Why do the seasons retain their place?
Observation
One of the most common astronomical and geographical fallacies is that the seasons are caused by
the distance of the Earth from the Sun: since the Earth rotates in an elliptical orbit, it would be
closest to the Sun during summer and furthest away from it during winter.
This theory is proven false simply by the fact that the seasons are always in an opposite phase to
each other on the Northern and the Southern hemishpheres: when it is summer up North, in the
South it is winter, and vice versa.
In reality, the seasons of the
Earth are caused by the fact
that the Earth’s axis is tilted in
relation the Earth’s orbit. When
it is summer in the North, the
Sun shines straight on the
Northern hemisphere, giving it
more warmth and light than to
the Southern hemisphere. In
half a year’s time the tables are
turned.
Thus the question is not ”How
do the seasons come about?”
but rather ”Why are the seasons so regular and their position so unchangeable?” Or is their position
as unchangeable during the year as it seems?
Exploration
Finding out why the seasons retain their place means, in
physical terms, exploring why the direction of the Earth’s axle
remains the same as the Earth swirls at a rabid pace on its
orbit around the Sun. It almost seems as if the axle around
which the Earth rotates were attached somewhere, as it is in a
globe.
The way a heavy spherical object rotates is demonstrated well
by the rock ball in the Galilei Park in Heureka. It rotates very
smoothly and retains its direction once it has been set going.
There is no motor rotating this rock.
A video clip of the rock ball rotating in Heureka
The forces affecting the rotating rock ball are friction and the
Earth’s gravitational force. The friction is very small, since
there is a layer of water between the carefully grinded rock
ball and the platform. The Earth’s gravitational force, on the other hand, affects every part of the
ball to an equal degree, so its total effect on the ball’s rotation is +/ – 0.
The rock ball in Heureka is a rather good model of the Earth in at least two ways. The energy of the
rock ball is dwindled on the water on its surface; likewise, the tides are constantly decreasing the
speed at which the Earth rotates. The rock ball is kept to the ground by the Earth’s gravitational
force; the Earth is kept on its orbit by the Sun’s gravitational force. Neither one of these forces
affects the rotation in any way.
Concept introduction
The physical principle according to which a rotating object attempts to maintain both the velocity
and the direction of its rotation (unless some other force attempts to slow down the rotation or
change its angle) is called conservation of angular momentum. It is the angular momentum’s
equivalent to the conservation of momentum of linear movement.
The angular momentum increases as the mass, the angular radius and the angular speed of the
rotating object increase. The Earth does not rotate very fast, but its mass and angular radius are
large, and there are no significant forces that would have the effect of inverting it or slowing it
down. That is why it rotates very steadily, maintaining its speed and direction.
A smaller rotating object, for example a spinning top, needs to be set spinning much faster in order
to have it maintain its direction and not fall down. The forces opposing the rotation of a spinning top
are so powerful that a spinning top usually isn’t able to maintain its rotation for a very long time.
Generalization of the concept
The large globe in the Science Center
Heureka allows us to examine what are the
factors affecting the conservation of the
Earth’s direction of rotational motion. (The
direction of motion of a rotating object is the
same as the direction of its axle. If this
remains the same, the direction of motion
remains the same as well, even if the object
moves otherwise.)
Change the direction of the Earth’s axle when
the globe is not rotating.
Change the direction of the Earth’s axle when
the globe is rotating.
Why does the globe react differently in these two cases?
When the globe is not rotating, it is fairly easy to change its position. When one tries to change the
position of a rotating globe, one can observe two interesting things. First, the globe is much heavier
to move than when it’s still: because of the conservation of angular movement, the globe strives to
maintain the direction of its rotation. Another observation, perhaps puzzling at first, is that the globe
does not strive to turn to the direction where it’s being turned, but to a 90 degree angle of this
direction. This can also be observed by the gyroscope in the Heureka suitcase.
This phenomenon is known as precession. Precession refers to the change in the direction of the
rotating axis. A precessing object rotates around its own axle, and apart from that the direction of
the axle changes to the effect that the angle of incline remains the same but the direction of incline
spins. If the axis of a spinning top is not strictly vertical, precession can be observed in its movement.
Since the Earth is not entirely spherical and its axle is tilted in relation to the ecliptic, the impact of
the Sun’s and the Moon’s gravitational forces isn’t greatest at the centre of the Earth, but a bit more
to the side. These gravitational forces strive to straighten the axle of rotation, so that it would be
perpendicular in relation to the level of the orbit. The Earth resists these efforts: the Earth’s axle
rotates in a spherical movement, akin to that of a spinning top, in a period of 25,725 years, which is
known as the Platonic year. The direction of this movement is opposite to that of the Earth’s
rotation and movement on its orbit. In the course of one Platonic year the celestial poles seem to
draw a circle around the poles of the ecliptic, the radius of this circle being the same as the angle
between the Earth’s axle and the perpendicular of the Earth’s orbital plane: 23.5 degrees. During this
period of time the seasons travel through the year – in other words, their position is not fixed either.
As a spinning top rotates, one can notice particularly near the end of the spin a tilting of the rotating
direction and a slight fluttering motion. This last phenomenon is known as nutation
This cone movement is caused in the Earth’s axle by the Moon. The nutation deflects the axle from
its central position to the degree of 14–18", this movement taking place during a period of 18.6
years – the exact same time as that of the precession of the Moon’s orbital nodes. Because of
nutation even the Arctic Circle does not remain at the same place, but travels back and forth, to the
amount of the degree mentioned above, during the period of the nutation. This amounts to
approximately 0.3 miles.
In human time scale, precession cannot be observed in the Earth’s movements, but nutation can –
though even these observations can only be made by exact astronomical measuring devices, not by
human senses.