Those of us living in the Western U.S. will witness a solar eclipse today (Sunday, May 20) from around 5-7 pm Pacific time.  A lucky few will get the full “ring of fire” spectacle, while many more will see some sort of partial eclipse.  If you wish to see this event, please follow the appropriate safety precautions.

Some fun facts:

• Solar eclipses happen when the moon gets between the sun and the Earth, so why don’t we have an eclipse with every new moon?  It’s because the moon-Earth orbital plane and the Earth-sun orbital plane are slightly (~5 degrees) tilted with respect to each other.  Most of the time when the moon passes by the sun’s direction, it’s slightly off the Earth’s orbital plane so its shadow misses us.  Every once in a while, though, the moon crosses the sun on plane.
• The moon’s orbit is slowly changing as tidal torques transfer angular momentum from the Earth’s spin to the moon’s orbital motion through a process first worked out by Charles Darwin’s son.  So, the day is slowly getting longer and the moon is slowly getting farther away.
• When the moon blocks out the sun’s photosphere (the yellow ball in the sky) in a total eclipse (which is not what we’re having today–we’ll only be seeing the sun 3/4 covered), we get to see the sun’s corona.  The sun doesn’t have a sharp surface where density goes to zero.  Low density gas actually extends quite far from what looks like the edge of the sun.  The “edge” we see (the photosphere) is really the surface of last scattering beyond which the gas is essentially transparent and photons travel unimpeded into deep space.  It’s sort of like the edges we seem to see on clouds.  The photosphere itself isn’t a sharp surface–it’s 600km thick–but that’s tiny compared to the sun’s radius, so the surface appears thick.
• First odd thing:  in the interior of the sun, temperature decreases as you go out, as one would sort of expect since the fusion is happening only in the center, but in the chromosphere (the layer just outside the photosphere), the temperature starts to increase again, reaching a million degrees Kelvin in the corona.  Why does this happen?  Presumably, it’s the magnetic field dumping energy into the gas through resistive effects like reconnection.
• Second, the corona and other outer layers of the sun are arguably more dynamically interesting than the interior.  The great Eugene Parker once noted that without the sun’s magnetic field, it would be a rather uninteresting dynamical system, and it’s the outer layers that are dominated by the magnetic field.  Reconnection in the corona drives enormous and violent events like solar flares and coronal mass ejections.
• The corona has “holes” (called thus because of their low X-ray emission) where magnetic field lines, instead of curling back into the sun, shoot out into outer space.  Charged particles, being more-or-less stuck on whatever field line they find themselves on, flow outwards along these open field lines, resulting in the solar wind.  This wind blows throughout the solar system to beyond the orbits of the planets.  (The Earth is protected by its magnetosphere.)  Thus, in a sense, the sun’s gas extends all the way through the solar system.  Signs of this solar wind blowing outward are seen in the tails of comets.
• The solar wind creates a sort of “bubble” in the interstellar medium.  The wind ends at a termination shock which we now know is about 94 astronomical units (AU–the distance between the Earth and sun) from the sun.  We know this because on December 2004, the Voyager 1 probe crossed the termination shock.  It is now 120 AU away in a region in between the solar and interstellar winds called the “heliosheath”.  Voyager 1 is expected to cross the outer edge of the heliosheath, called the “heliopause”, in around 2015, at which time it will become the first man-made object to definitively leave the solar system.  At this time, the probe is still taking data and transmitting messages to Earth (messages that take 16.5 hours to reach us traveling at the speed of light), although rundown of its power supply will increasingly restrict its ability to function in the coming years.  Voyager 2 crossed into the heliosheath in August 2007 and is also on a path out of the solar system.
• Where does the sun’s magnetic field come from?  That’s an interesting story, not yet entirely worked out.  Basically, magnetic field lines are stuck to parcels of gas (a general property of fluids with high electrical conductivity) and get “wound up” by the gas’s rotation and convective churning, a so-called “dynamo” process.  This is also where the Earth’s magnetic field comes from and presumably most other astrophysical magnetic fields.  The key to the process is thought to be a thin region in the sun’s interior called the “tachocline” where convective motions stop.  This is very interesting if true, because it means the magnetic flux tubes that create sunspots must rise a long way–a third of the sun’s radius.