Physics 335 Astronomy
Chapter 2: The Sky
Instructor: Dr. M. Azad Islam
In chapter 1, we studied the incredible scales of distances. From campus scene of 50 ft. to village scene, parts of New York State to the earth, solar system, galaxy like our own called Milky Way (contains over 100 billions of stars). Each of the billions of galaxies contains billions of stars. Group of galaxies called Cluster (example, Local Group of which Milky Way is one) and, cluster of clusters called Supercluster. Super Clusters are linked with filament or string like long structures in presence of what seemed voids (empty space). These structures made of Superclusters seemed to be the largest structures of the known universe.

In this chapter, we must answer questions like, "Why the sky looks the way it is?"

Day and night caused by the rotation of the earth.

Earth rotates eastward (from west to east). Sun (at the center of earth's orbit) rises in the east and sets in the west. Sun is a star and all stars behave the same way. All the stars appear in the eastern horizon and set in the western horizon. Summer and winter seasons caused by earth's tilting (at 23.5 degrees) of its axis joining the north and south poles.

Constellations: There are 88 of them covering the entire celestial sphere, that is 44 constellations in each hemisphere. In many of the constellations, the sky contains a number of less formally defined groupings called Asterism (Catchy Figures). The Big Dipper as a part of the constellation Ursa Major is a well-known Asterism.
Names of the Stars: Whereas the names of the constellations are mostly Latin, the names of the stars are mostly Arabic. Betelgeuse in the Orion comes from "hand of Jawza (Orion)" from the Arabic phrase "yad al-jawza." Aldebaran, meaning the follower (of the Pleiades), which is the bright red eye of the Taurus the bull and Fomalhaut "mouth of the fish." Such naming served the purpose of stories built upon them for the bright stars. We will soon run out of names trying to give individual star a name, because there are billions of them. Another way to name was to identify the constellation in which the star is present and use sequential Greek letters to name the bright ones. For example alpha for the brightest and beta the next bright and so on.

Brightness of Stars: From naked eye observation of the brightness of stars, Hipparchus in about 2nd century B.C. classified them into 6 classes. The fainter stars have greater number for their magnitude. There are 21 first magnitude brightest stars in the sky, all except 3 have proper names of their own. These are measured as apparent brightness as measured from the earth, without consideration for their distance. In the recent refined scale the brightest of all bright stars, known as Sirius, has a magnitude of -1.47.

The modern scale is a variant of Hipparchus idea. It is defined the following way:

A difference of 5 visual magnitudes = factor of 100 x difference in brightness.

Stars that differ by 1 magnitude differ by a brightness factor of 2.512 (2.5 is usually close enough). More technically we should speak of the intensity for a given wavelength or wavelength range. For now it is sufficient to call it brightness. So that it is easily understood by all. We will continue to use this terminology until circumstances require more precise language.

The Celestial Sphere: This is the imaginary huge clear crystalline hollow sphere surrounding the earth, just as ancient Greeks and others before them thought it to be.

Angles on the sky: Separation of two stars by angle, is a measure of location. Angles in degrees, minutes of arc and seconds of arc are used.
60 minutes of arc = 1 degree
60 seconds of arc = 1 minute of arc
Sun and moon have diameters that measure an angle of 0.5 degree.
The two pointer stars in the bowl of the Big Dipper is about 5 degrees apart.
The bowl of the Big Dipper is about 30 degrees from the North Star.

REFERENCE MARKS ON THE SPHERE: North and south pole axis of the earth, is also the north and south pole axis of the Celestial sphere. This axis lies on the Celestial north pole directly above the earth's north pole, and Celestial south pole directly below the earth's south pole. The North star, also called "Polaris" is located almost exactly on the axis of the Celestial sphere. The circumference of the Celestial sphere in the sky directly above the earth's equator is called the Celestial equator. The Celestial Equator divides the sky into North and South hemispheres just as earth's equator divides northern and southern hemispheres.

Like the Celestial equator, the latitudes in the sky are exactly the same as those directly above the earth's latitudes.

An observer at any latitude will ideally observe, half of the celestial sphere of the sky overhead at any time. The ground level of which is seen as a circumference, and the immediate neighborhood above it is called the Horizon. The circumference of the horizon at the ground level, always intersects the Celestial equator at the exact east point and the exact west point of the horizon.

During night time navigation, sailors locate the North Star (also called Polaris) using the Pointers in the outer bowl of the Big Dipper. If you look up and face the Polaris and then drop down a line from it to the horizon, you reach the exact north point of the horizon. By measuring the angle between Pole star and the north point of the horizon you find the latitude at the point of observation.

The System of coordinates described by the Celestial equator and latitudes corresponding to earth's, is widely used as Celestial Coordinate System.

SOME STARS ARE ALWAYS VISIBLE IN THE SKY. Those stars that are always visible in the north
hemisphere are called the North Circumpolar Stars. Among them are Ursa Major, Ursa Minor, Cepheus, Cassiopeia and Perseus. These stars rotate around the Celestial north pole. There are similar stars in the south hemisphere called South Circumpolar Stars, seen to rotate around the south Celestial pole.
This chapter focuses on the appearance of the night sky. Many of the concepts presented were common knowledge before time became quantified on clocks and watches, and city lights blocked our nightly view of the sky. The concept of a scientific model should be stressed in this chapter. The concepts of model, theory, and hypothesis are used consistently and coherently throughout the book, and model is defined and discussed in section 2-2. If you are going to have students make outdoor observations, stress the measurement of angles in the sky. The cause of the seasons is one of the most basic concepts that is most misunderstood. Section 2-3 discusses the seasons. This is a good section to work on developing the skill of three dimensional visualization. Most students have trouble understanding the seasons and precession, that stems from not being able to understand the 3-D figures or to picture them. Section 2-5 presents a good example of how science is done


Last Updated: 07/23/05