The phenomenon we call "precession" was discovered by Greek astronomer
Eratosthenes when he compared his own circa 200 BC records with older charts.
What he saw was that the equinoxes in his day (where the sun's path crosses the
celestial equator) were in a different position among the stars than the
150-year-old comparison charts showed. This is due to a gyroscopic wobble of
earth's spin axis that takes 26,000 years to complete. In this wobble motion, the
tilt of the earth stays roughly constant at 23.4 degrees but the orientation is
always changing.
Consequences of precession:
- The north star Polaris is drifting. It is only "north star" by coincidence
today. Vega will be our north star for a time in the distant future.
http://www.jgiesen.de/astro/argelander/SkyApplet/
- There is a difference between a sidereal (real) year and a tropical (by
the sun) year because during the course of one year the position of the
equinox changes slightly.
The physical cause of the precession is a torque (twisting) of the earth,
caused mostly by the sun's and the moon's gravity pulling on the equatorial
bulges of the earth. If earth were NOT spinning, the sun and moon would pull the
earth so that the bulges were flat in the sun-earth orbital plane.
The planets have some gravitational influence also, but insignificant
compared to the sun and moon. Still, the planets manage to cause the orbit of
earth about the sun to precess, with a period of 71000 years. They also cause an
oscillation in the ellipticity of earth's orbit.
Taken in conjunction with the 26,000-year spin-axis precession, the 71,000-year
orbit precession causes a 41,000-year oscillation in the tilt of the earths
axis, about plus or minus 1.3 degrees from its average value of 23.3 degrees.
This number is not absolutely stable - it depends on the combined positions of
all the planets through time.
The tilt reached a maximum of 24.2 degrees about 9,500 years ago, and has been
decreasing ever since. The tilt is now near the average value, but the rate of
change of the obliquity is near a maximum.
The most startling consequence of this is that the tropics (the tropics of
Cancer and Capricorn, where the most northerly or southerly vertical rays of the
sun strike) are MOVING TOWARD THE EQUATOR. The rate is more than 14 meters per
year! Example: the government of Taiwan erected a monument in a park marking the
tropic in 1908. The actual tropic is now more than a kilometer south of this
location! The arctic circles are likewise traveling toward their respective
poles. The temperate zones gain 1,550 square kilometers of territory every year!
Precession Mechanism
Due to the interaction between the Earth and the Moon, the rotation axis of
the Earth is not stationary. The interaction arises because the Moon is not in
the equatorial plane of the Earth; there is a misalignment between the spin axis
of the Earth and the orbital axis of the Moon
In the interaction the Moon tries to force the two axes to coincide. This
force causes the direction in which the rotation axis points to change slowly.
The motion is quite regular; the path on the Celestial Sphere is roughly
circular. The radius of the circle is large, ~ 23.5 degrees (is this a
surprise?). The period of the motion is long, 26,000 years, due to the weakness
of the effect.
Nutation. The principal sources of Earth's tidal force are
the Sun and Moon, which continually change location relative to each other and
thus cause nutation in Earth's axis. The largest component of Earth's nutation
has a period of 18.6 years, the same as that of the precession of the Moon's
orbital nodes. [ Source:
http://en.wikipedia.org/wiki/Nutation ]
Influence of precession on position of the Polaris
Since the point where the projection of the rotation axis of the Earth
strikes the Celestial Sphere moves around (the stars are still not in motion),
this means that the North Celestial Pole moves around through the stars. This
produces changes in the coordinates of stars (even though they are not moving).
Today, it is fortuitous that the North star is Polaris. At other times, other
stars were or will be the North star:
Periodic Fluctuations in Earth's Orbit About the Sun
The advance and retreat of ice sheets during times of icehouse climate
appears to be strongly influenced by three periodic fluctuations in Earth's
orbit about the sun.
The eccentricity (deviation from a circle) of Earth's
elliptical orbital path around the Sun varies in a cycle of approximately
100,000 years. A greater amount of eccentricity means that Earth-Sun distance
varies substantially over the course of a year from perihelion (closest approach
to the sun) to aphelion (orbital point farthest from the Sun).
The Earth is currently tilted 23.45° from a perpendicular axis to Earth's
orbital plane about the Sun.
The axis is tilted in the same direction throughout a year; however, as the
Earth orbits the Sun, the hemisphere tilted away from the Sun will gradually
come to be tilted towards the Sun, and vice versa. This effect is the main cause
of the seasons. Whichever hemisphere is currently tilted toward the Sun
experiences more hours of sunlight each day, and the sunlight at midday also
strikes the ground at an angle nearer the vertical and thus delivers more heat.
As Earth progresses in its orbit, this
axial tilt creates the familiar seasonal variation (northern winter occurring
when the north pole is tilted away from the Sun).
Summary of the seasons (caused by the 23.5°
tilt of the Earth axis).
The following images explain why Summer days are longer than nights:
The current amount of tilt is approximately in the middle of the range
of historical tilt values, which vary by a few degrees over a cyclical span of
about 41,000 years. Greater amounts of tilt increase the magnitude of
seasonality (leading to hotter summers and colder winters).
The Earth's axial tilt varies between 21.5° and 24.5° with a 41,000 year
periodicity (currently decreasing: 24.049 in 3300 BC, 23.443 in 1973, 23.439 in
2000), while the direction of the tilt gradually undergoes precession, moving in
a slow circle over a period of about 25,800 years. However, other factors may
change the axial tilt of Earth (and of other planets).
Through time, axial precession changes the position of the Earth in its orbit at
which the seasons occur (precession of the equinoxes).
The third and final cyclical change in Earth's orbit (with a period of about
26,000 years) is precession (sometimes termed the precession of the equinoxes).
The Earth wobbles in its orbit like a spinning top, so the direction (with
respect to the fixed background of stars) in which the north pole points varies
over time. This change in direction also brings a change in the timing of the
onset of the seasons with respect to Earth's position in its orbital path.
Currently (part A of this figure) the northern summer solstice occurs near
aphelion, and the north pole points toward a star in Ursa Minor named Polaris
(the famous north star).
About ten thousand years ago, the north pole pointed
toward the bright blue star Vega in the constellation Lyra, and the onset of
northern summer occurred near perihelion (part B).
During icehouse conditions, glacial advance is favored by relative warm, wet
(snowy) winters (at very cold temperatures, little snow falls because the air
cannot hold much moisture) and cool summers that minimize glacial melting. Thus
conditions are optimal for glaciation in the Northern Hemisphere when the orbit
is highly eccentric (which is opposite the state of the present orbit, which is
relatively circular), when tilt is at a minimum, and when the northern summer
occurs at aphelion (as in the present) so that the northern summer is moderated
by a great distance between the Earth and Sun. That we are currently in an
interglacial episode suggests that orbital eccentricity is a more important
factor than precession.
Changes in Earth-Sun Interaction - Milankovich Cycles
Milankovich cycles are cycles in the Earth's orbit that influence the
amount of solar radiation striking different parts of the Earth at different
times of year. They are named after a Serbian mathematician, Milutin
Milankovitch, who explained how these orbital cycles cause the advance and
retreat of the polar ice caps. Although they are named after Milankovitch, he
was not the first to link orbital cycles to climate. Adhemar (1842) and Croll
(1875) were two of the earliest.
While we're all familiar with the axis of the earth pointing
toward the North Star (Polaris) at an angle of 23.45° and that the
earth is approximately 91-94 million miles from the sun, these facts
are not absolute or constant. The interaction between the earth and
sun, known as orbital variation, changes and has changed throughout
the 4.6 billion year history of our planet.
Eccentricity - Orbital shape
Eccentricity is the change in the shape of the earth's orbit
around the sun. Currently, our planet's orbit is almost a
perfect circle: the present eccentricity is 0.017. There is
only a 6% difference in distance between the time when we're closest
to the sun (perihelion) and the time when we're farthest from the
sun (aphelion). Perihelion occurs on January 3 and at that point,
the earth is 91.4 million miles away from the sun. At aphelion, July
4, the earth is 94.5 million miles from the sun.
Over a 95,000 year cycle, the earth's orbit around the sun
changes from a thin ellipse (oval) to a circle and back again. When
the orbit around the sun is most elliptical, there is as much as a
30% difference in the distance between the earth and sun at
perihelion and aphelion. Though the current three million mile
difference in distance doesn't change the amount of solar energy we
receive much, a 30% difference really would modify the amount of
solar energy received and would make perihelion a much warmer time
of the year than aphelion.
Obliquity - Axial Tilt
On a 42,000 year cycle, the earth wobbles and the angle of
the axis, with respect to the plane of revolution around the sun,
varies between 22.1° and 24.5° . Less of an angle than our current
23.45° means less seasonal differences between the Northern and
Southern Hemispheres while a greater angle means greater seasonal
differences (i.e. a warmer summer and cooler winter).
When the obliquity is low, the polar regions get less sunlight,
cool, and accumulate ice and snow. The total amount of sunlight
caught by the earth remains the same, so it is premature to
positively identify obliquity changes as the root cause of the ice
ages.
Precession - Axial Orientation
12,000 years from now the Northern Hemisphere will experience
summer in December and winter in June because the axis of the earth
will be pointing at the star Vega instead of it's current alignment
with the North Star or Polaris. This seasonal reversal won't happen
suddenly but the seasons will gradually shift over thousands of
years.
Milankovitch Cycles
Astronomer Milutin Milankovitch developed the mathematical
formulas upon which these orbital variations are based. He
hypothesized that when some parts of the cyclic variations are
combined and occur at the same time, they are responsible for major
changes to the earth's climate (even ice ages). Milankovitch
estimated climatic fluctuations over the last 450,000 years and
described cold and warm periods. Though he did his work in the first
half of the 20th century, Milankovich's results weren't proven until
the 1970s.
A 1976 study, published in the journal Science examined deep-sea
sediment cores and found that Milankovich's theory corresponded to
periods of climate change. Indeed, ice ages had occurred when the
earth was going through different stages of orbital variation.
Though Milankovitch cycles do explain long-term climate change,
they can't account for changes being made by humans, which appear to
have an even greater effect than variations in earth-sun
interaction.
Related Links:
