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Precession, the Ecliptic, and
Objective: This activity will investigate the precession of Earth’s rotation axis.
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 the15006
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 Sun15006
and Moon
on this bulge produce the Precession (Figure 1).
Year 15006
Precession of
the North Pole
Year 2006
North Pole
at o
Equ 0 0 6
2 0 0 t or
Figure 1
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 move every thousand years.
360 
26,000 yrs
 0.0138 
As was mentioned in Activity two, with The Sky 6 software it is possible to simulate what the sky
looks like at any given time between 4173 B.C.E. and 10000 C.E.. 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. 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, sports, mathematics.
The Ecliptic
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. Figure 2 depicts the motion of the Sun for a few months moving through
the background Zodiac constellations.
The Path of the Sun
Figure 2
Part 1:
In the first column of data table 1, record the traditional astrological sign corresponding to the given dates.
You may use as a reference.
Open the Sky 6 software.
Turn on the constellation boundaries. By clicking the
Turn on the constellation labels.
Under the View menu choose Labels. Then make sure the constellations button is checked and all
other buttons are not checked. Then click the apply button. Then close the screen.
Then, you can turn on the common labels by clicking the
Open the time window: Data  Time
 Set the date to April 1 2007.
 Set the time to 12:00 PM.
 Record the constellation in which the Sun actually appears.
 Repeat for the other given days until the chart is complete.
Part 2:
1. Reset the year to 1000.
2. Set the time to 12:00 AM. Face north (Click N)
3. Activate the equatorial grid (click the blue gridded globe icon).
4. Locate Polaris. Remember it is at the end of the tail of Ursa Minor. If you have difficulty locating it you
may turn on the constellation stick drawings.
5. Record the RA and DEC for Polaris on the data sheet.
6. Continue recording all this information until the year 10,000.
Part 3:
1. Plot the first RA and DEC for Polaris on the polar chart. Label the point with the number.
2. Plot the second RA and DEC for Polaris and label the point 2.
3. Continue doing this until all 14 points are identified.
4. Answer the question on the back of your data sheet.
A video demonstration of Polar Coordinate chart reading and plotting is provided in DEMO PREC30 using
the year 1 AD as an example.