Aurora is an atmospheric phenomenon of light which most frequently appears at polar zones. Aurorae can have many different shapes: arcs, threads, stripes, coronae, aurora clouds, aurora glow, drapes and flames. The color of an aurora is determined by the properties of atmospheric gases.
|In the ionosphere, where aurorae actually form, particles in the solar
wind collide with particles of Oxygen and Nitrogen. As a consequence,
Oxygen atoms emit green and red light, while Nitrogen atoms emit red
light. This is why the color of an aurora is most commonly green, but it
can also be red.
What causes aurorae?
During the activity of sunspots on the surface of the Sun, particles are ejected with high speeds. They form the solar wind. Two invisible entities surround the Earth: the atmosphere, which is composed of a few gas layers; and the magnetic field. After 2-5 days of traveling through space, a certain proportion of particles in the solar wind reaches the Earth magnetic field. The magnetic field then drags the particles in. The particles continue moving in a spiral trajectory, determined by the lines of the magnetic field, converging towards the poles. Great amounts of energy are released in collisions between these particles and the particles in the atmosphere. This energy is perceived by us as colored light. The greater the solar activity, the bigger the aurora.
How do we see it?
Aurora begins as a fluorescent glow, and afterwards arcs of light appear one by one. After that rays of light come from outer space and form "folds" with red and purple edges, which resembles a curtain in the wind. This is followed by rays of light which are scattered in all directions and which make up the auroraís corona. 10 to 20 minutes later, the activity has decreased and the aurora is reduced to pale pulsing flames which appear and suddenly disappear after a few seconds.
An energy source
When an aurora appears in the sky, a tremendous electric discharge occurs high in the Earthís atmosphere. A huge amount of energy is released , only 4% of which is seen as light. The rest of the energy is dissipated into the vast emptiness of space. This is a good reason to believe that aurorae could be a significant energy source in the 21st century.
Aurorae and legends
Since the ancient times, various beliefs related to aurorae have existed. In eastern parts of North America people thought that auorae are torches which souls from heaven lit to light the way for the newcomers. In medieval Europe, a belief existed that an aurora is actually the breath of a soldier who died for his country and king. In the old times weather forecasts were based on observations of aurorae. Some people considered them to be a sign of good weather coming, while others thought that aurorae precede bad weather. Today scientists believe that aurorae affect the global warming process, but there still isnít enough proof to support this claim.
An eclipse happens when the light from the Moon or the Sun is temporarily blocked. Eclipses occur when a celestial body is positioned in the shadow of another body, or when a body is between another body and the Sun.
Lunar eclipse happens when the Moon, in its orbit around the Earth, is aligned opposite to the Sun (full moon), inside the Earthís shadow. A total eclipse occurs when the whole Moon passes through the shadow shed by the Earth. It can last up to one hour and forty minutes.
A total eclipse can be seen by all people on the dark side of Earth. It is not harmful to look at it directly.
Partial eclipses happen if only a part of the Moon passes through Earthís shadow. The size of a partial eclipse ranges from an almost total eclipse to a small eclipse, where only a minute part of Earthís shadow is shed on the Moonís surface. During an eclipse, the Moon is usually not completely dark, but radiates a faint red glow which is refracted by Earthís atmosphere (other colors are reflected by the atmosphere).
Lunar eclipses are observable from the whole hemisphere where the Moon is above the horizon. Lunar eclipses are of great importance for determination of structure of upper layers of Earthís atmosphere
Total eclipse of the Sun happens when the whole Moon is between the Earth and the Sun (new moon) so that the Sun is totally or partially covered by its shadow.
Due to the movement of the Moon and the Earth, the top of the Moonís shadow is moving on the surface of Earth with a speed of 3200km/h and in that way the shadow describes the zone from which the eclipse is visible. During the eclipse, the Moon appears on the Sunís bright disc and moves slowly across it. At the moment of the total eclipse, a thin red ring is visible around the Sun from which protuberances are ejected, resembling flames, and around all that appears a corona with a pale silver glow. The sky becomes colored in a dark shade of blue, and the daylight becomes so dim that bright stars and planets are visible from the surface of Earth. That is one of the most impressive sights in nature.
After a few minutes have past, the Sun appears again. Total eclipse of the Sun is usually visible for about 3 minutes . It can last up to 7.5 minutes, but this happens very rarely - once in a few thousand years.
A total eclipse of the Sun cannot be observed directly, because direct exposure to radiation can be very harmful to naked eyes. In the case of Moon being positioned in the furthest point of its trajectory around the Earth, its shadow does not reach the Earth, but it falls under its surface. Then a ring - shaped eclipse occurs. A bright ring of uncovered surface of the Sun is visible around the dark disc of the Moon. A partial eclipse of the Sun happens when the Moon covers only a part of the Sun.
Refraction in the Atmosphere
A ray of light refracts in the Earth's atmosphere when passing through layers of air with different densities. The trajectory of the refracted ray of light is not a broken line segment, but a continuous curve. This is due to the gradual changing of air density. Refraction of light in the atmosphere: