10 most difficult Sun riddles

The first experimental photovoltaic cell has been built by Edmund Becquerel, French physicist aged 19 at the time, in his father garage in 1839. In 1883 Charles Fritts created the first solid state cell.

The sun performs a full turn around the center of our galaxy during 225 million years.

Halo is the name for a family of optical phenomena produced by light interacting with ice crystals suspended in the atmosphere. Halos can have many forms, ranging from colored or white rings to arcs and spots in the sky. Among the best known halo types are the circular halo (properly called the 22° halo), Other common optical phenomena involving water droplets rather than ice crystals include the glory and the rainbow.

The photosphere is a layer below which the Sun becomes opaque to visible light. It is tens to hundreds of kilometers thick, and is slightly less opaque than air on Earth. Because the upper part of the photosphere is cooler than the lower part, an image of the Sun appears brighter in the center than on the edge or limb of the solar disk, in a phenomenon known as limb darkening.

The Moon's orbit is elliptical, and its apparent diameter varies by 10%. This is why both total eclipses (if the Moon is larger) and annular eclipses (if the Sun is larger) happen.

Kepler is a space observatory launched by NASA to discover Earth-size planets orbiting other stars. Named after astronomer Johannes Kepler, the spacecraft was launched on March , 2009, in. Designed to survey a portion of our region of the Milky Way to discover Earth-size exoplanets in or near habitable zones and estimate how many of the billions of stars in the Milky Way have such planets,

Earth's perihelion occurs around January 3, and the aphelion around July 4. The distance change is about 3.5%, which gives 6.9% increase in solar energy reaching the Earth at perihelion relative to aphelion. Since the southern hemisphere is tilted toward the perihelion, the southern hemisphere receives more energy from the Sun than does the northern over the course of a year.

Solar cycles have an average duration of about 11 years. The periodic changes in the Sun's activity include changes of the levels of solar radiation and the number and size of sunspots.

The Sun is a nearly perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. Hot plasmas (thermal plasmas) have electrons and the heavy particles at the same temperature, i.e. they are in thermal equilibrium with each other. Cold plasmas (nonthermal plasmas) on the other hand have the ions and neutrals at a much lower temperature, whereas electrons are much "hotter".

Tropic of Cancer and Tropic of Capricorn mark the northernmost and southernmost latitudes at which the sun may be seen directly overhead. Since the whole Europe is north of the Tropic of Cancer, it is impossible to see the sun at the zenith there.