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    The Sun is by far the largest object in the solar system. It contains more than 99.8% of the total mass of the Solar System. (Jupiter contains most of the rest).

    It is often said that the Sun is an "ordinary" star. That's true in the sense that there are many others similar to it. But there are many smaller stars than larger ones; the Sun is in the top 10% by mass. The mediah size of stars in our galaxy is probably less than half the mass of the Sun.

    The surface of the Sun called the photosphere, is at a temperature of about 5800 K. Sunspots are cool regions, only 3800 K (they look dark only by comparison. with the surrounding regions). Sunspots can be very large, as much as 50,000 km in diameter. Sunspots are caused by complicated and not very well understood interactions with the Sun's magnetic field.

    It just happens that the Moon and the Sun appear the same size in the sky as viewed from the Earth. And since the Moon orbits the Earth in approximately the same plane as the Earth's orbit around the Sun sometimes the Moon comes directly between the Earth and the Sun. This is called a solar eclipse; if the alignment is slightly imperfect then the Moon covers only part of the Sun's disk and the event is called a partial eclipse. When it lines up perfectly, the entire solar disk is blocked and it is called a total eclipse of the Sun. Partial eclipses are visible over a wide area of the Earth but the region from which a total eclipse is visible, called the path of totality, is very narrow, just a few kilometers (though it is usually thousands of kilometers long). Eclipses of the Sun happen once or twice a year.

When does a total eclipse of the Sun happen?