02-29-2016, 04:51 AM
Irradiation of the Moon:
Irradiation is the phenomenon whereby there is an apparent enlargement of the edges of a strongly illuminated object when viewed against a dark background. The effect would make it more difficult to accurately observe the moon by night, particularly when low on the horizon - but, back to my original argument, why would we want to observe the moon by night when the sky is already full of stars? On nights with a particularly bright moon I have successfully taken star sights against a sufficiently moonlit horizon.
Irradiation is mentioned in older editions of Bowditch, and a correction for irradiation was included in the NA until around 1970; it was dropped from later editions because it was difficult to quantify accurately.
My follow up question perhaps belongs in a different thread - Who bothers with Noon sights, and why?
I ask because there seems to be an almost religious attachment to the noon sight amongst many navigators and a belief that it is necessary to determine latitude to obtain a fix! Consequently we hang around on deck for 20 minutes awaiting the magical zenith moment while our lunch is going cold in the galley below, only to have the only clouds of the day obscure the sun right about LAN. Why not snap off and reduce a quick series of sights whenever we please and cross the resultant LOP with that of our advanced morning sight. This still renders our position; we can still determine Lat and Long. We only NEED a Latitude if we are performing a Time Sight - an observation by which we determine LHA and thence Longitude - but when using the intercept method precise Lat is not necessary.
Lunars? Great in theory, terribly difficult in practice, particularly from the heaving deck of a small sailing vessel!!
Jeremy
Irradiation is the phenomenon whereby there is an apparent enlargement of the edges of a strongly illuminated object when viewed against a dark background. The effect would make it more difficult to accurately observe the moon by night, particularly when low on the horizon - but, back to my original argument, why would we want to observe the moon by night when the sky is already full of stars? On nights with a particularly bright moon I have successfully taken star sights against a sufficiently moonlit horizon.
Irradiation is mentioned in older editions of Bowditch, and a correction for irradiation was included in the NA until around 1970; it was dropped from later editions because it was difficult to quantify accurately.
My follow up question perhaps belongs in a different thread - Who bothers with Noon sights, and why?
I ask because there seems to be an almost religious attachment to the noon sight amongst many navigators and a belief that it is necessary to determine latitude to obtain a fix! Consequently we hang around on deck for 20 minutes awaiting the magical zenith moment while our lunch is going cold in the galley below, only to have the only clouds of the day obscure the sun right about LAN. Why not snap off and reduce a quick series of sights whenever we please and cross the resultant LOP with that of our advanced morning sight. This still renders our position; we can still determine Lat and Long. We only NEED a Latitude if we are performing a Time Sight - an observation by which we determine LHA and thence Longitude - but when using the intercept method precise Lat is not necessary.
Lunars? Great in theory, terribly difficult in practice, particularly from the heaving deck of a small sailing vessel!!
Jeremy