© 2007 Jones and Bartlett Publishers Chapter 1 Sections 1-1 thru1-3 The Quest Ahead Courtesy of Hubble Space Telescope Comet Team and NASA Courtesy of STScI/NASA

© 2007 Jones and Bartlett Publishers Science and Astronomy 1. It is not easy to define what science is. However, any effort to define it must include its methods, its historical development, its social context, and a clear understanding of its language. 2. Astronomy is the oldest of the sciences. Its long history and recent advances make it a great example of the progressive nature of science.

© 2007 Jones and Bartlett Publishers 1-1 The View from Earth 1. The Milky Way, a great number of stars, the Moon, and some of the planets are some of the objects that you could see during clear nights. 2. Nebulae, giant clouds of gas and dust, are involved in both the birth and death of stars. Photo by Dave Palmer Courtesy of T.A.Rector (NRAO/AUI/NSF and NOAO/AURA/NSF) and B.A.Wolpa (NOAO/AURA/NSF)

© 2007 Jones and Bartlett Publishers 1-1 The View from Earth 3. Ancient observers wondered about these objects as we do today along with a number of even more exotic ones. 4. These are but examples through which we will study the basic methods of inquiry of not only astronomy but of all the natural sciences. 5. In our quest to understand the universe we will first study our neighborhood (Earth, Moon, and the planets in our solar system), then our Sun (the closest star to us), then the stars and finally galaxies.

© 2007 Jones and Bartlett Publishers 1-2 The Celestial Sphere 1. Celestial sphere is the imaginary sphere of heavenly objects that seems to center on the observer. 2. Celestial pole is the point on the celestial sphere directly above a pole of the Earth. In the Northern Hemisphere one can see the north celestial pole directly above the Earth’s North Pole. In the Southern Hemisphere the south celestial pole is located above the South Pole. Figure 1.07: Celestial sphere

© 2007 Jones and Bartlett Publishers Constellations 1. A constellation (from the Latin, meaning “stars together”) is an area of the sky containing a pattern of stars named for a particular object, animal or person. 2. The earliest constellations were defined by the Sumerians as early as 2000 B.C. 3. The 88 constellations used today were established by international agreement. They cover the entire celestial sphere and have specific boundaries. 4. Constellations are simply accidental patterns of stars. The stars in a constellation are at different distances from us and move relative to each other in different directions and with different speeds. 5. Astronomers use constellations as a convenient way to identify parts of the sky.

© 2007 Jones and Bartlett Publishers Measuring the Positions of Celestial Objects 1. The angular separation of two objects is the angle between two lines originating from the eye of the observer toward the two objects. 2. One degree is divided into 60 arcminutes. One arcminute is divided into 60 arcseconds. 3. A fist held at arm’s length yields an angle of about 10°. A little finger held at arm’s length yields an angle of about 1°. Figure 1.12: Two stars, when viewed from Earth, have an angular separation as shown

© 2007 Jones and Bartlett Publishers Celestial Coordinates 1. Longitude and latitude uniquely define the position of an object on Earth. Similarly, right ascension and declination uniquely define the position of an object on the celestial sphere. 2. The declination of an object on the celestial sphere is its angle north or south of the celestial equator (a line on the celestial sphere directly above the Earth’s equator); the scale ranges from  90  to +90 . Figure 1.16a: Declination measures the angle of a star north or south of the celestial equator.

© 2007 Jones and Bartlett Publishers Celestial Coordinates 3. The right ascension of an object states its angle around the celestial sphere, measuring eastward from the vernal equinox (the location on the celestial equator where the Sun crosses it moving north). It is stated in hours, minutes, and seconds (with 24 hours encompassing the entire celestial equator). Figure 1.16b: Right ascension measures the angle around the celestial equator eastward from the vernal equinox.

© 2007 Jones and Bartlett Publishers Question 1 Record all the answers on a word document and when completed to Put your name and this class period in the subject If we could visit the other side of the Milky Way Galaxy would we see the same constellations as we see here on Earth? Why or why not?

© 2007 Jones and Bartlett Publishers Question 2 Why do they use angles to measure the distance between stars? Explain.

© 2007 Jones and Bartlett Publishers 1-3 The Sun’s Motion Across the Sky 1. The Sun seems to rise in the east and set in the west just like the rest of the stars. However, as time goes on, the Sun appears to move constantly eastward among the stars. 2. The time the Sun takes to return to the same place among the stars is about days.

© 2007 Jones and Bartlett Publishers The Ecliptic 1. The ecliptic is the apparent path of the Sun on the celestial sphere. 2. The zodiac is the band that lies 9° on either side of the ecliptic on the celestial sphere and contains the constellations through which the Sun passes. Figure 1.17: A map of the stars within 30 degrees of the equator.

© 2007 Jones and Bartlett Publishers The Sun and the Seasons 1. For an observer in the Northern Hemisphere, the Sun rises and sets farther north in the summer than in the winter. 2. The Sun is in the sky longer each day in summer than in winter. This is one of the reasons for seasonal differences. 3. In summer, the Sun reaches a point higher in the sky, than in winter. This results in each portion of the Earth’s surface receiving more energy in a given amount of time in the summer than in winter. Also, sunlight passes through more atmosphere in winter than in summer, resulting in more scattering and absorption in the atmosphere. 4. For an observer in the Southern Hemisphere the above explanation is backward.

© 2007 Jones and Bartlett Publishers Figure 1.19: The Sun's apparent path across the sky of the Northern Hemisphere in (a) December, (b) March or September, and (c) June.

© 2007 Jones and Bartlett Publishers 5. The distance of the Earth from the Sun does not vary too much during the year and thus is not a determining factor for the seasons. 6. The orientation of the Earth with respect to the Sun is the main reason for the seasons. 7. Altitude is the height of a celestial object (such as the Sun) measured as an angle above the horizon. 8. The summer and winter solstices are points on the celestial sphere where the Sun reaches its northernmost and southernmost positions, respectively. 9. The vernal and autumnal equinoxes are the points on the celestial sphere where the Sun crosses the celestial equator while moving north and south, respectively.

© 2007 Jones and Bartlett Publishers Historical Note: Leap Year and the Calendar 1. The tropical year ( days) determines the seasons and is the time the Sun takes to return to the vernal equinox. 2. The Julian calendar was 365 days long and added one day at the end of February every four years. Thus it had an average of days. 3. The difference between the tropical and Julian year caused the calendar to get out of synchronization with the seasons. The Gregorian calendar has an average of days. 4. The leap year rule: every year whose number is divisible by four is a leap year, except century years, unless they are divisible by 400.

© 2007 Jones and Bartlett Publishers Scientific Models 1. A scientific model is a theory that accounts for a set of observations in nature. 2. The idea that stars reside on a giant celestial sphere is a model. 3. A scientific model is not necessarily a physical model. 4. The Sun’s motion along the ecliptic can be explained by a geocentric model.

© 2007 Jones and Bartlett Publishers Question 3 Describe what the solstices and equinoxes have to do with the changing temperatures throughout the year.