1. Our Place in the Universe

Reading: Chapter 1, especially pages 1-20

This is an exciting era to be studying astronomy - new technologies for ground-based telescopes and telescopes in space are providing new information about the universe - from the neighboring planets to the most distant galaxies. You are about to embark on a journey to explore these exciting places.

The first chapter of the book aims to give an introduction to astronomy - both ancient and modern - and discuss the cycles of the sky that have been observed since ancient times. It is a long chapter that is particularly full of detailed material. We will make our way through the chapter, but bear in mind that subsequent chapters are somewhat less 'dense'.

What Is Astronomy?

First things first - we need to think about the very basic issues of what is astronomy and what are the different types of objects we are going to study.

(1) What sorts of questions do you think astronomers/astrophysicists attempt to answer? (2) Fill in each blank with the most appropriate word from the list below. Our sun is a _______________ and belongs to a galaxy called the ________________ . The Sun is just one of billions of ________________ that make up the galaxy. Our galaxy is one of about two dozen galaxies that make up a _______________, known as the ___________________. In turn, these larger groups of galaxies are linked to form ____________________, which may be the largest structures in the _________________. In addition to containing so many stars, most galaxies are located millions, if not billions of ______________ away. When we study these objects , we must consider these great distances because it takes _______________ for the light from these distant objects to reach Earth. Thus, when we see these objects, we do not see them as they are but as they were ___________ or ____________ of years ago.   billions millions star stars cluster supercluster filaments galaxy galaxies thousands Milky Way universe light Local Group time light years planets black hole

The Obvious View

First of all, take the time to go check out the "Obvious View" of the night sky outside. This is an astronomy course, after all. It is an opportunity to learn your way around the sky and become familiar with some of the constellations. Once you know a handful of constellations you can often pick out one or two out even if the sky is partially blocked by clouds or city lights.

(3a) Go outside and find Polaris and the celestial pole in the night sky. The easiest way is to look roughly north and use the 2 "pointer stars" of the Big Dipper. (Click on the diagram for a larger view) (b) Point with your left arm to Polaris. Make a right angle to your left arm with your right arm. Your right arm is now pointing to somewhere on the celestial equator. Keeping your left arm pointing to Polaris and your right arm at a rightangle, rotate about your left arm--your right arm will be sweeping out the celestial equator. (c) Point to the south celestial pole. (d) Look for a bright star (preferable 2 or 3 different ones) that is fairly close to the horizon or an object on the skyline--such as the roof of a house or a tree or a lampost. Note where you are standing, where the star is located and the time. Come back in about 1 hour and note how the stars have moved.

The Celestial Sphere

The celestial sphere is an imaginary sphere that we use to help visualize the motion of celestial bodies in the entire sky. When we look out into the sky, there is nothing to tell us how far the stars, the planets, the Sun, or the Moon are from us. Thus it is reasonable to suppose that they all lie at the same distance from us on the surface of transparent sphere, with the Earth at its center. Although we no longer believe that the celestial sphere is a real entity, it is still a useful tool for thinking about how the sky moves in relation to the Earth.

Let's start with a celestial sphere and the Earth as in Figure 1.7. If we extend the North and South poles of the Earth outward until they intersected the sphere, the intersection points are defined as the North Celestial Pole and the South Celestial Pole. These always point back to the North and South poles on the Earth. Thus if you were standing at the North Pole and looked straight up, you would be looking at the North Celestial Pole in the sky. If you were at the South Pole, you would see the South Celestial Pole directly overhead. Similarly if we took the Earth's equator and projected it outward until we get to the celestial sphere, we would end up with a ring called the Celestial Equator. If you were at the equator, the Celestial Equator would be directly overhead in the sky at all times. The star Polaris is located very close to the North Celestial Pole. As a result, as the Earth rotates about its axis in its diurnal cycle, all the stars appear to rotate about us in the sky except for Polaris which stays fixed in its position near the North Celestial Pole.

Angles

Astronomy involves thinking about angles--often very tiny, tiny angles. A circle is divided into 360 degrees (or 360°). Each degree is divided into 60 arcminutes (or 60'). Each arcminute can be divided into 60 arcseconds (or 60"). Therefore there are 360x60x60 = 1,296,000 ~ 1.3 x 10⁶ arcminutes in a full circle. Yes, this seems a crazy system and not very `metric'--it goes back to the Babylonians who were hung up on the number 60. No one has been bothered to convince everyone else to make a better system. Try these exercises to get a feel for these angles.

(4) Hold your hand out at arms length (preferably against the sky towards the Big Dipper at night). Think of the long, skinny triangle made by the width of your fist and your eye. The width of your fist subtends and angle of 10° at your eye. The Big Dipper is about 20° (two fists at arms length) from end to end in the sky. (5) Now hold up a finger at arms length. The width of your finger subtends an angle of about 2° at your eye. So one degree would be about 1/2 a finger (or about the width of your little finger nail). Imagine dividing that angle into 60 arcminutes--or 3600 arcseconds--tiny angle, eh? (6) Think of a place (shop, house, landmark, etc.;) that is about 2.5 miles (4 kilometers) away. Imagine someone holding up a penny there. If you could see that penny 4 kilometers away, the angle the penny subtends at your eye is one arcsecond. This is the sort of tiny angle that astronomers like to work with.

Daily (or Diurnal) Motion

(7) Go back outside and note the motion of the stars. When you are facing Polaris, which way do the stars appear to go around the celestial north pole? (8) Face (a) east, (b) south, (c) west--in each case, describe the way the stars move over the night.

The Earth spins on its north-south axis which gives us day and night. If all the stars were fixed on the celestial sphere, then as the Earth spun, we would see that during different times of the day, we would see different parts of the sky or different parts of the celestial sphere. This is illustrated on the right (B) below.

Another way to look at this is if we assumed the Earth was stationary and if we have the celestial sphere rotating about the axis defined by the North and South Celestial Poles. This is in fact what we apparently see when we go outside and view the heavens. We appear to stand on an immobile Earth while all the stars and the Sun rotate around us. Thus the diurnal motion of day and night can be imagined to be due to the rotation of the celestial sphere around the Earth. This is illustrated on the left (A) below.

It is the apparent rotation of the celestial sphere that results in the Sun and stars rising in the east and setting in the west.

How do we KNOW that the apparent motion of the stars is due to the Earth spinning? The space-age view of the Earth as a "spinning blue marble" make this obvious. But before the space age it was not so clear. There was little concrete evidence to distinguish which of the following views was reality. A clever way of actually demonstrating the Earth's spin was devised by the French physicist Foucault who hung a huge pendulum that kept swinging for several hours. The pendulum keeps swinging to and fro in its initial direction and the Earth spins underneath. There is a Foucault pendulum in the Gamow Tower (opposite the stadium, across the road from the Ralphie statue) on the CU Boulder campus.

Apparent Motion of the Stars

This link shows you the apparent motion of the stars as viewed from a northern latitude location, at the equator, and a southern latitude location. For each location, we see the motion of the stars in the four cardinal directions: North, East, South, and West. Each animation shows you a winter sky through an entire day, with the Sun and Moon "turned off" so you can watch the stars move.

Constellations

The constellations of western cultures have their origins in Mesopotamia in the 3rd millennium BC. If you divide up the sky into constellations, you would see different constellations at night at different times of the year. The ancient Mesopotamians also realized that the Sun would appear in different constellations throughout the year, even though the constellations themselves would not be visible because it would be daytime when the Sun was up. (The path that the Sun appears to move amidst the stars is called the ecliptic.)

To learn more about constellations check out these web sites: The Constellations and Their Stars; The Constellations; Constellation Pronunciation Guide; The Constellations for maps and info, and a program to generate on-line star maps; Constellations article from the Astronomical Society of the Pacific; Aboriginal Star Knowledge for Native American astronomy; Ancient Astronomical Cosmology web site at Pomona University. This site contains information on how the ancients of many different cultures viewed the skies. Astronomy in Japan with information for both the history of astronomy in Japan as well as astronomical activities today. Inventing the Solar System, an essay on early Greek cosmology. The Maya Astronomy Page.

The Zodiac

The Sun doesn't wander all over the sky, but travels during the course of the year through a set of 12 constellations that make up the zodiac. You can see a picture of it here. In this illustration, it is March, and a person standing on the Earth will be able to see Leo in the nighttime sky (the side facing away from the Sun), and the Sun will be in Aquarius. If it was September, Aquarius would be in the nighttime sky and the Sun would be in Leo.

The ancient astrologers actually divided the zodiac into twelve equal sections called "Houses." The constellations are of different sizes in the sky, and if you went by where their boundaries are, it turns out the Sun would spend a week or two in some constellations of the zodiac, while it would spend more than a month in others. By making equal sized houses, they gave each constellation "equal time."

Model answers to the comprehension questions.