NavList:

On 5/4/2013 1:59 PM, I wrote:
> The true time of sunrise is nominally when the sun's lower limb is one
> half a semi-diameter above the horizon.

Sorry, got it wrong, misspoke. I meant to say a semi-diameter, not one half a semi-diameter.

This time I'll quote directly from no less an authority than the Admiralty Manual of Navigation Volume 2, Astro Navigation, 10th Edition, 2011, Section 0702:

"The time of True (Theoretical) Rising and Setting occurs when the centre of a heavenly body is on the observer's Celestial Horizon, to the east or west of his Meridian. At these times the True Zenith Distance is 90 degrees. Except in the case of the Moon, this phenomenonn cannot be observed directly from the Earth's surface due to Atmospheric Refraction raising the image of the body appreciably above the Visible Horizon.

The Sun. It is shown at Para 0723 below, that when the Sun's centre lies on the Celestial Horizon (ie at the moment of True (Theoretical) Rising and Setting), the Sun's Lower Limb appears one Semi-Diameter above the Visible Horizon. It is for this reason that Sunrise / Sunset compass checks are taken when the Sun's Lower Limb is one Semi-Diameter above the Visible Horizon."

Sorry for the earlier error.

wmh
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