NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: "Differential" Altitude Measurement (?)
From: Frank Reed CT
Date: 2006 Jun 1, 05:26 -0500
Hi Greg, you wrote:
"Venus is really close to the moon tonight (Wednesday, on the west coast of
the US)"
A little typo there, yes? You meant to right, "Saturn is really close to the
moon tonight". No problem. It's obvious that that is the case.
And you wrote:
"but I wasn't able to find its reflection in the artificial horizon (might
work better when it's darker, I've been able to bring down Jupiter that way).
So as a substitute technique, I got a good altitude on the Moon in the AH,
then quickly measured the vertical angle between it and Saturn (or really
guesstimated by eyeball, it's hard to be accurate since the moon is still a
crescent tonight), subtracted that from the first reading, and used it for
Saturn's Hs. Navigator software gives me an intercept of 1.1 NM on this, therefore
I'm guessing that it either works or all of the errors fell my way for a
change... ;-) "
OK. I think I see what you're saying. But let me ask you this: if you have
an altitude of the Moon tat you trust, why would you want an altitude of
Saturn that is based on that Moon altitude? Whatever error you have in the Moon's
altitude will not go away. Throwing in a separate measurement of Saturn's
additional altitude above the Moon will only make things worse (on average). Why
not just take that Moon altitude and work IT up for your line of position? Is
it just because clearing Moon sights is a little more work? If so, then your
procedure will work, but not with reliable accuracy, as long as the second
object is very close to the same vertical line as the Moon.
And:
"The navigation texts talk about using a sextant to measure horizontal
angles to get a fix in the piloting section, so wouldn't this just be a variation
on that technique? If so, does it have a "real" name of its own?"
No and no. :-) Your 1.1 mile intercept was probably a lucky accident. Yes,
they do happen sometimes!
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
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From: Frank Reed CT
Date: 2006 Jun 1, 05:26 -0500
Hi Greg, you wrote:
"Venus is really close to the moon tonight (Wednesday, on the west coast of
the US)"
A little typo there, yes? You meant to right, "Saturn is really close to the
moon tonight". No problem. It's obvious that that is the case.
And you wrote:
"but I wasn't able to find its reflection in the artificial horizon (might
work better when it's darker, I've been able to bring down Jupiter that way).
So as a substitute technique, I got a good altitude on the Moon in the AH,
then quickly measured the vertical angle between it and Saturn (or really
guesstimated by eyeball, it's hard to be accurate since the moon is still a
crescent tonight), subtracted that from the first reading, and used it for
Saturn's Hs. Navigator software gives me an intercept of 1.1 NM on this, therefore
I'm guessing that it either works or all of the errors fell my way for a
change... ;-) "
OK. I think I see what you're saying. But let me ask you this: if you have
an altitude of the Moon tat you trust, why would you want an altitude of
Saturn that is based on that Moon altitude? Whatever error you have in the Moon's
altitude will not go away. Throwing in a separate measurement of Saturn's
additional altitude above the Moon will only make things worse (on average). Why
not just take that Moon altitude and work IT up for your line of position? Is
it just because clearing Moon sights is a little more work? If so, then your
procedure will work, but not with reliable accuracy, as long as the second
object is very close to the same vertical line as the Moon.
And:
"The navigation texts talk about using a sextant to measure horizontal
angles to get a fix in the piloting section, so wouldn't this just be a variation
on that technique? If so, does it have a "real" name of its own?"
No and no. :-) Your 1.1 mile intercept was probably a lucky accident. Yes,
they do happen sometimes!
-FER
42.0N 87.7W, or 41.4N 72.1W.
www.HistoricalAtlas.com/lunars
--~--~---------~--~----~------------~-------~--~----~
To post to this group, send email to NavList@fer3.com
To , send email to NavList-@fer3.com
-~----------~----~----~----~------~----~------~--~---
