Constellations
STARRY, STARRY NIGHT ... Star Shapes Are Called Constellations. Some
of the most familiar constellations are shown in the image below. Click on the
image to see a larger version of the same image. The image is from sorossys.com
In this unit, students hopefully will learn about the night sky and begin to
recognize groups of stars called constellations. Various websites will access
students to star charts and evening observation highlights, while the text portion
of this unit will acquaint students with the way the night sky works. Here are
a few websites with hthe constellations so you can get a look at what is up
there:
Anyone who ventures out for a good look at a dark site away from city lights
cannot help but be amazed by all of the cosmic lights which light up the night
sky. They can find stars, planets, clusters of stars, fuzzy "clouds,"
and the occasional moving object or aurora display. Upon closer look, stars
appear to be grouped and form imaginary shapes just light daytime clouds look
like various animals and other objects. The ancients gave names to star groupings
like Orion the Hunter, Taurus the Bull, Cygnus the Swan, and Ursa Major the
Big Bear. They became familiar friends when navigating the open seas, planting
spring crops, and preparing for festivals. While these star patterns did not
change their shape, they were not always in the same place throughout the year,
and through careful study, early astronomers began to watch the motion of the
stars and make sense of the Universe.
Stars are grouped into various groupings according to the patterns that they
appear in the sky. A constellation is one of the 88 historically named groups
of stars. There are 44 major constellations visible in the sky north of the
equator, and another 44 in the southern sky. An asterism is a pattern of stars
(e.g., the Big Dipper) that is not one of the original 88. Stars shine by the
light they give off and in most cases the stars will remain with their pattern
partners throughout your lifetime. The planets of our solar system shine by
reflected light and move on a daily basis, some more noticeably than others.
They appear to wander in and out of different constellations, and, in fact,
the name "planet" comes from a root word meaning "wandering star."
Some of the differences between planets and stars are:
1. Stars twinkle because Earth's atmosphere is changing, while planets generally
do not twinkle because they are closer to us and appear "larger" in
the sky.
2. Planets, if you can find them with the naked eye, must always be on the ecliptic
plane ... the imaginary line traced by the sun in the sky. Stars will be anywhere
in the sky, including the ecliptic plane.
3. The planets you can see with your naked eye are Mercury, Venus, Mars, Jupiter,
Saturn, and of course, Earth. Uranus and Neptune require good binoculars, and
Pluto requires a fairly large telescope. A person with 20/20 vision sitting
on a chair under a moonless and dark night sky can perhaps see over 3,000 stars!
This may seem like a big number, but there may be almost 200,000,000,000 stars
in our galaxy alone, and there may be almost 100,000,000,000 galaxies. We really
do not see too many stars.
4. Planets "move" slowly against the background constellations due
to their various orbital velocities around the sun, while stars appear to not
move at all. In actuality, stars do move, and some pretty fast, but because
they are so incredibly far away, we do not notice their motion even during your
lifetime.
STAR LIGHT, STAR BRIGHT ... Measuring Brightness
When you look out at the night sky, you are seeing stars which are relatively
close, and others which are very far away. Some stars are very bright while
others appear to be quite dim. The difference in brightness is deceiving because
a bright star very far away will appear to be more dim than a dim star which
is very close. The 2nd century B.C. astronomer Hipparchus
defined a magnitude scale of brightness to help others compare one star to another.
He grouped stars into six categories, according to perceived brightness. Magnitude
1 stars were the brightest according to Hipparchus, and he called these "first-class
stars." The "second-class stars" were given a Magnitude of 2,
and the most faint stars Magnitude 6. Like golf where the lowest score wins,
the smaller number means the star is brighter. You actually will be thinking
of magnitude as measuring "faintness." According to Hipparchus, a
star of magnitude 2 was halfway between a magnitude 1 star and a magnitude 3
star.
The system developed by Hipparchus was used for 1600 years with little or
no changes. However, astronomers soon found a few stars brighter than magnitude
1 and with the invention of a telescope found many stars fainter than magnitude
6. To correct for the new findings, the scale was extended beyond magnitude
6 to describe stars invisible to the naked eye, and the opposite direction (0
or even negative numbers) to describe the brightest stars.
Furthermore, Hipparchus used his own eye to be the judge of magnitudes, but
electronic instruments made the magnitude system more precise and less prone
to errors of different vision abilities. These instruments found that a difference
in 5 magnitudes corresponds to a factor of 100 in brightness. In other words,
a magnitude 1 star is 100 times more bright than a magnitude 6 star. A difference
of 1 in magnitude means a factor of 2.512 in brightness (2.512 is the fifth
root of 100). A star of magnitude 5 is 2.512 times brighter than a magnitude
6 star. A star of magnitude 4 is 2.512e2 times (or 6.3) times brighter than
a magnitude 6 star because it is 2 magnitudes brighter. A star of magnitude
3 is 2.512e3 times (or 15.85) times brighter because it is 3 magnitudes brighter,
and so forth. Therefore, when you look at stars, notice that they are different
in their brightness.
The
sun appears to rise from the east and set in the west because the earth rotates
counterclockwise in 24 hours. This rotation of the earth causes the sun to appear
to move in the sky, and the path which the sun takes through the sky is called
the ecliptic. If you were to trace the sun's path during the complete 24 hour
day, it would move through 12 different constellations, of which 6 are visible
during the night and 6 are hidden by the sun's glare during the day. These 12
constellations make up the Zodiac, and all sorts of ideas have abounded over
the years as to the importance of the sun's position in the zodiac on particular
dates. Indeed, the astrologers developed their horoscopes on the basis of the
location of celestial objects in the Zodiac on various days of the year.
The planets are revolving around the sun in orbits very close to this ecliptic
because their orbits are very closely aligned with earth's orbital plane (with
the exception of Pluto). This means that you can often find planets within the
constellations of the Zodiac.
Beyond the basic constellations of the ecliptic, early astronomers gave names
to others which were above or below the sun's path. While the ancient Babylonians
gave many of the names to the original constellations which lie in the Zodiac,
most now have Latin names with which we are more familiar. However, many of
the stars still retain their Babylonian names, such as Aldeberan, Antares, Saiph,
and so forth.
The ancients gave names to the major constellations which they could see,
and later, as mariners returned from journeys south of the equator, others were
named. With the advent of telescopes and a need to make a better sky coordinate
system, more minor constellations were named which are not as bright as the
major groups, but help astronomers find their way in the dark.
To look at a complete map of the night sky, or individual constellations click
on the image at left. In addition to finding some nice manners to manipulate
the sky charts with a virtual telescope, this site also will let you look at
the earth from a space telescope, watch the changing of lunar phases, and even
watch the fall of night on the rotating earth. You can enter your city from
anywhere in the world and make an accurate map of the constellations as they
appear at any moment. Try not to stay away from the course too long though,
since better things are yet to come. Hopefully, after a few visits outside at
night with a chart, you can come to recognize the constellations yourself.
NOW WHAT? Get Outside And Look At The Stars ... The Assignment is found
at: First
Observation. What is written below are some general introductory words about
the first observation for this course.
Your assignment is
to observe
constellations. Within those pages you will find a link
to a monthly starchart where you can print out a chart for the month you
are doing this assignment. There are some constellations in the north sky that
you can see at some time during any night of the year. These are the "circumpolar"
constellations. You will always be able to find the circumpolar constellations.
All of the other constellations are seasonal. For instance, during the summer
months, the sun moves across Gemini in the daytime, and Sagittarius is visible
at night. During the winter months, the sun moves across Sagittarius, and Gemini
is visible at night. The constellations are always present, but sometimes they
are hidden in the sun's glare depending on the season.
First,
it would be a good idea to get a nice night sky chart to help you find things.
Afterall, the night sky does not have yellow lines connecting constellation
stars, nor are the star names labeled up there for you to see. To make matters
worse, you will sometime have trouble recognizing constellations in terms of
name and appearance. Please click on the image to get to the constellation
website and then print out a chart. The same site is are accessed from the
the icon and the highlighted words. If you do not have a printer, send me an
e-mail and I will get one to you ASAP.
You want to see something really cool
... click on the image of the star chart at the top of this page.
I would like you to go outside on a dark night and try
to observe
as many constellations as possible. Try to find circumpolar and zodiacal
constellations. Try to find asterisms as well as constellations. Learn how to
recognize obvious constellations by their shapes, and laugh as you wonder how
the ancients named some constellations after animals and people when the star
groupings do not look like that at all. Try to compare the location of particular
constellations one month at a given time with the location of the same constellations
at a different month. How have they moved, and at what time did you find them
at a specific point? What causes this apparent motion?
After
you have finished this part of the course, move to a site
which teaches important information about the celestial
sphere and how we mark the positions of celestial objects, or you can go
back to the Introduction to the Starry
Sky.
The
sun appears to rise from the east and set in the west because the earth rotates
counterclockwise in 24 hours. This rotation of the earth causes the sun to appear
to move in the sky, and the path which the sun takes through the sky is called
the ecliptic. If you were to trace the sun's path during the complete 24 hour
day, it would move through 12 different constellations, of which 6 are visible
during the night and 6 are hidden by the sun's glare during the day. These 12
constellations make up the Zodiac, and all sorts of ideas have abounded over
the years as to the importance of the sun's position in the zodiac on particular
dates. Indeed, the astrologers developed their horoscopes on the basis of the
location of celestial objects in the Zodiac on various days of the year.