CHAPTER 1

The concept of scale is important and you should spend an appropriate amount of time getting comfortable with it. If you can get access to some of the objects described in the text to show the scale of the universe then use them; the relationships are more meaningful when viewed in three dimensions than when described in a textbook. If you cannot get access to them then you should try to picture the objects rather than memorize the scale. Even better, in order to better visualize the scale of the solar system and then of the universe, it is worthwhile to make a replica like the one suggested as an activity at the end of this chapter.

The following two sites include good animations related to the collision of comet Shoemaker-Levy 9 with Jupiter and to "Powers of 10," respectively:

1) http://www.jpl.nasa.gov/sl9/sl9.html

2) http://micro.magnet.fsu.edu/primer/java/scienceopticsu/powersof10/

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.

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.

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.

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°.

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°.

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).

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 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.

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.

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.

Advancing the Model: 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 356.2425 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 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.

1-4 The Moon’s Phases

1 The rotation and revolution period of the Moon are equal and can be explained by the law of universal gravitation.

2 Rotation is the spinning of an object about an axis that passes through it.

3 Revolution is the orbiting of one object around another.

4 Phases of the Moon—the changing appearance of the Moon during its cycle—are caused by the relative positions of the Earth, Moon, and Sun.

5 The phases follow the sequence of waxing crescent, first quarter, waxing gibbous, full Moon, waning gibbous, third (or last) quarter, waning crescent, new Moon.

6 Elongation is the angle of the Moon (or planet) from the Sun in the sky.

7 A sidereal revolution of the Moon takes about 27 1/3 days.

8 A sidereal period is the amount of time required for one revolution (or rotation) of a celestial object with respect to the distant stars.

9 A synodic period is the time interval between successive similar alignments of a celestial object with respect to the Sun.

10 A synodic revolution of the Moon takes about 29 1/2 days.

11 Lunar month is the Moon’s synodic period, or the time between successive phases: 29d12h44m2s.

1-5 Lunar Eclipses

1 Lunar eclipse is an eclipse in which the Moon passes into the shadow of the Earth.

2 Umbra is the portion of a shadow that receives no direct light from the light source.

3 Penumbra is the portion of a shadow that receives direct light from only part of the light source.

4 Eclipse season is a time of the year during which a solar or lunar eclipse is possible.

5 A lunar eclipse does not occur at each full Moon because the Moon’s plane of revolution is tilted 5° compared to the Earth’s plane of revolution around the Sun. Only during the two eclipse seasons that occur each year are the Earth and Moon positioned so that the Moon will enter the Earth’s shadow during a full Moon.

Types of Lunar Eclipses

1 Penumbral lunar eclipse is an eclipse of the Moon in which the Moon passes through the Earth’s penumbra but not through its umbra.

2 Total lunar eclipse is an eclipse of the Moon in which the Moon is completely in the umbra of the Earth’s shadow.

3 Partial lunar eclipse is an eclipse of the Moon in which only part of the Moon passes through the umbra of the Earth’s shadow.

4 A total eclipse of the Moon is never totally dark because some light is refracted toward the Moon by the Earth’s atmosphere. Most of this refracted light reaching the Moon is red; the blue portion has been scattered out.

1-6 Solar Eclipses

1 Solar eclipse is an eclipse of the Sun in which light from the Sun is blocked by the Moon.

2 Total solar eclipse is an eclipse in which light from the normally visible portion of the Sun (the photosphere) is completely blocked by the Moon.

3 The corona—the outer atmosphere of the Sun—is visible during a total solar eclipse.

The Partial Solar Eclipse

1 In a partial solar eclipse only part of the Sun’s disk is covered by the Moon.

The Annular Eclipse

1 An annular eclipse is an eclipse in which the Moon is too far from the Earth for its disk to cover that of the Sun completely, so the outer edge of the Sun is seen as a ring or annulus.

1-7 Observations of Planetary Motion

1 Five planets are visible to the naked eye: Mercury, Venus, Mars, Jupiter, Saturn.

2 The planets lack the simple, uniform motion of the Sun and Moon. They sometimes stop their eastward motion among the stars and move westward for a while. This is called retrograde motion.

3 The planets always stay near the ecliptic. In addition, Mercury and Venus never appear very far from the position of the Sun in the sky. Thus their elongation (the angle in the sky from an object to the Sun) is small.

4 Any model for the planets must explain these observations.

1-8 Rotations

1 Solar day is the amount of time that elapses between successive passages of the Sun across the meridian.

2 Meridian is an imaginary line that runs from north to south, passing through the observer’s zenith.

3 Sidereal day is the amount of time that passes between successive passages of a given star across the meridian.

4 The Earth’s solar day and sidereal day differ by about 4 minutes.

1-9 Units of Distance in Astronomy

1 To measure distances in a planetary system such as ours we use the astronomical unit (AU), which is the average distance between Earth and Sun.

2 For greater distances we use the light-year, the distance light travels in one year.

1-10 The Scale of the Universe

1 If we represent the Sun by a basketball, the Earth is a head of a pin 84 feet away. The entire solar system inside Pluto’s orbit occupies an area of about

2 2 miles in diameter with a basketball-size Sun at the center.

3 The nearest star other than the Sun is about 4460 miles away.

4 The diameter of the Galaxy on this scale would be about 164,000,000 miles

Tools of Astronomy: Powers of Ten

The powers-of-ten notation is a powerful tool in describing the immense distances and scales involved in the universe.

1 Advances in our knowledge of the universe are being made every day, with new telescopes and satellites bringing us new information about planets, stars and galaxies.

2 We have entered the era of precision cosmology, and we are now able to seriously address the questions about the origin and fate of the universe.

3 However, whether or not you should support this quest for knowledge depends on what you feel about the value of a pure science such as astronomy.

After studying the material in this chapter you should be able to:

1 Identify the basic classes of objects seen when looking at the night sky.

2 Explain the concept of the celestial sphere and the celestial poles.

3 Be aware that constellation patterns are of an arbitrary nature.

4 Have a working knowledge of the units of angular distance and the conversions between degrees, arcminutes, and arcseconds.

5 Describe the basis for the seasons on Earth.

6 Describe the basis for our calendar.

7 Give a demonstration that illustrates the phases of the Moon as it relates to the Moon’s orbital position relative to the Sun and Earth.

8 Draw a diagram illustrating a lunar eclipse.

9 Explain why there is not a lunar eclipse every month.

10 Draw a diagram illustrating a solar eclipse.

11 Describe the observed motions of the planets (including retrograde motion) that must be explained by a scientific model explaining planetary motion.

12 Form a mental picture of a scale model of the solar system.

13 Be able to extend this mental model to incorporate galaxies.

14 Understand that astronomy is a pure rather than applied science and be able to explain what this means.