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Re: What do "d" and "v" really stand for?
From: Lu Abel
Date: 2008 Jun 19, 17:09 -0700
From: Lu Abel
Date: 2008 Jun 19, 17:09 -0700
I think Jim's comment may provide the key:
"d" is pretty obviously related to declination, and we know it's the hourly change in declination, which leads us to applying a "d corr" (as it is called on some sight reduction forms) to compute the exact declination of a body at the time of a sight.
Calling this declination change "d" always seemed pretty obvious. But why "v?"
It appears that whomever coined the term was thinking of the horizontal "velocity" (or more correctly, change in "velocity") of a body over time. A v correction applies only to the planets. And let's remember that they do appear to "wander" through the sky. What to call their speed against the fixed background of the stars? How about "velocity?" It's pretty large too -- a glance at the NA pages for today shows that while d is pretty small for the planets, v is comparatively large -- 1 to 3 minutes per hour which works out to almost a half up to more than one degree per day. The change in the planet's positions relative to the stars (their "velocities") would be pretty obvious to ancient observers after just a few day's time.
Lu Abel
James N Wilson wrote:
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"d" is pretty obviously related to declination, and we know it's the hourly change in declination, which leads us to applying a "d corr" (as it is called on some sight reduction forms) to compute the exact declination of a body at the time of a sight.
Calling this declination change "d" always seemed pretty obvious. But why "v?"
It appears that whomever coined the term was thinking of the horizontal "velocity" (or more correctly, change in "velocity") of a body over time. A v correction applies only to the planets. And let's remember that they do appear to "wander" through the sky. What to call their speed against the fixed background of the stars? How about "velocity?" It's pretty large too -- a glance at the NA pages for today shows that while d is pretty small for the planets, v is comparatively large -- 1 to 3 minutes per hour which works out to almost a half up to more than one degree per day. The change in the planet's positions relative to the stars (their "velocities") would be pretty obvious to ancient observers after just a few day's time.
Lu Abel
James N Wilson wrote:
Greg:Sorry. That makes for a more interesting question, to which I don't know the answer. My Navigation instruction referred to v as the velocity correction, but I don't know why.Jim Wilson
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