NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: refraction
From: Lu Abel
Date: 2007 Jan 04, 20:22 -0800
From: Lu Abel
Date: 2007 Jan 04, 20:22 -0800
Guy: When you say "how do I know if I'm viewing with or without refraction to start the analysis," let's consider two cases: If there was a constant distortion of the position of a body throughout its arc across the sky, it likely would be of little consequence, since it would be a constant and uniform error and almanacs could simply include this constant error. The key words here are constant and uniform. But that's not the case in the real world. When we observe a body with a reasonably high altitude there is no refraction and as a result the body exhibits a uniform rate of movement in its arc across the sky.* To a terrestrial observer, though, the body seems to slow down as it approaches the horizon. I lack a knowledge of the history of astronomy, but even pre-telescope Greek, Roman, Arab, and medieval astronomers were able to do some pretty darn precise measurements. I'm sure they could observe the motion of a body across the sky precisely enough in location and time to detect this apparent slowing of the body. A bit of 17th century science is applied (the body does move at a uniform rate and, oh, there is this phenomenon called refraction that occurs when light passes through dense media) and we conclude that a body's apparent position when it is near the horizon is affected by refraction. Lu Abel * Before anyone on the list objects, I'm assuming there is little or no motion of the body itself, which is not precisely true for solar system bodies. Guy Schwartz wrote: > > Lets take the sun for example, how would one know if they were viewing > it with or without refraction to start the analysis? > > ----- Original Message ----- From: "Lu Abel"> To: > Sent: Thursday, January 04, 2007 7:11 PM > Subject: [NavList 1977] Re: refraction > > >> >> Not much different than measuring bending of the stick, Guy, except >> with a bit of patience required. >> >> If I track the rising or setting of a body, I should see a uniform >> rate of motion in the sky as long as refraction is insignificant. >> Once it becomes significant, however, the rate should change. >> >> I could, for example, calculate the expected time a body would >> disappear below the horizon based on its motion through the sky. But >> I would observe it to set later. The time of observed setting would >> lead me to a value for refraction, ie, the amount the body is below >> the horizon and therefore the amount of refraction. >> >> Measurement of factors such a pressure, humidity, and temperature >> could then be used to refine the parameters. >> >> Lu Abel >> >> Guy Schwartz wrote: >> >>> Inserting a 4 foot stick half way into a big tank of water at a 45 >>> degree angle, the refraction can be measured or closely approximated. >>> How were the refraction adjustments initially calculated for the >>> atmosphere? >>> What was the methodology used? I don't think it was as easy as the >>> stick in the water method? >>> Thank you. >>> Guy >>> > >> >> >> > >> >> >> -- >> No virus found in this incoming message. >> Checked by AVG Free Edition. >> Version: 7.5.432 / Virus Database: 268.16.5/616 - Release Date: >> 1/4/2007 1:34 PM >> >> > > > > > --~--~---------~--~----~------------~-------~--~----~ To post to this group, send email to NavList@fer3.com To , send email to NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---