasons, spring rains, annual flooding of rivers and lakes, and the location and migration of birds, insects, and herds of animals. Fishers, mariners, and travellers used the fixed patterns of the stars to help them navigate on land and water.
Early Calendars and Sky Observations (Page 272) Early Aztec calendars http://en.wikipedia.org/wiki/file:azteccalendarmuseoantropologia.jpg http://en.wikipedia.org/wiki/file:aztec_calendar.svg With the invention of the calendar, the first civilizations were born. Calendars led to organized agriculture, which led to more food; that in turn led to time and resources being freed up to develop knowledge and skills in a variety of areas.
Early Astronomers (Page 272) Our earliest ancestors paid a great deal of attention to the sky, taking care not to offend the deities (gods) they believed ruled the skies. Changes in the heavens were thought to be signs that the gods might be getting restless. Celestial objects are objects that exist in space, such as a planet, star, or the Moon. Celestial priests and priestesses studied these objects and learned how to predict seasons and eclipses.
Mesopotamian Astronomers, Years and Days (Page 273) Astronomers are scientists who study astronomy, which is the study of the night sky. The first astronomers were the Mesopotamians, who kept detailed records of the sky as early as 6000 years ago. Our year (365 days) is determined by counting the number of days it takes the Sun to return to exactly the same place in the sky with respect to background stars. This is the time it takes for Earth to make one revolution around the Sun. Revolution is the time it takes an object to orbit another object. One day (24 hours) is the time it takes for Earth to make one rotation. A rotation is the turning of an object around an imaginary axis running through it.
Early Clocks (Page 273) Early clocks, such as the sundial shown below, tracked shadows to tell the time of day. As Earth rotates, the position of the Sun in the sky changes, and the sundial casts shadows in different directions. Time was told by reading the position of the shadow on the sundial.
Reviewing Rotation and Revolution Click the Start button to review the revolution and rotation of Earth.
Inferring Earth s Spherical Shape (Pages 274-5) Most early peoples thought that Earth was flat. Two ancient Greek philosophers, Eratosthenes and Aristarchus (310-230 BCE) hypothesized that Earth was spherical. They based their hypothesis on three pieces of evidence. 1. The hull and then the masts of ships appeared to descend below the horizon as ships sailed away. 2. The appearance of the sky changed as travellers journeyed farther north or south. 3. During a lunar eclipse, the shadow of Earth on the moon was curved.
Section 7.1 Review (Page 276) Concepts to be reviewed: How did early sky watchers develop the first calendars? What were calendars used for? Why were the calendars developed by early peoples so useful? What is the difference between a revolution and a rotation? What evidence was used by ancient Greeks to infer the spherical shape of Earth?
7.2 The Constellations (Page 277) Most cultures imagined that the patterns formed by the stars in the night sky represented different people, animals, and objects. Constellations are groups of stars that seem to form a distinctive pattern in the sky. A light-year is the distance that light travels in one year, about 9.5 x 10 12 km. Because they lie in the same line of sight, the stars in a constellation appear to be close to each other and at exactly the same distance from Earth. They may in fact be light-years apart.
Stars that appear as a constellation when viewed from Earth may appear to be unrelated when viewed from space. Random Stars in Space (Page 278) Star maps such as the one shown to the right show constellations and individual stars. The larger the dot, the brighter the star. A star s apparent magnitude is its brightness as seen from Earth. Stellar magnitude scales compare the brightness of stars.
Reviewing Star Rise and Star Set Click the Start button to review how the rotation of Earth affects star rise and star set.
Names of Constellations (Page 279) The International Astronomical Union (IAU) is the group that names and classifies celestial objects, including the 88 official constellations. Many of the constellation names, particularly in the northern hemisphere, are from ancient Greek or Latin. Orion Ursa Major Libra The star pattern in the Big Dipper was recognized by many cultures, and a variety of stories attempt to explain its existence and motion. The Big Dipper is considered to be an asterism, a pattern within a constellation, Ursa Major.
Polaris and the Pointer Stars (Page 280) The Big Dipper s two end stars are called pointer stars because they point towards Polaris, the North Star. The distance from the pointer stars to Polaris is about five times the distance between the two pointer stars. During the night, the stars seem to revolve counterclockwise around Polaris. In the northern hemisphere, Polaris seems to stay stationary in the north sky, making it useful for navigation.
Viewing Different Constellations (Page 281) Due to Earth s revolution around the Sun, you see different constellations in the evening sky at different times of the year. The constellations you see also depends on where you are in relation to the equator (the latitude where you re making your observations). view from Ottawa view from Miami
Viewing Different Constellations Click the Start button to review how the revolution of Earth around the Sun affects the constellations observed.
Section 7.2 Review (Page 282) Concepts to be reviewed: What are constellations? How are the positions of stars within them related in space? What is a star s apparent magnitude? What is the significance of the Big Dipper? Why is it considered to be an asterism? How can Polaris be found in the night sky? Why is it useful for navigation? Why do different cultures have different interpretations of the night sky. What is a light-year?
7.3 Movements of Earth and the Moon (Page 283) The study of the movement of Earth and Moon are important to understanding seasons, the cause of tides, the observed phases of the Moon, and eclipses. Earth s orbit around the Sun is not a perfect circle. It is an ellipse (an oval or egg-like shape). The Sun is found at one of the ellipse s two focal points.
Why Do We Experience Seasons? (Page 284) Earth rotates with its axis tilted 23.5 o from its flat orbital plane. In the summer, the northern hemisphere is tilted towards the Sun, while in winter it is tilted away. It is this difference in tilt that causes the seasons. As a result of these tilts, Earth receives sunlight at a larger angle for longer periods of time during the summer, and at a smaller angle for shorter periods of time in the winter. Winter Summer
Why Do We Experience Seasons? (Page 284) The approximate height of the Sun in the sky around the start of each season is shown in the diagram below. The length of the day and how high the Sun rises in the sky is directly related to the tilt of Earth.
Why Do We Experience Seasons? (Page 284) Click the Start button to review how the movement of Earth and its tilt affect the seasons.
The Moon s Motion (Page 286) The Moon makes a complete orbit around Earth in about 29.5 days. As the Moon completes one orbit, it rotates only once on its axis. As a result, you always see the same side of the Moon. The phases of the Moon, which are the monthly progression of changes in the Moon s appearance, results from different portions of the Moon s sunlit side being visible from Earth. The dark side of the moon was not observed by humans until 1959, when a Russian spacecraft passed behind the Moon and took photos.
Phases of The Moon Click the Start button to review the cause of the Moon s phases.
Lunar Eclipses (Page 287) An eclipse is a phenomenon in which one celestial object moves directly in front of another celestial object. In a total lunar eclipse. the full Moon passes in Earth s shadow. If the Moon passes through only the penumbra or part of the umbra, a partial eclipse results. On average, lunar eclipses only occur about twice a year because the Moon s orbit is tilted about 5 o to Earth s orbit.
Solar Eclipses (Page 288) In a solar eclipse, the shadow of the Moon falls on Earth. Solar eclipses also happen about twice a year, but only people living in a very small area can observe the phenomena.
Reviewing Lunar and Solar Eclipses Click the Start button to review lunar and solar eclipses.
Tides (Page 289) The Moon s motion is responsible for the tides. The gravitational force (a force of attraction between all masses in the universe) exerted by the Moon and Earth pulling on each other causes tides. High Tide Low Tide The highest tides are on the side of Earth that faces the Moon. The difference between the force of gravity on the side of Earth nearest the Moon and the force of gravity on the side of Earth farthest from the Moon results in a stretching effect called the tidal force.
Reviewing Tides Click the Start button to review tidal forces.
Section 7.3 Review (Page 290) Concepts to be reviewed: Describe and explain the following by referencing the relative motion and position of Earth, Moon, and the Sun. Earth s seasons the phases of the Moon lunar and solar eclipses tides
7.4 Meet Your Solar System (Page 291) A solar system is a group of planets that circle one or more stars. A planet is an object that orbits one or more stars (and is not a star itself), is spherical, and does not share its orbit with another object. The current heliocentric (Sun-centered) model of the solar system was first introduced in the 1500s by Polish astronomer Nicolaus Copernicus. Previous models of the solar system were geocentric (Earth-centered), originating with the Greek astronomer Ptolemy.
Planetary Motion (Page 292) When we observe planets in the night sky, Venus and Mercury stay near the Sun and can thus only be seen in the early evening or morning. Mars, Jupiter, and Saturn usually appear to move westward as Earth rotates but at times seem to wander westward in a slow looping motion. This unusual movement from east to west is called retrograde motion. Retrograde motion is caused by Earth catching up to and then passing an outer planet in its orbit. Earth is on an inside track and thus moves faster than the outer planets.
Reviewing Retrograde Motion Click the Start button to review retrograde motion.
Distances Between Planets (Page 293) The distances between planets are so large that units such as kilometres cannot represent them in a meaningful way. For this reason, astronomers created a unit for measuring distances in the solar system: the astronomical unit (AU). One AU is approximately equal to the distance between Earth and the Sun, about 150 million kilometres. Earth is 1 AU from the Sun. The average distance between the Sun and an object orbiting the Sun is called the object s orbital radius. The orbital radius is expressed in astronomical units.
Classification of the Planets (Pages 294-5) Mercury, Venus, Earth, and Mars are called the inner planets. These planets are also called the terrestrial (Earth-like) planets. They are relatively small and have solid cores and rocky crusts. Mercury Venus Earth Mars Saturn, Jupiter, Uranus, and Neptune are called the outer planets or the gas giants. These planets were formed from large clumps of gas, ice, and dust. They are also known for their large gaseous bands and cold temperatures. Jupiter Saturn Uranus Neptune
Solar System Data (Pages 294-5) Inner Planet Data Outer Planet Data
Reviewing The Planets of the Solar System Click the Start button to review characteristics of the planets of the solar system.
Section 7.4 Review (Page 296) Concepts to be reviewed: How does the geocentric model of the solar system compare to the heliocentric model? What are the basic differences and similarities between the planets of the solar system? How do the inner and outer planets differ from each other? What units are used to measure distances within the solar system?
7.5 Other Objects in the Solar System (Page 297) In addition to the Moon and planets, other important objects in the solar system include comets, meteoroids, and asteroids. Comets are composed of rocky material, ice, and gas that originate in the Kuiper Belt and the Oort Cloud. The Kuiper Belt is a disc-shaped group of millions of small objects orbiting the Sun beyond the orbit of Neptune (trans-neptunian objects)
Visualizing the Kuiper Belt (Page 298)
The Plight of Pluto (Page 299) In 2006, the IAU demoted Pluto to dwarf planet status because its orbit sometimes crosses Neptune s orbit. Other Kuiper Belt objects, such as the dwarf planet Eris, are actually larger than Pluto.
The Oort Cloud (Page 299) The Oort Cloud is a spherical cloud of icy fragments of debris 50 000 to 100 000 AU from the Sun. This region is thought to be home to many comets. It marks the outer boundary of the Sun s gravitational influence. Wikipedia/AZcolvin 429
Comets (Pages 299-300) Most comets originate in the Kuiper Belt and the Oort Cloud. Occasionally Jupiter s gravitational influence will nudge a comet to change its orbit and enter the inner solar system. When a comet comes too close to the Sun, the radiation from the Sun causes it to release gases and particles, forming a tail that always points away from the Sun. While some comets visit the Sun just once, periodic comets orbit the Sun. Periodic Comets
Asteroids (Page 300) Asteroids are small, non-spherical objects that range in size from a tiny speck, like a grain of sand, to 500 km wide. Most asteroids originate in the asteroid belt between Mars and Jupiter. They are believed to be composed of debris left over from the formation of the solar system. Asteroid Ida Moon Dactyl Asteroids can have their own moons, as shown in the image above.
Meteoroids, Meteors, and Meteorites (Page 301) A meteoroid is a piece of rock moving through space, while a meteor is a meteoroid that hits Earth s atmosphere and burns up. Meteorites are meteoroids that are large enough to pass through Earth s atmosphere and reach the ground without being totally burned up. The most famous meteor shower is the Perseid meteor shower, which occurs around August 12 every year. It results from Earth passing through debris left along the path of Comet Swift Tuttle.
Asteroid and Meteor Impacts (Page 302) An asteroid the size of a mountain hit Earth 65 million years ago. Many scientists believe that this impact led to climate changes that resulted in the global mass extinctions of thousands of species. Impact craters in Nunavut (A) and in Arizona (B) are evidence of impacts many years ago.
Tunguska Devastation (Page 303) A more recent impact occurred on June 30, 1908, in Tunguska, Siberia, when an object entered Earth s atmosphere and destroyed an area of more than 2000 km 2. The object (thought to be about 50 m in diameter) flattened nearly 100 million trees and killed thousands of forest animals. Many astronomers around the world are currently working to discover and map the courses of any Near Earth Objects (NEOs) that could pose an impact risk to Earth. In 2010, Canada will launch NEOSSat to help find NEOs.
Section 7.5 Review (Page 306) Concepts to be reviewed: In addition to planets, what other objects are found in the solar system? What distinguishing characteristics do these objects have? Why do scientists think that an asteroid or large meteor will hit Earth in the future? What measures could be taken to protect Earth from future impacts?