1. Chapter 1 Astronomy Today 7th Edition
Lecture Outlines
Chapter 1
Astronomy Today
7th Edition
Chaisson/McMillan
© 2011 Pearson Education, Inc.

2. Chapter 1 Charting the Heavens
High overhead on a clear, dark night, we can see a rich band of stars known as the Milky Way—so-called for its resemblance to a milky band of countless stars. All these stars (and more) are part of a much larger system called the Milky Way Galaxy, of which our star, the Sun, is one member. This single exposure, dubbed “the Going to the Stars Road,” was made at night with only the Moon’s light illuminating the terrain on the continental divide at Logan Pass in Glacier National Park, near the Montana/Alberta border. (© Tyler Nordgren)

3. Units of Chapter 1 1.1 Our Place in Space
1.2 Scientific Theory and the Scientific Method
1.3 The “Obvious” View
Angular Measure
1.4 Earth’s Orbital Motion
1.5 Motion of the Moon
1.6 The Measurement of Distance
Measuring Distances with Geometry

4. 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
Figure 1-1. Caption: Earth. Earth is a planet, a mostly solid object, although it has some liquid in its oceans and core and gas in its atmosphere. In this view, the North and South American continents are clearly visible, though most of the scene shows Pacific waters. (NASA)

5. 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
Figure 1-5. Caption: Size and Scale. This artist’s conception puts each of the previous four figures in perspective. The bottom of this figure shows spacecraft (and astronauts) in Earth orbit, a view that widens progressively in each of the next five cubes drawn from bottom to top—Earth, our planetary system, the local neighborhood of stars, the Milky Way Galaxy, and the closest cluster of galaxies. The image at the top right depicts the spread of galaxies (white dots) in the universe on extremely large scales—the field of view in this final frame is hundreds of millions of light-years across. The numbers indicate approximately the increase in scale between successive images: Earth is 500,000 times larger than the spacecraft in the foreground, the solar system in turn is some 1,000,000 times larger than Earth, and so on. (D. Berry)

6. 1.1 Our Place in Space
This galaxy is about 100,000 light-years across.
Figure 1-3. Caption: Galaxy. A typical galaxy is a collection of a hundred billion stars, each separated by vast regions of nearly empty space. Our Sun is a rather undistinguished star near the edge of another such galaxy, called the Milky Way. (NASA)

7. 1.2 Scientific Theory and the Scientific Method
Scientific theories:
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.

8. 1.2 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.
Figure 1-6. Caption: Scientific Method. Scientific theories evolve through a combination of observation, theoretical reasoning, and prediction, which in turn suggests new observations. The process can begin at any point in the cycle (although it usually starts with observations), and it continues forever—or until the theory fails to explain an observation or makes a demonstrably false prediction.

9. 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
Figure 1.8a. Caption: Constellation Orion. (a) A photograph of the group of bright stars that make up the constellation Orion. (See the preface for an explanation of the icon at the bottom, which simply indicates that this image was made in visible light.)

10. 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
Figure 1.8b. Caption: Constellation Orion. (b) The stars are connected to show the pattern visualized by the Greeks: the outline of a hunter. The Greek letters serve to identify some of the brighter stars in the constellation (see also Figure 1.9). You can easily find Orion in the northern winter sky by identifying the line of three bright stars in the hunter’s “belt.” (S.Westphal)

11. 1.3 The “Obvious” View
Stars that appear close in the sky may not actually be close in space
Figure 1-9. Caption: Orion in 3-D. The true three-dimensional relationships among the most prominent stars in Orion. The distances were determined by the Hipparcos satellite in the 1990s. (See Chapter 17.)

12. 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
Figure Caption: Celestial Sphere. Planet Earth sits fixed at the hub of the celestial sphere, which contains all the stars. This is one of the simplest possible models of the universe, but it doesn’t agree with all the facts that astronomers now know about the universe.

13. 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

14. 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
Figure Caption: Solar and Sidereal Days. A sidereal day is Earth’s true rotation period—the time taken for our planet to return to the same orientation in space relative to the distant stars. A solar day is the time from one noon to the next. The difference in length between the two is easily explained once we understand that Earth revolves around the Sun at the same time as it rotates on its axis. Frames (a) and (b) are one sidereal day apart. During that time, Earth rotates exactly once on its axis and also moves a little in its solar orbit—approximately 1°. Consequently, between noon at point A on one day and noon at the same point the next day, Earth actually rotates through about 361° (frame c), and the solar day exceeds the sidereal day by about 4 minutes. Note that the diagrams are not drawn to scale; the true 1° angle is in reality much smaller than shown here.

15. 1.4 Earth’s Orbital Motion
Seasonal changes to night sky are due to Earth’s motion around Sun
Figure Caption: Typical Night Sky. (a) A typical summer sky above the United States. Some prominent stars (labeled in lowercase letters) and constellations (labeled in all capital letters) are shown. (b) A typical winter sky above the United States.

16. 1.4 Earth’s Orbital Motion
Twelve constellations Sun moves through during the year are called the zodiac; path is ecliptic
Figure Caption: The Zodiac. The view of the night sky changes as Earth moves in its orbit about the Sun. As drawn here, the night side of Earth faces a different set of constellations at different times of the year. The 12 constellations named here make up the astrological zodiac. The arrows indicate the most prominent zodiacal constellations in the night sky at various times of year. For example, in June, when the Sun is “in” Gemini, Sagittarius and Capricornus are visible at night.

17. 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
Figure Caption: Seasons. In reality, the Sun’s apparent motion along the ecliptic is a consequence of Earth’s orbital motion around the Sun. The seasons result from the inclination of our planet’s rotation axis with respect to its orbit plane. The summer solstice corresponds to the point on Earth’s orbit where our planet’s North Pole points most nearly toward the Sun. The opposite is true of the winter solstice. The vernal and autumnal equinoxes correspond to the points in Earth’s orbit where our planet’s axis is perpendicular to the line joining Earth and the Sun. The insets show how rays of sunlight striking the ground at an angle (e.g., during northern winter) are spread over a larger area than rays coming nearly straight down (e.g., during northern summer). As a result, the amount of solar heat delivered to a given area of Earth’s surface is greatest when the Sun is high in the sky.

18. 1.4 Earth’s Orbital Motion
Precession: rotation of Earth’s axis itself; makes one complete circle in about 26,000 years
Figure 1-19a. Caption: Precession. (a) Earth’s axis currently points nearly toward the star Polaris. About 12,000 years from now—almost halfway through one cycle of precession—Earth’s axis will point toward a star called Vega, which will then be the “North Star.” Five thousand years ago, the North Star was a star named Thuban in the constellation Draco.

19. 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

20. 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
Figure Caption: Lunar Phases. Because the Moon orbits Earth, the visible fraction of the lunar sunlit face varies from night to night, although the Moon always keeps the same face toward our planet. (Note the location of the small, straight arrows, which mark the same point on the lunar surface at each phase shown.) The complete cycle of lunar phases, shown here starting at the waxing crescent phase and following the Moon’s orbit counterclockwise, takes 29.5 days to complete. Rising and setting times for some phases are also indicated. (UC/Lick Observatory)

21. 1.5 Motion of the Moon
Eclipses occur when Earth, Moon, and Sun form a straight line
Figure 1-26a. Caption: Eclipse Geometry. (a) An eclipse occurs when Earth, Moon, and Sun are precisely aligned. If the Moon’s orbital plane lay in exactly the plane of the ecliptic, this alignment would occur once a month. However, the Moon’s orbit is inclined at about 5° to the ecliptic, so not all configurations are favorable for producing an eclipse.

22. 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
Figure Caption: Lunar Eclipse. A lunar eclipse occurs when the Moon passes through Earth’s shadow. At these times, we see a darkened, copper-colored Moon, as shown by the partial eclipse in the inset photograph. The red coloration is caused by sunlight deflected by Earth’s atmosphere onto the Moon’s surface. An observer on the Moon would see Earth surrounded by a bright, but narrow, ring of orange sunlight. Note that this figure is not drawn to scale, and only Earth’s umbra (see text and Figure 1.24) is shown. (Inset: G. Schneider)

23. 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
Figure Caption: Types of Solar Eclipse. (a) The Moon’s shadow consists of two parts: the umbra, where no sunlight is seen, and the penumbra, where a portion of the Sun is visible. (b) If we are in the umbra, we see a total eclipse; in the penumbra, we see a partial eclipse. (c) If the Moon is too far from Earth at the moment of the eclipse, the umbra does not reach Earth and there is no region of totality; instead, an annular eclipse is seen. (Note that these figures are not drawn to scale.) (Insets: NOAA; G. Schneider)

24. 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
Figure 1-28b. Caption: Eclipse Geometry. (b) For an eclipse to occur, the line of intersection of the two planes must lie along the Earth–Sun line. Thus, eclipses can occur just at specific times of the year. Only the umbra of each shadow is shown, for clarity (see Figure 1.24).

25. 1.6 The Measurement of Distance
Triangulation: Measure baseline and angles, can calculate distance
Figure Caption: Triangulation. Surveyors often use simple geometry and trigonometry to estimate the distance to a faraway object by triangulation. By measuring the angles at A and B and the length of the baseline, the distance can be calculated without the need for direct measurement.

26. 1.6 The Measurement of Distance
Parallax: Similar to triangulation, but look at apparent motion of object against distant background from two vantage points
Figure Caption: Parallax. (a) This imaginary triangle extends from Earth to a nearby object in space (such as a planet). The group of stars at the top represents a background field of very distant stars. (b) Hypothetical photographs of the same star field showing the nearby object’s apparent displacement, or shift, relative to the distant undisplaced stars.

27. 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.
Figure Caption: Measuring Earth’s Radius. The Sun’s rays strike different parts of Earth’s surface at different angles. The Greek philosopher Eratosthenes realized that the difference was due to Earth’s curvature, enabling him to determine Earth’s radius by using simple geometry.

28. Measuring Distances with Geometry
More Precisely 1-2:
Measuring Distances with Geometry
Converting baselines and parallaxes into distances
Figure: More Precisely 1-2.

29. Measuring Distances with Geometry
More Precisely 1-2:
Measuring Distances with Geometry
Converting angular diameter and distance into size
Figure: More Precisely 1-2.

30. 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

31. 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