Chapter 1 Charting the Heavens Chapter 1 opener. Stars are perhaps the most fundamental component of the universe that we inhabit. Roughly as many stars reside in the observable universe as grains of sand in all the beaches of the world. Here, we see displayed one of the most easily recognizable star fields in the winter sky—the familiar constellation Orion. Near center are three blue-white diagonally aligned stars hotter than our Sun, below which is a red-white nebulosity where stars are now forming. At top left is an old giant red star probably about to explode. The entire field of view spans about 100 light-years, or 1015 kilometers. (J. SANFORD/ASTROSTOCK-SANFORD)

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 Celestial Coordinates 1.4 Earth’s Orbital Motion 1.5 Astronomical Timekeeping 1.6 The Motion of the Moon 1.7 The Measurement of Distance Measuring Distances with Geometry

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. (NASA)

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, the planetary system, the local neighborhood of stars, the Milky Way Galaxy, and the closest cluster of galaxies. The top image depicts the distribution of galaxies 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)

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)

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% certainty.

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 can 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.

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.)

1.3 The “Obvious” View Group stars into constellations: Figures having meaning to those doing the grouping Useful: Polaris, which is almost due north Not so useful: 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 More Precisely 1-2 and 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)

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 early 1990s. (See Chapter 17.)

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 1-11. 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.

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 actual size and distance away

More Precisely 1-2: Celestial Coordinates Declination: degrees north or south of celestial equator Right ascension: measured in hours, minutes, and seconds eastward from position of Sun at vernal equinox

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, one sidereal day has passed Figure 1-13. 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 four minutes. Note that the diagrams are not drawn to scale; the true 1° angle is in reality much smaller than shown here.

1.4 Earth’s Orbital Motion Seasonal changes to night sky are due to Earth’s motion around Sun Figure 1-14. 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.

1.4 Earth’s Orbital Motion 12 constellations Sun moves through during the year are called the zodiac; path is ecliptic Figure 1-15. 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.

1.4 Earth’s Orbital Motion Ecliptic is plane of Earth’s path around Sun; at 23.5° to celestial equator Northernmost point (above celestial equator) is summer solstice; southernmost is winter solstice; points where path cross 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 1-17. 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.

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.

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

1.5 Astronomical Timekeeping Solar noon: when Sun is at its highest point for the day Drawbacks: length of solar day varies during year; noon is different at different locations Figure 1-20. Caption: Variations in the Solar Day. (a) Because Earth does not orbit the Sun at a constant speed, the rate at which the Sun appears to move across the sky, and hence the length of the solar day, also varies. (b) In addition, the length of the solar day changes with the season, because the eastward progress of the Sun in, say, 1 hour (indicated here), is greater at the solstices than at the equinoxes.

1.5 Astronomical Timekeeping Define mean (average) solar day—this is what clocks measure. Takes care of length variation. Also define 24 time zones around Earth, with time the same in each one and then jumping an hour to the next. Takes care of noon variation.

1.5 Astronomical Timekeeping World time zones Figure 1-21. Caption: Time Zones. The standard time zones were adopted to provide consistent times around the world. Within each time zone, all clocks show the same time, the local solar time of one specific longitude inside the zone.

1.5 Astronomical Timekeeping Lunar month (complete lunar cycle) doesn’t have whole number of solar days in it, and tropical year doesn’t have whole number of months Current calendar has months that are close to lunar cycle, but adjusted so there are 12 of them in a year Figure 1-23. Caption: Sidereal Month. The difference between a synodic and a sidereal month stems from the motion of Earth relative to the Sun. Because Earth orbits the Sun in 365 days, in the 29.5 days from one new Moon to the next (1 synodic month), Earth moves through an angle of approximately 29°. Thus, the Moon must revolve more than 360° between new Moons. The sidereal month, which is the time taken for the Moon to revolve through exactly 360°, relative to the stars, is about 2 days shorter.

1.5 Astronomical Timekeeping Year doesn’t quite have a whole number of solar days in it—leap years take care of this. Add extra day every 4 years Omit years that are multiples of 100 but not of 400 Omit years that are multiples of 1000 but not of 4000 This will work for 20,000 years.

1.6 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° around Earth, sidereal month, is about 2 days shorter Figure 1-22. 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)

1.6 Motion of the Moon Eclipses occur when Earth, Moon, and Sun form a straight line Figure 1-28a. 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.

1.6 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 1-24. 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 1.26) is shown. (Inset: G. Schneider)

1.6 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 1-26. 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)

1.6 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 1.26).

1.7 The Measurement of Distance Triangulation: Measure baseline and angles, can calculate distance Figure 1-30. 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.

1.7 The Measurement of Distance Parallax: Similar to triangulation, but look at apparent motion of object against distant background from two vantage points Figure 1-32. 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.

1.7 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 1-34. 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.

Measuring Distances with Geometry More Precisely 1-3: Measuring Distances with Geometry Converting baselines and parallaxes into distances Figure: More Precisely 1-3.

Measuring Distances with Geometry More Precisely 1-3: Measuring Distances with Geometry Converting angular diameter and distance into size Figure: More Precisely 1-3.

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