Understanding the Seasons
& Its Measurement
& Lunar Eclipses
Astronomy is one of the few sciences that allows
for active participation by the nonprofessional. The night sky is accessible
to most individuals, and it is here that an enjoyable start can be made
to comprehend the beauty, as well as the history of this oldest science.
Unfortunately, in many urban regions the glow of civilization has caused
a dimming of our senses toward nature and a decrease in our enthusiasm
to look skyward and enjoy the majesty of this natural phenomenon.
The ability of humans to realize that the rhythmic
frequencies of the heavens could serve as an accurate time monitoring
device allowed for the establishment of agriculture on a more orderly
basis, a division of labor among the populace, the functioning of governments,
and the eventual blossoming of our technological society. The consistency
of the sky, however, has had even greater ramifications than just our
own personal refinement. Philosophically, it is our link to the past,
for everyone who has lived within the time frame of recorded history
has gazed upon the same stars and planets that we can observe today.
That is an amazing amount of time by human standards, and because the
sky changes so gradually, the stars and their patterns will remain a
unifying element to the history of humankind tens of thousands of years
into the future.
The ancients organized the starry firmament
into a series of regions -- constellations -- representing creatures,
deities, and inanimate objects. Knowledge of some of these figures date
back almost 5000 years. Many of these divisions were arbitrary in nature.
Areas of the sky were marked off to honor a special deity; then attempts
were made to produce a representative picture artificially. Even with
this capricious attitude, an amazing number of constellations visible
to northern hemispheric observers bear striking resemblances to the
figures which they are supposed to portray.
And, of course, there was great entertainment
value in the sky. Consider the starry firmaments as the original TV
set, simple to operate, and requiring no electricity. It had one channel,
could only be seen on clear nights, and its program took one year to
cycle through in its entirety. Yet its story potential seemed endless
as witnessed by the continuation of the use of mythologies today. Consider
the popular television programs of Hercules and Xena (Xena
is not a constellation) or movies such as Clash of the Titans,
and it is obvious that good stories endure.
Unfortunately, the same cannot be said for
the southern sky. These regions were unknown to the ancient civilizations
and were first delineated by European explorers beginning in the sixteenth
century. Many of the star figures deal with items familiar to the technology
of that era, such as furnaces (Fornax), compasses (Circinus), air pumps
(Antlia), pendulum clocks (Horologium), and telescopes (Telescopium)
among others. Needless to say, the people who originated these star
patterns did not possess the same richness of imagination as their predecessors,
and this may be, in part, one of the reasons why the southern sky does
not hold the same appeal as its northern counterpart.
The most important group of constellations
known to the ancients were those through which the seven principal deities
moved: the sun, the moon, and the five naked eye planets. The Greeks
named this region the zodiac (circle of animals), and it was most likely
the first zone of the sky to be formalized. Astrology had its origin
with the movements of the sun, moon, and planets within this tract,
and much of the drama of western mythology, as it is related to the
sky, was formulated around these objects.
Eighty-eight constellation boundaries encompassing
both hemispheres were officially sanctioned in 1928 by the International
Astronomical Union, a worldwide congress of professional astronomers.
All of the stars within a particular boundary are members of the same
constellation, regardless of whether or not they are incorporated into
forming the pattern. The constellation pictures have not been standardized
which has led to a great deal of variations, depending upon the source.
Most modern star atlases simply ignore the problem by only representing
the boundaries and not the figures themselves.
To further complicate matters, there are numerous
star groupings which are considered to be constellations by the general
public, but are not recognized as such by the professional community.
The pattern most frequently mistaken for a constellation by Americans
is the Big Dipper. It is, in essence, composed of the brightest members
of the Great Bear, the constellation of Ursa Major. Such unofficial
groupings of stars are termed asterisms. The Little Dipper and
the Pleiades are two other examples of well-known asterisms often mistaken
The reason for standardizing the constellation
boundaries has not stemmed from any incentive to memorialize their ancient
lore, but rather as a means of rapidly identifying the region of the
sky in which an object of interest is located. Since the constellation
regions are large with respect to the position of individual objects,
there exist more exact methods for accurately positioning a telescope
so that an observer can identify a particular star or object. The primary
coordinate system which is used by astronomers is termed right ascension
and declination. It is earth's grid system, longitude and latitude,
projected into the sky.
Right ascension and declination allow for the
precise location of everything in the heavens, and, at the very least,
the numerical designation or naming of any celestial object. An example
of this might be the location of a star at RA 023150.4; Dec +891551,
better known to most individuals as the North Star or Polaris. Brighter
stars, such as the Polestar, have proper names, usually Arabic translations,
coming originally from the Greek language. There are also Greek and
Latin exceptions, and Polaris happens to be one of them. It is an abbreviation
from the medieval Latin, Stella Polaris or Pole Star.
With the ascendancy of Islam during the seventh
and eighth centuries AD, and its near conquest of Europe, many of the
Arabic translations were brought into more common usage by western medieval
astronomers. These names have been unofficially adopted by professional
and amateur observers. The brightest stars in any one particular constellation
also have Greek letter designations as well as Arabic numerals. This
really represents a combination of nomenclature systems.
The earliest identification system utilized
upper case letters of the Greek alphabet introduced by Johannes Bayer
(died 1625) in 1603. With few exceptions, stars received letters in
order of descending brightness, followed by the genitive form of the
constellation. Polaris, the brightest star in Ursa Minor, the little
bear, was designated as alpha Ursa Minoris in Bayer's scheme. Once the
24 characters of the Greek alphabet were exhausted, Bayer repeated the
procedure using first lower case letters, then the upper case letters
of the Roman alphabet. John Flamsteed (1646-1719) devised an alternate
method by which the positions of stars of any constellation were numbered,
starting from their westernmost boundaries, in order of their increasing
eastward position or right ascension. Today, it is common practice to
list the brightest stars of a constellation with Bayer's Greek letters
(lower case) and the fainter luminaries by their Flamsteed numbers.
This can be seen in the figure of Orion, the hunter to the left.
All of this terminology can become rather confusing,
especially when one considers the variety of other objects such as variable
stars, galaxies, and clusters which must be properly symbolized and
identified on star charts. For the purpose of Sky Literacy
this extra information has been kept to a minimum.
The star maps selected for this unit contain
only the brightest stars which are visible from an urban location, away
from the direct view of distracting streetlights. The different sized
dots are representative of the various magnitudes or brightnesses of
the stars. Characteristic stars are designated with numbers on each
chart. It should be noted that fainter stars have more positive magnitudes
than the brighter stars. A star of the third magnitude is about 2-1/2
times fainter than a second magnitude star, which is about 2-1/2 times
fainter than a first magnitude star. The change in light intensity between
a first magnitude star and a sixth magnitude star, a difference of five
magnitudes, is exactly 100. The average person can observe stars of
the sixth magnitude in an environment which is not light polluted. In
Sky Literacy, essentially all of the stars plotted on
the charts are brighter than fifth magnitude. Each chart is also provided
with an approximate scale in degrees. For easy conversion to the sky,
a 12-inch ruler held at arm's length is about 30 degrees in length.
For the moment, let us return to the nighttime
sky and discover for ourselves the beauty and lore of a few prominent
constellations, as well as some of the names of their brightest members.
What others might be tempted to call the mysterious may then be revealed
to be the same source of simple amusement and joy that once motivated
earlier groups of skywatchers to look up and marvel.