Eclipse

Eclipse

An eclipse is a phenomenon in which the light from a celestial body is temporarily obscured by the presence of another.

A solar eclipse occurs when the Moon is aligned between the Sun and Earth. The trace of the lunar shadow (where the solar eclipse is visible) is less than 270 km (168 mi) wide. A partial eclipse is visible over a much wider region. When the Moon is further away from Earth, the lunar disc has a smaller visible diameter than the solar disc, so a narrow ring of the Sun remains uncovered, even when the three bodies are aligned. This produces an annular solar eclipse. The ratio between the visible lunar and solar diameters is called the magnitude of the eclipse. At the beginning of the solar eclipse, the Moon progressively covers the solar disk. Illumination of Earth's surface rapidly diminishes. The air temperature falls a few degrees. Seconds before the totality of the eclipse, shadow bands appear. Shadow bands are irregular bands of shadow, a few centimeters wide and up to

a meter apart, moving over the ground. The diamond ring phase of the eclipse then shines for few seconds and later, Bailey's beads appear on the solar limb. Bailey's beads are a string of bright beads of light produced by the uneven shape of the lunar limb.

In the first two to three seconds of the total phase of the eclipse (totality), the chromosphere is visible as a pink halo around half of the limb. Maximal duration of the totality varies from eclipse to eclipse, up to 7.5 minutes. The brightest stars and planets are observable on the sky during the totality. Prominences are the brightest objects visible continuously during the totality. They are clouds of relatively cold (10,000K) and dense matter with the same properties as that of the chromosphere matter. They emit in lines of hydrogen, helium and calcuim, which produce the pink color of prominences and the chromosphere, and can always be observed in monochromatic light.

White corona can be observed from Earth only during total solar eclipses, because its intensity is much lower than the brightness of the sky. It has several components emitting in the entire visible region of spectra. The K- (Electron or continuum) corona is due to scattering of sunlight on free high-energy electrons, which are at a temperature of 1 million degrees, and contain continuous spectra and linear polarization of the light. The K-corona dominates in the corona, have distinct 11-year cycles, and have variable structures depending on the level of solar activity. During the solar maximum, it is circular. During the solar minimum, it is symmetrical and elongated in the equatorial region, while

in the polar regions, it has bunches of short rays or plumes. During intermediate phases, it has asymmetric structure with many streamers of different lengths. The F- (Fraunhofer or Dust) corona is due to scattering of sunlight on dust particles. An F-corona has Fraunhofer spectra with absorption lines. Due to heating of dust particles close to the Sun, the F-corona evaporates, producing a large cavity in the dust distribution. An F-corona has oval shape. Its intensity decreases slowly with the distance from the Sun, and it predominates over the K-corona at long distances. The F-corona reaches near-Earth space , producing Zodiacal light (a faint conical glow extending along the ecliptic, visible after sunset or before sunrise in a dark, clear sky). The Thermal (T) corona is due to thermal emission of dust particles heated by the Sun.

Solar corona also have components emitting linear spectrum. The E- (Emission) corona is due to emission lines of highly ionized atoms of iron , nickel, and calcium. The E-corona intensity decreases rapidly with its distance from the Sun and is visible up to a 2-solar radius in monochromatic light. The S- (Sublimation) corona, was recently found, but as of 2002, its existence is still debatable. It consists of emission of low ionized atoms of Ca(II) produced by sublimation of dust particles in relatively cold parts of the corona. All these components are visible together in the corona during total eclipses.

The last and most mysterious component of the corona is giant coronal streamers observed only from the orbital coronagraph LASCO and from stratospheric flights during total eclipses. The giant coronal streamer shape and properties are

different from those of any other component of the corona. Animations of their timed development look similar to visualizations of gusts of solar wind . In the last few years, evidence has arisen demonstrating that its nature is the same as that of plasma tails of comets , fluorescence of ionized gas molecules (originated by evaporation of comets near the Sun), and is due to interaction with the solar wind and sunlight. This component of the corona is called Fluorescent (Fl) corona, but this hypothesis needs further scientific verification. The corona is divided arbitrarily to Internal corona (up to 1.3 radius), which can be observed any time by coronagraph, Medium (1.3-2.3 radius), and External corona (over 2.3 radius) where F-corona dominates. Edges of the corona gradually disappear in the background of the sky. Therefore, the size of the corona greatly depends on the spectral region of observations and clearness of the sky.

Lunar eclipses occur when the Moon passes into Earth's shadow. The Moon does not normally disappear completely; its disc is illuminated by light scattered by the Earth's atmosphere. Color of the lunar eclipse depends highly on the composition of the atmosphere (amount of ozone and dust). The full shadow (umbra) cast by Earth is surrounded by a region of partial shadow, called the penumbra. Some lunar eclipses are visible only as penumbral, other as partial. The length of the Moon's path through the umbra, divided by the Moon's diameter, defines the magnitude of a lunar eclipse.

See also Coronal ejections and magnetic storms