Comments on: Lunar eclipse Dec. 10 https://habitablezone.com/2011/12/08/lunar-eclipse-dec-10/ Fri, 03 Apr 2026 18:20:54 -0700 hourly 1 http://wordpress.org/?v=3.3.1 By: ER https://habitablezone.com/2011/12/08/lunar-eclipse-dec-10/#comment-9303 ER Fri, 09 Dec 2011 00:34:31 +0000 http://habitablezone.com/?p=5826#comment-9303 The atmospheric refraction effect is very real, and must be corrected for in sextant observations in celestial navigation. The effect is non-linear, 0 at the zenith, and increases rapidly towards the horizon, to as much as a half degree at the horizon. Remember, a minute of arc is a nautical mile of latitude. Unfortunately, the effect is very temperature and pressure dependent and so is difficult to model accurately for extremely low elevations. It can be as much as approx 30 minutes of arc, (about the angular size of the solar or lunar disc). This means that when the orb is seen just clear of the horizon, it is actually just below it. Still, this is good enough to derive a rough longitude, so its a good thing to know if your chronometer is down. The atmospheric refraction effect is very real, and must be corrected for in sextant observations in celestial navigation. The effect is non-linear, 0 at the zenith, and increases rapidly towards the horizon, to as much as a half degree at the horizon. Remember, a minute of arc is a nautical mile of latitude.

Unfortunately, the effect is very temperature and pressure dependent and so is difficult to model accurately for extremely low elevations. It can be as much as approx 30 minutes of arc, (about the angular size of the solar or lunar disc). This means that when the orb is seen just clear of the horizon, it is actually just below it.

Still, this is good enough to derive a rough longitude, so its a good thing to know if your chronometer is down.

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