Sun

Specifics

Solar Eclipse
During the phase of new moon, the Moon passes between Earth and the Sun. If the Moon casts a shadow on Earth, an eclipse occurs. When the Moon completely covers the extremely bright photosphere of the Sun, it is a total eclipse. At this time, the outer edge of the Sun, or corona, appears. When the distance to the Moon is far enough Earth that the complete photosphere is not covered, it is an annular eclipse. If the observer is not quite in the shadow, it is a partial eclipse.

A complete shadow is called the umbra. A partial shadow is called the penumbra. Only small portions of Earth fall into the umbra. The duration at any given point on Earth is from a split second to seven and one-half minutes.

Sunspots
They are dark spots on the Sun, appearing so because these areas are cooler than the rest of the photosphere. Even so, the dark centre, or umbra, has a temperature of 4200°K, while the grey area, or penumbra, has a temperature of 5700°K. They are often found in groups. In these areas the magnetic field of the Sun is extremely intense. There seems to be an interaction between the rotation of the Sun and the currents that create the magnetic field. The average number of sunspots varies in a cycle of eleven years. They refract back to Earth radio waves that would have otherwise escaped into space.

Ecliptic
This is the Sun's path across the sky. It is tilted, relative to the celestial equator.

All planets move around the Sun in paths that are ellipses. The eccentricity of an ellipse is the measure of how close the orbit is to being circular. If it is 0, it is a circle. If it is less than 1, it is an ellipse. It it is 1, it is a parabola. That of Earth is 0.07, nearly a circle. Only Venus and Neptune have a lower eccentricity. Perhelion is the point on the orbit closest to the Sun. Aphelion is the point farthest away. Since all the planets revolve around the Sun inclined only slightly to the plane of Earth's orbit, they will be seen close to the ecliptic.

Aurora Borealis (Northern Lights)
It originates when solar flares on the Sun's surface free enormous amounts of charged particles into space. When they come close to Earth, they follow the natural magnetic field and form a large ring around the magnetic North Pole in Canada. As they interact with air molecules a few hundred kilometers above Earth, energy in the form of light is released. Near the horizon, it will be a pale greenish-white. Higher in the sky, red, green, and purple appear, constantly on the move.

A similar phenomenon appears around the magnetic South Pole. Here it is known as Aurora Australis.

The strongest displays usually occur in March, April, September, and October. Usually seen in the northern sky only, they can infrequently be seen across the whole sky in the northern latitudes. Auroras tend to peak and subside with the eleven-year siunspot cycle.

Seasons
Earth's axis is tilted relative to its orbital plane. Scientists have not been able to explain why this is so. This tilt causes Earth to have its seasons.

On December 22, the Sun fully illuminates the area below the Antarctic Circle. At this time, the area near the North Pole receives no sunlight. Relative to Earth, the Sun is directly above the Tropic of Capricorn, an imaginary line encircling Earth at 22.5°S. latitude. This is the Winter Solstice, the first day of winter.

On June 22, the opposite occurs. The Sun fully illuminates the area above the Arctic Circle. At this time, the area near the South Pole receives no sunlight. Relative to Earth, the Sun is directly above the Tropic of Cancer, which is at 22.5°N. latitude. This is the Summer Solstice, the first day of summer.

On March 21, the Sun is above the equator, and heading north. Everywhere else, there are equal hours of daylight and darkness. This is the Vernal Equinox, the first day of spring.

On September 23, the same occurs, except that the Sun is heading south. This is the Autumnal Equinox, the first day of autumn.