Page 299 - NS-2 Textbook

294 NAUTICAL SCIENCES

penumbra penumbra
total eclipse

Sun

A solar eclipse. The umbra is the darkest part of the shadow of the Moon on the Earth's surface; the penumbra is the outer, or lighter, part of
the shadow.

mil thick and is the layer from which the visible light of Sunspots may often be seen projecting well beyond
the Sun originates. Next is the chromosphere, a 10,000- the chromosphere as a prominence. An eruptive promi-
km (6,000-mi) thick region in which the Sun's tempera- nence extending more than 400,000 kilometers above the
ture goes from about 6,000°C at the innermost edge to surface of the Sun was photographed in 1973 by the
about 10,000°C at the outer edge. TI,e outermost layer is Naval Research LaboratOlY's telescope mounted on Sky-
the corona, extending many rnillions of miles out into lab. Sunspots may last only a few minutes or as long as a
space. Temperatures in the corona reach more than a mil- year and a half. There are times when few are seen, and
lion degrees Celsius. other times when there are many. There seems to be a
TI,e Moon has almost the same visible size in the sky sort of sunspot cycle, with the greatest munber occmring
as the photosphere of the Sun. TIms, when the Moon about every eleven years.
passes between the Earth and the Sun, it can partially or It is believed that the slmspots are responsible for
totally block most sunlight from reaching Earth, a phe- the beautiful and spectacular Aurora Borealis visible in
nomenon called a solar eclipse. During a total eclipse of the higher northern latitudes. Similar polar lights, the
the Sun, its chromosphere-and, to a lesser extent, its Aurora Australis, appear in the southern hemisphere.
corona~becomes visible to observers on Earth. Here is how these lights are created. The radiation pres-
sme of the Slm pushes some of the hot sunspot material
completely away from the chromosphere. TI,ese gases
SUNSPOTS
are electrically charged, since they came from the Sun's
Sllnspots are whirling fountains of hot gas that have come interior, -where atomic nuclei and electrons are sepa-
out of the interior of the Slm. Hotter than the surround- rated. Some of these particles are drawn into Earth's
ing gases of the photosphere, these fOlmtains of gas rise atmo-sphere near the magnetic poles. There they form
through the chromosphere, expanding and then cooling. an electric field. When elements of oxygen and nitrogen
When cooling, they appear darker than the hotter and collide with hydrogen, or reform into complete atoms,
brighter environment behind them-thus earning the radiation In the form of light is produced. This causes an
nanle JlSlUlspotS." aurora.

Sunspots photographed by a Navy balloonist from an altitude of A photograph of the Sun taken on 19 December 1973 from NASA's
80,000 feet. The black spots are dark cores of relatively cool gases Skylab 4. This shows one of the most spectacular solar flares
embedded in a strong magnetic field. Sunspots produce magnetic ever recorded, spanning more than 367,000 miles across the solar
storms and major disturbances in radio broadcasts on Earth. surface.