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*Subject*: RE: [amsat-bb] AO40 velocity*From*: "Tom Clark, W3IWI" <w3iwi@xxxxxxxx>*Date*: Thu, 29 Aug 2002 21:32:36 -0400*In-reply-to*: <3D6EB162.4080408@voicenet.com>

Margaret is correct in stating that the reference frame needs to be defined. The answer that Stacey (W4SM) gave applied to an observer at the center of the earth. Let me expand on this with a couple of simple examples (which also give you some fun "magic numbers" to help the next time you have to answer these questions in a dingy bar): ----------------------------------------- The earth rotates on its axis every 24 hours = 86400 seconds. The original definition of the meter is that 10,000,000 meters is the distance from pole to equator along the meridian of Paris. Therefore I can take this number (and ignoring the fact that the earth is flattened), I immediately know that the circumference of the earth is 40,000 km. So if you are an observer on the equator, you are spinning at a speed of 40000/86400 = 463 meters/second. If you are at a higher latitude, then your spin velocity is reduced by cosine(latitude), so at 40 degrees (like Boulder), you are moving at 355 meters/second. So if the 'how fast "those things" go' question is answered for an observer on the earth you need to account for this effect, which is ~5% of the speeds that Stacey gave. ----------------------------------------- Now lets move the observer into interstellar space (perhaps at Alpha Centauri). The earth's distance from the sun (one AU = Astronomical Unit) is about 150,000,000 km and the earth's orbit is nearly circular, so the circumference of the orbit is 2*pi*150,000,000 km. The earth goes around the sun in one year which just happens to be pi*10,000,000 seconds (to within a small fraction of a percent) So the earth's orbital velocity can add as much 30 km/sec for part of the year and subtract 30 km/sec 6 months later when seen from outside. This is nearly 4 times the speeds that Stacey answered. Define the observer -- It's all relative! 73, de Tom, W3IWI ---- Via the amsat-bb mailing list at AMSAT.ORG courtesy of AMSAT-NA. To unsubscribe, send "unsubscribe amsat-bb" to Majordomo@amsat.org

**Follow-Ups**:**Re: AO40 velocity***From:*Margaret Leber

**RE: AO40 velocity***From:*Vince Fiscus, KB7ADL

**References**:**Re: AO40 velocity***From:*Margaret Leber

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