NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
The Sun's limb
From: Frank Reed CT
Date: 2007 Apr 16, 19:22 -0700
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 --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---