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例3.次の英文は 「磁極の移動」について述べたものです。(ア) ~ (オ) に入れ
る最も適当なものを選択肢から選びなさい。
Earth has two sets of poles: the geographic poles and the magnetic poles. The geographic
poles are located at the northernmost and southernmost ends of the earth. The location of the
magnetic poles, on the other hand, are ( ア ) moving. This fact has several implications
for navigation and transportation.
Shifts in the position of the North Magnetic Pole are nothing new. Since 1831, scientists
have been tracking its location. The movements are caused by changes in the flow of swirling
molten iron in the earth's core which affect how the earth's magnetic field behaves. What is
surprising to scientists is the speed at which the pole has been moving in recent years.
For most of the 20th century, magnetic north was located around the northern parts of
Canada. It drifted around, moving slowly north at an ( イ) speed of approximately 10
kilometers per year. In recent decades, that rate has increased significantly to about 55
kilometers per year, The North Maggnetic Pole is now moving away from Canada toward eastern
Russia.
While scientists can't fully explain how changes in the earth's molten core are affecting
the pole's movement, they can map the earth's magnetic field. Doing so allows them to
( ウ ) the rate of change over time. This, in turn, provides information on how the
magnetic field may shift in the future.
Information on changes to the earth's magnetic field is used to produce the World
Magnetic Model (WMM). This model is used as the basis for all forms of modern navigation,
from how ships move at sea to mapping software in smartphones. ( エ ) inthe model、
caused by shifts in the real location of the North Magnetic Pole, can seriously impact almost all
foms of modern transportation.
The model is updated every five years, but in 2018, during routine checks of the 2015-
2020 model cycle, scientists noticed a problem, Because of the rapidly shifting pole, the WMIM
was close to being outside the acceptable limits for navigational errors. This forced the release
of an updated version of the model, It was revised (オ ) in2020, and the current version
is expected to last until 2024,
As well as kecping track of the WMM's accuracy, scientists continue investigating why
the magnetic ficld is changing so dramatically, This essential work will ensure a safe joumey
for everyone navigating their way through the world.
