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    The Sun's limb
    From: Frank Reed CT
    Date: 2007 Apr 16, 19:22 -0700

    NOTE: this post has almost nothing to do with navigation.
    
    We've all seen the very sharp limb of the Sun through a sextant, and
    many of us on the list have seen that it is still extraordinarily
    sharp and smooth when viewed at high power through a telescope. But
    it's really something of a miracle. There's no "surface". The sharply-
    defined photosphere is just a point in the Sun's atmosphere where the
    gases shift rather abruptly from being opaque (like a dense, glowing
    fog) to transparent. Other stars which have been "imaged" in various
    ways do not necessarily have this abrupt transition. They have "hazy"
    limbs which would be much less convenient for navigation than our
    star's limb. So what's responsible for the abrupt change in opacity?
    Why does the Sun have a sharp limb?
    
    Some decades ago (I don't know the historical details),
    astrophysicists figured out the principle source of opacity in the
    atmospheres of G type stars, like our Sun. Its source is a strange ion
    of hydrogen. Hydrogen, as we all know, is the simplest atom possible.
    Its nucleus consists of a single proton (and rarely an extra neutron
    or two) with a single electron bound to it --one positive, the other
    negative, held together by electrostatic attraction in a simple
    quantum state. That doesn't appear to leave much room for variation.
    If we take away the electron, we're left with a simple proton. Ionized
    hydrogen is simply a plasma of protons and electrons. But that's not
    responsible for the opacity at the Sun's limb. Instead, it turns out
    that there is another meta-stable bound state involving hydrogen --
    another ion of hydrogen. It's the H- ion, the negatively charged
    hydrogen ion. That is, it's a hydrogen atom with an extra electron
    bound to it. This is a weakly bound state with an ionization energy of
    a mere 0.75 eV, and it can only exist at certain temperatures and
    densities of hydrogen gas. It turns out that those conditions are just
    met at a particular height in the Sun's atmosphere. It turns out that
    absorption by the H- ion is directly responsible for the existence of
    the sharp, visible limb of our Sun. Were it not for the existence of
    this strange species of hydrogen ion, the visible edge of our star
    might look very different, and the sharply-defined limb that we see
    might not exist at all. But because the ion does exist, and because
    the Sun is hydrodynamically quite stable and rotates so slowly, the
    diameter of the Sun can be measured very accurately. While the limb of
    the Moon shows visible variations from mountain ranges of two and
    three seconds of arc (with implications for lunar distance
    observations), the limb of the Sun is smooth down to a fraction of an
    arcsecond.
    
    -FER
    
    
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