© 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

© 2007 Jones and Bartlett Publishers

Chapter 1Sections 1-1 thru1-3

The Quest Ahead

Courtesy of H

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omet Team

and NA

SA

Courtesy of S

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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.


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

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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.


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


Constellations1. 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.


Measuring the Positions of Celestial Objects1. 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


Celestial Coordinates1. 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.


Celestial Coordinates3. 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.


Question 1

Record all the answers on a word document and when completed e-mail to [email protected]. Put your name and this class period in the subject line

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?

mailto:[email protected]


Question 2

Why do they use angles to measure the distance between stars? Explain.


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 365.25 days.


The Ecliptic1. 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.


The Sun and the Seasons1. 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.


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.


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.


Historical Note: Leap Year and the Calendar

1. The tropical year (365.242190 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 365.25 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 365.2425 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.


Scientific Models1. 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.


Question 3

Describe what the solstices and equinoxes have to do with the changing temperatures throughout the year.

Documents

© 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