Download Precession of Earth

Precession of Earth: Spinning on the Axis
Environmental Science
Name: ____________________________________________Period:__________ Date: ___________
Essential Question: Why is earth’s axis wobbling? Een 1.1.1
Instructions: Study the selection and answer the questions.
Just like a toy top that wobbles before it falls, the Earth
wobbles as it rotates on its axis, the imaginary line that
extends through the poles. This wobbling motion is due to
the slight bulge at the equator. If you traced the Earth's axis
out into space, you would see the extension of the axis
slowly tracing a cone shape. The wobble is very slow; it
takes the Earth 26,000 years to trace one complete conical
shape. This wobbling motion of Earth on its axis is called
precession. Because of precession, Polaris and Vega
alternate as the North Star every 13,000 years.
Polaris: The Current North Star
Today the Earth's axis points within one degree of Polaris,
the brightest star in the constellation Ursa Minor (also
called the Little Bear or the Little Dipper). Polaris appears
to be in a fixed position in the sky throughout the year. All
other stars and constellations seem to revolve around the
North Star.
Why isn't the North Star Fixed?
Over the course of time, the North Star changes. Right now Polaris is within one degree of true north,
but at other times the North Star has been and will again be Thuban (the brightest star in the
constellation Draco), Vega (the brightest star in the constellation Lyra), and Alpha Cephei (the brightest
star in the constellation Cepheus).
The North Star changes over time because the direction of the earth's axis changes slowly over time.
Since by definition the North Star is the star most closely aligned with the earth's axis, as the axis moves
the nearest star changes too.
This type of axis movement is similar to that of a spinning top. As the top slows, the axis of rotation
changes as the top draws out each rotation; that is to say that the stem of the top itself traces out a circular
pattern rather than pointing at a single spot or staying mostly still. If you draw an imaginary line of the
earth's axis and continue it up to the sky, it will make a similar path. This type of axis rotation is called
precession.
In the case of the earth, precession is caused by the gravitational pull of the sun and the moon. The earth's
axis makes one complete rotation over the course of approximately 26,000 years. If you trace the path of
the axis in the sky, you will find that Polaris, Vega, Thuban, and Alpha Cephei all fall on or very close to
it. So when the earth's axis is at a point on the path near Vega, Vega becomes the North Star while
Thuban is the North Star when the axis is near it on the path.
Five thousand years ago, Thuban was the North Star. Five thousand years from now, the North Star will
be Alpha Cephei. Seven thousand years after that, it will be Vega. Nine thousand years after that, Thuban
will be the North Star again. At these dates, the various stars will be at the closest to absolute north. For
some time before, the relevant star will be approaching due north and it will be receding for some time
after the time listed. In these interim times, the North Star is whichever star is closest to north.
Answer the following questions
1. What is earth’s axis?
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2. What is precession?
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3. How long does it take for earth’s axis to wobble into a complete conical shape?
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4. What two stars alternate every 13,000 years as earth’s northern star?
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5. What is earth’s current northern star?
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6. What is the brightest star in the constellation Ursa Minor?
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7. What is the brightest star in the constellation Draco?
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8. What is the brightest star in the constellation Lyra?
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9. What is the brightest star in the constellation Cepheus?
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10. Why do earth’s northern star change over time?
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11. What is precession and what causes it?
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12. What are the four stars that can become earth’s northern star?
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13. What was earth’s northern star before Polaris?
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14. What will be earth’s northern star five thousand years from now?
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15. What will be earth’s northern star twelve thousand years from now?
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16. Which do we consider as our northern star and why is our northern star changing?
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http://www.education.com/reference/article/precession-earth/
http://www.essortment.com/north-star-61725.html
Precession and Nutation
Environmental Science
Name: ____________________________________________Period:__________ Date: ___________
Essential Question: Why is Polaris fixed on its position throughout the year?
What is the difference between precession and nutation?
Instructions: Encircle and label the constellations where Polaris, Vega, Thuban, and Alpha Cephei are
found.
Polaris, the brightest star in the constellation Ursa Minor (also called the Little Bear or the Little Dipper).
Polaris appears to be in a fixed position in the sky throughout the year. All other stars and constellations
seem to revolve around the North Star. Thuban (the brightest star in the constellation Draco), Vega (the
brightest star in the constellation Lyra), and Alpha Cephei (the brightest star in the constellation
Cepheus).
http://www.msnucleus.org/membership/html/jh/earth/stars/lesson4/stars04c.html
Come up with your Hypothesis: How come Polaris seems to be stationary in the sky throughout the year
while the others stars in their constellations seem to revolve around it?
Hypothesis:
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Try this out!
1. Stand directly below a smoke alarm or anything that you can consider as your
northern star. Directly above you is your north, your right side is your east,
down is south, and your left side is your west.
2. Look around and observe anything that you can consider as markers ( like
ceiling fan, light bulb, air vent, spot on the ceiling etc. ) and consider them as
the other stars.
3. Slowly turn around 360 degrees from west to east direction while looking at
your northern star. Did your northern star move to another direction in
reference to your position ( N-E-S-W)? Explain.
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4. Again, slowly turn around 360 degrees from West to East direction while
looking at your other star markers. Did your other stars move to another
direction in reference to your position? ( N-E-S-W)? Explain.
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5. Why is the northern star stationary and the rest of the stars seemingly
rotating around it? Explain.
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Nutation is, in astronomy, a small irregularity in the precession of the
equinoxes. Precession is the slow, top-like wobbling of the spinning Earth,
with a period of about 26,000 years. Nutation (Latin nutare, "to nod").
The cause of nutation lies chiefly in the fact that the plane of the Moon's
orbit around the Earth is tilted by about 5 degrees from the plane of the
Earth's orbit around the Sun. The Moon's orbital plane precesses around
the Earth's in 18.6 years, and the effect of the Moon on the precession of
the equinoxes varies with this same period. The British astronomer James
Bradley announced his discovery of nutation in 1748.
Rotation (R), Precession
(P) and Nutation,
jagged line (N) of the
Earth.
http://en.wikipedia.org
/wiki/Nutation
http://nutation.org
Have you noticed your head nodding as you turned around at 360
degrees? This can be compared to the earth’s nutation ( jagged line in the
figure). What caused your nutation was your movement but what is
causing earth’s nutation?
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