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Laboratory 1, The Night Sky and an Introduction to the Constellations
The purpose of this laboratory is to acquaint your self with the night sky: to learn how to
find various objects, identify some constellations and bright stars by using the
Planisphere and celestial charts, to understand how the night sky changes with time and
to learn something about the history of the naming of the constellations.
In your report include date, time, place and lab partners with whom you were working.
Introduction to the history of the constellations
Since time immemorial people all over the world have studied the sky and given
names to certain groupings of stars. Small groups of stars that are close together are
called “asterisms” (astron is Greek for star). Examples are: the Pleiades, also called the
seven sisters, a cluster of six bright stars ( about 200 in a telescope) known all over the
world but now counted as part of the larger group called the constellation Taurus (the
Bull) and the asterism “the Big Dipper”, the seven brightest stars in the larger group
called Ursa Major (the Big Bear). Western astronomy recognizes relatively few such
asterisms and from early times on gave names to rather large groups of stars called
“constellations” (con means together, with, and stella means star in Latin). The stars in
such constellations form a pattern that often does not much resemble the name given to it,
though H.A.Rey, the author of “the Stars” and children books like Curious George, found
original ways to connect the stars in a constellation in a way different from the usual one
so that it often resembles more the name given to the constellation.
Other cultures developed their early astronomy quite differently and independent from
our Babylonian/Greek culture. So did for example the Chinese during the Han dynasty
(206 BC – 220 AD) recognize 283 constellations/asterisms and had 1464 star names.
Their constellations were not named after celebrities such as gods, kings, queens, heroes
or animals as most Western constellations were, but instead named after daily life objects
such as: the emperor, his five palaces, the house his concubines lived in, food and
weapon storage facilities, his servants and ministers and even garbage and dung heaps.
The reasons the ancients were interested in the nightly beauty of the sky were:
1) To keep a calendar based on the three periodicities namely the day/night cycle (only
since Copernicus in the 16th century accepted as resulting from the rotation of the earth
around its axis), the monthly repeating phases of the moon and the year. The Chinese
(and Greek) knew, about two thousand years ago, that the year consisted of 365 ¼ days
so they divided the circle in 365 ¼ Chinese degrees, while in the West the circle was
divided in 360 Babylonian degrees because early Babylonian and Egyptian years had 360
days with extra days inserted occasionally to keep their moon and sun timekeeping on
track. Keeping a good calendar was necessary to celebrate religious ceremonies at the
correct day yearly, to plant and harvest at the right time and to regularly collect taxes in
the urbanized ancient societies.
2) Seafaring societies such as the ancient Phoenicians and Polynesians used stars
(Polaris, the Southern Cross) as night time compasses.
3) Another major reason for study of the heavens for most cultures was astrology, a set of
rules by which the future was believed to be somewhat predictable. Ancient sky watchers
believed that the god(s) used the stars, comets, novae, eclipses etc. to communicate with
us mortals and use some such omens to warn or advise us.
Already more than 2000 years ago several cultures, independent from each other, had
created tables and maps in which position and brightness of stars and planets were
recorded such as in Babylon, China, India, Greece, Guatemala. When travel became
easier and more frequent, scientific exchanges took place, and a need was felt to create
one universally accepted system of naming the stars. One example of several was
ambiguity regarding the star called Alpharetz. You see that star in your planimeter as one
of the four making up the great square of Pegasus. It is the star where Pegasus contacts
Andromeda. Some maps showed it belonging to Pegasus, others to Andromeda. The latter won the day (or night) and it is now named α Andromedae. In 1930, at a conference of
the International Astronomical Union, standard boundaries were drawn for all of the 88
agreed upon constellations as drawn by E.Delporte. The celestial globe is now divided in
88 patches into which every star finds the place of the constellation it belongs to.
Exercises and Questions.
1) By how many % is the Chinese degree smaller than the Babylonian one? (This issue
comes up when comparing coordinates in ancient tables presented by these two cultures).
2) Take out your planimeter. Turn the disc around the hole which represents Polaris
around which the stars seem to rotate. Some constellations come up in the East and- as
you turn the disc- disappear in the West, but six constellations stay always in their
entirety above the horizon. These constellations are called the circumpolar constellations.
Write down their names. One consists of only weak stars and is almost impossible to see
from Providence. What is the name of that constellation? Why is it not quite correct to
say that Ursa Major belongs to the circumpolar constellations?
3) As you travel North, will you see more circumpolar constellations or less. How many
would you see at the North pole, using your planimeter ? (Only count constellations that
are completely above the horizon at all times).
4)a) Set your planimeter for midnight January 1. Notice that the visible sky is identical to
the one on February 1 at 10 p.m. and at March 2 at 8 p.m.. Prove that this is to be
expected, by making a sketch of the rotating and orbiting earth.
b) Rotate the disc from midnight January 1 forward, to later hours (a.m.) Notice that the
stars seem to rotate counter clockwise around Polaris, some rise in the East and set in the
West. Explain this, knowing that the earth rotates from West to East around its axis.
c) Take a position from where you can observe one bright star close to which seems to be
a far away object, such as a house, chimney or tree. Notice the time and the angular
distance between star and far away object at that time. Repeat this observation next day
and notice the time that the star is again in the same orientation with respect to your
reference object. Look at your watch and read the time. It should be roughly 4 minutes
earlier. Why is this and why 4 minutes? How is this related to sidereal vs. solar time?
5) You won an astronomy competition and got a free trip to the Southern hemisphere.
a) Would you still see Polaris or, if not, around which point would the stars seem to
rotate? Do the sun, moon and stars still seem to rise in the East and set (=disappear
behind the horizon) in the West? Do the stars still seem to rotate counter clockwise?
b) In which direction would you see the sun there at noon time? Explain all this.
6) Find the North in the sky . Identify on your planimeter and in the sky the following
constellations and stars. Also record the colors of these stars and whether they are very
bright, bright or mediocre. How do we now indicate brightness of stars? Sketch the
constellations as you saw them in the sky and give date and time in the sketch:
Ursa Major with the asterism the Big Dipper, Ursa Minor and its asterism of the Little
Dipper with the α star Polaris, Cassiopeia and its bright stars arranged in the shape of the
letters W or M. Find Cepheus and Draco, which twists between the two Bears. Find
Auriga(Charioteer in English) with Capella a yellow star and nearby Gemini (the twins)
with its two bright stars Castor near Capella and Pollux near Procyon, a bright star in
Canis Minor (the Little Dog), also see the brightest star in the heavens, Sirius, in Canis
Major. Some people like to connect the stars Sirius, Procyon and the red supergiant
Betelgeuse in Orion and call that the winter triangle. Orion contains another very bright
star called Rigel and the famous Orion Nebula, just below the three starred belt. Look for
Taurus, the Bull, with in it the Pleiades and the red giant star Aldebaran Look for another
constellation on the zodiac or ecliptic, the apparent path of the sun, namely Leo with
bright star Regulus, the dot in the inverted question mark. Some constellations are
typically “seasonal” and not visible in winter and summer. Find those also on your
planimeter. They are: Bootes, the Herdsman, a kite like figure with the bright star
Arcturus, Virgo with star Spica, and three prominent summer/fall constellations Lyra
with star Vega, Cygnus, the Swan, with star Deneb, the tail of the Swan, and Aquila, the
Eagle, with bright star Altair. These last three stars are often connected in what is called
the “Summer Triangle” a very bright group high in the summer sky. Finally identify
Pegasus, with the characteristic square and Andromeda with the bright star Alpharetz and
the spiral galaxy, the furthest object one can see with the naked eye.
It is 2.9 million light-years away from us and also called M 31. If it is a clear winter
night, try to identify and connect in a sketch the stars Sirius, Procyon, Pollux, Castor,
Capella, Aldebaran, Rigel which form the winter hexagram.
7) Set the planimeter for midnight December 22 and notice that you can see at your
Eastern horizon the autumnal equinox in Virgo and at your Western horizon the spring
equinox in Pisces . What is the significance of the equinoxes and why do they slowly
move along the zodiac so that in a few centuries the spring equinox will be in Aquarius?
8) Try to measure a few angles using fig. 1-9 and 1-10 in the book. The diameter of the
moon, the angle between the two pointer stars in the Big Dipper.