In this laboratory the motion of precession will be investigated and how it affects the appearance
of the night sky over a long period of time, particularly the North Star.
Precession refers to a change in direction of the axis of a rotating object. The most familiar
example of Precession can be observed in spinning tops. As a top begins to slow down it axis
begins to rotate in a circle. The same phenomenon is observed for the Earth, but in the case of the
Earth it takes 26,000 years for a complete revolution. Earth is not completely spherical; it has a
slight bulge at the Equator. Gravitation forces of the Sun and Moon on this bulge produce the
Currently the Earth’s northern axis points almost exactly to the star, Polaris which is more
commonly referred to as the North Star. However, because of precession the position of stars
will change slowly but continuously over a period of 26,000 years. In approximately 13,000
years Earth’s northern axis will point to the star Vega, at which time it will be referred to as our
Because of precession, the position of the stars in the sky change slowly but continuously as
viewed from Earth. Since a circle is divided into 360°, each year corresponds to a change in the
sky of 0.0138°, or 49.8″ per year, or 1° 23’ per century, or 13° 1’ 23″ per millennia. This is quite
a significant change. For example the full moon only takes up an angle of 0.5° in the sky.
Therefore all of the celestial objects have moved approximately 26 times the diameter of the full
moon every thousand years.
With the Starry Night software it is possible to simulate what the sky looks like at any given time
between 4173 B.C.E. (BC) and 10000 C.E. (AD). It is this flexibility in time that we will utilize
today, and verify the precession of Earth’s axis.
Astrology is a set of traditions or beliefs in which the knowledge of apparent positions of celestial
objects is considered useful in understanding human affairs. If you place faith in Astrology you
believe your life is determined by the position of the Sun and Planets at the time of your birth. That
is, your astrological sign (zodiac sign) is determined by what constellation the Sun was in on the
day of your birth. Nearly all the early civilizations had some form of Astrology. Statistical
experiments have found no correlation with time of birth and different characteristics, including
occupation, anxiety levels, marital status, aggressiveness, sociability, IQ levels and ability in art,
As the Earth orbits the Sun the position of the Sun against the background stars changes. If the
Sun could be turned off you would be able to see the position of the Sun shift throughout the year.
During a year the Sun moves on the celestial sphere along a path called the ecliptic. The ancients
believed that the constellations though which the Sun moved had special powers and dubbed these
the constellations of the Zodiac. Traditionally, there were twelve constellations of the zodiac and
each one was granted the same period of time. However, if we look at the actual path of the sun
through the stars we find that there are actually 13 constellations through which the sun moves.
Furthermore, the sun’s spends more time within some constellations than others.
EXERCISE #6-1: “What’s your sign?”
Ancient astrologers divided the ecliptic into twelve equally spaced zodiac signs. These signs
were then said to govern the lives of those born during the month at which the Sun was said to be
in that constellation. These dates did not take into account the different lengths of the ecliptic
path that actually fell within that constellation. Also, because of the precession of the earth over
the 3000 or so years the system was used.
So what sign were you REALLY born under? In what constellation was the Sun on the day of
your birth? Let’s compare the two.
Go to the Wikipedia website: https://en.wikipedia.org/wiki/Zodiac
Scroll down to the bottom of the website to the “Table of Dates.” This table shows the dates for which the
Traditional (Tropical) Zodiac correspond and are what has been taught in western astrology as your
“sign”. On the right side of the table are the dates when the Sun is within that constellations boundaries as
determined by the International Astronomical Union.
The Zodiac Table has a list of dates down the left hand side. Determine which Traditional constellation
somebody born on that day would have as well as the actual constellation in which the Sun was located
according to the IAU boundaries. At the bottom include your birthday!
Once you are finished Sign the table and upload it to Canvas.
EXERCISE #6-2 “Precession and Polaris”
Now let’s examine the effect of precession on our view of the Stars. Today we consider the star
Polaris to be the “North Star” because of its location near the North Celestial Pole. However,
Polaris wasn’t always the North Star and won’t be in the future. Let’s see how the Earth’s
precession moved Polaris in the Celestial Sphere.
Download the data of the Right Ascension and Declination of Polaris over the years.
Plot and label each point of data on the blank Polar Star chart (review the second half of the
Polar Plotting Video for help). Trace a smooth curve between the dots using your favorite
colored pen or marker. Notice the simple geometric shape that this path appears to make. Sign
and date the plot and upload it to Canvas.
LAB #7 ANSWER SHEET Name _________________________
THE ZODIAC TABLE
|Traditional (Tropical) Zodiac Constellation||Zodiac Constellation Using IAU Boundaries|
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