The Sun is an absolutely massive body with an equatorial diameter of 1.392 million kilometers. Over 1.3 million Earths could fit inside the Sun and it would take a third of a million Earths to equal the Sun's weight! It is estimated that the Sun accounts for more than 99% of the mass of the solar system. The Sun rotates in 25.38 days with respect to the stars. As we orbit the Sun, we see it rotate in a slightly longer period, namely, 27.28 days. At our closest to the Sun (perihelion occurs around January 2), we are 147.1 million kilometers away. At our furthest (aphelion occurs around July 5) we are 152.1 million kilometers away. Put another way, we orbit the Sun at an average distance of 149.6 million kilometers ± 1.7%. Light takes 8.3 minutes to travel from the Sun to the Earth. The brilliant surface of the Sun is called the photosphere and the most notable features on this blinding yellow surface are the sunspots. Sunspots look dark because, at around 4000°C, they are cooler and dimmer than the 5000°C photosphere around them. However, if you could see them in isolation they would glow like an arc lamp. Sunspots are, primarily, the result of intense bipolar magnetic fields formed where concentrated fields emerge from the photosphere. If you place some iron filings on a sheet of thick card and move a powerful bar magnet under the card, the filings will move around in a very similar manner to high-resolution animations of sunspots evolving with time! Long-lived sunspot groups can rotate off the limb of the solar disc and emerge on the opposite limb, still in-tact, two weeks later. Indeed, the very largest spot groups survive for several solar rotations. The largest sunspot group recorded was that of April 1947, which covered an area of 18 billion square kilometers. In other words, it had an area almost 40 times larger than the surface area of the Earth! Such huge sunspot groups can span two or three arc-minutes in size, i.e., a larger angle than any planet spans as seen from Earth and equal to the angle subtended by the largest lunar craters. The Sun has an 11-year cycle of activity during which a lot of sunspot activity is observed at the start (solar maximum), followed by a virtually spot-free Sun (solar minimum) some 51/2 years later, and another maximum after 11 years. During total solar eclipses the very outer atmosphere of the Sun appears almost symmetrical at solar maximum, but highly east-west elongated at solar minimum. Huge solar prominences arching above the Sun can be seen at total eclipses or if you have expensive H-Alpha filters (read on). When a really major solar flare erupts on the Sun and charged particles head earthward in a CME (coronal mass ejection), the results can be awesome. If the mass hits the Earth's magnetic field (a rare event), a spectacular aurora can be seen even at temperate latitudes. The last solar maximum occurred in 2001/2002. The next should be in 2012/2013.
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