# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Lunars barometric pressure correction**

**From:**Frank Reed CT

**Date:**2005 Apr 22, 22:24 EDT

I wrote earlier: By the way, you can also use it to answer the question "where does space begin?" Just set the equation for density of the air: density = exp(-altitude/34,000feet) equal to 0 and solve for the altitude. :-)" Bill you replied: "Having little success with modifying the formula with my math skills. Trying to manipulate the exponent and or find the nth root of zero has me off in deep space. Did try the plug and chug method with the TI-30, and ran out of computing power approx. 1277 nm. Looks like with enough computing power it could go on like pi. Interesting as I would have thought the value would have been in the 60-120 nm range." Yep. It never ends. The equation 0=exp(-altitude/34000feet) has no solution. The atmosphere never ends. It just gets thinner and thinner until it becomes indistinguishable from the interplanetary medium. If you ever end up explaining this to a group of students, one of them will surely ask, "but, but isn't the International Space Station in space??" Isn't it? Well for various practical purposes, it is, yes. It is so nearly in freefall that for nearly all practical purposes the astronauts are weightless and free of the influence of air. But the station actually experiences considerable air resistance from the higher reaches of the Earth's atmosphere even "way up there". That air resistance slowly bleeds away its orbital energy. The station would burn up in thicker air within a year and a half or so unless actively reboosted every couple of months. That, of course, is exactly what happened to the abandoned US space station Skylab back in 1979 when it rained debris over Australia and the abandoned Soviet space station Salyut 7 back in 1991 which rained its debris over Argentina. Sorry for getting off-topic. Just to help understand the barometric pressure issue, there is another way of writing the equation. We can change the "base" from e to 2 and write it as relative density=(1/2)^(altitude/24000feet). It might not look much better and really it is exactly the same content as the original equation, but the verbal description is much easier to understand for most people. This equation is saying that the density is cut in half every 24,000 feet. It's a simple "geometric progression". And when you start out with any non-zero number (air density at sea level in this case), no matter how many times you cut it in half, you will never get to zero. -FER 42.0N 87.7W, or 41.4N 72.1W. www.HistoricalAtlas.com/lunars