- Physics 335
- Chapter 2: The
- Instructor: Dr. M.
- 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.
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.
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.
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
Sphere: This is the
imaginary huge clear crystalline hollow sphere surrounding the earth,
just as ancient Greeks and others before them thought it to
- 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
- 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
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
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
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
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
- 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
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Last Updated: 07/23/05