Re: GMT and UTC
Earlier I wrote: The Explanatory Supplement to the Astronomical Almanac, mentions alternatives to atomic time under study that may offer improvements. (I don't have the book handy and cannot recall what they are. I will try to post a follow-up on Monday.) I believe that atomic timing technology is about fifty years old now... [...] Jim On pages 60-61 of the Explanatory Supplement to the Astronomical Almanac, is a discussion of modern timing technologies. 2.321 Quartz-Crystal Oscillators ... The performance characteristics may range from 10^-4 to 10^-13 in frequency stability per day. ... 2.322 Cesium Beam Standards ... laboratory cesium-beam frequency standards ... realize the second with the utmost accuracy (currently, 1.5 x 10^-14) and are, therefore, stable in the long term. ... 2.323 Hydrogen Masers ... The optimum stability reaches about 1 x 10^-15 for integration times of 1000 to 1 seconds. ... [S]ome hydrogen masers equipped with automatic tuning of the cavity and kept in temperature-controlled rooms have a long-term stability of the same order as the best cesium standards. ... 2.324 Rubidium Vapor Cells The rubidium clock is an appropriate device when a relatively low-cost clock is needed that has better stability than a quartz crystal clock. The rubidium clock can reach a stability of 1 x 10^-13 per day under the best conditions, but is subject to temperature- and pressure-induced frequency variations. Ringer et al. (1975) describe the design and performance of a clock for the GPS satellites. This clock has achieved stability of 2 x 10^-13 per day. 2.325 Mercury Ion Frequency Standard The mercury-ion frequency standard uses ions that are confined in a small region of space by an electromagnetic field trap. Thus the particles can be observed without having them collide with the walls, which would disturb the atomic resonance. The mercury-ion isotope Hg-199 has an extremely narrow microwave resonance line at 40507 MHz. Although this type of frequency standard should be a large improvement over the cesium standard, it has an unfortunately low signal-to-noise ratio, resulting in limited short-term stability. However, the long-term stability is very good, since integration can take place over a number of days (Winkler, 1987). I suppose I was incorrect to call these alternatives to atomic timing. Quartz is molecular; the maser is ultimately atomic in nature (or perhaps molecular, I'm not sure); and the others are all atomic. But they are alternatives to atomic cesium clocks. Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- Have you noticed how the hole in the ozone layer has grown progressively larger since rap got popular? -- Dave Barry
Re: Sunset times (was: GMT and UTC)
[EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] wrote: snip ...as the time of sunset varies by as much as 6 hours from solstice to solstice (here in Michigan). end snip Hmmm... can someone help me out? A quick check of my astrolabe, with a plate for St. Paul, MN, gives sunset at about 1618hrs for the winter solstice, and about 1943 for summer - about a 3 1/2 hr variance. Am I missing something? Mike Blackwell For Detroit (assuimg EST is the time zone) xephem computes Limb 6/21/1999 4:55:05 EST: RiseTm SetTm HrsUp Sun 4:55 20:13 15:18 Limb 12/21/1999 4:55:05 EST: RiseTm SetTm HrsUp Sun 7:57 17:03 9:06 which agrees with your astrolabe computations. My guess is he was thinking of a six hour daylight delta. Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- Your emotions are often a reverse indicator of what you ought to be doing. -- John F. Hindelong
Re: GMT and UTC
Dear Dialists... Regarding the assertion by David Higgon that the Earth makes a better clock than the frequency of an arbitrarily chosen atom. Unfortunately Earth's rotation is slowing down, so the atom is preferable, though admittedly less romantic. Presumably, if the human timekeepers endure long enough on the Earth there would be a noticeable difference in the day of the equinoxes and solstices. (the Earth's revolution IS NOT slowing down, which makes the atomic measurement necessary...please correct me if this is wrong) Troy Heck Ortonville, Michigan The Explanatory Supplement to the Astronomical Almanac, mentions alternatives to atomic time under study that may offer improvements. (I don't have the book handy and cannot recall what they are. I will try to post a follow-up on Monday.) I believe that atomic timing technology is about fifty years old now... Clock technology marches on: First shadow measurement of diurnal rotation, then clepsydras, then spring/escapement mechanisms, then pendulum clocks (which were accurate enough to lead to the notion of the mean sun and the equation of time), then atomic clocks, then ???. Jim --- -- | Jim Cobb | 540 Arapeen Dr. #100 | [EMAIL PROTECTED] | | Parametric| Salt Lake City, UT | (801)-588-4632 | | Technology Corp. | 84108-1202 | Fax (801)-588-4650 | --- -- He that wrestles with us strengthens our nerves, and sharpens our skill. Our antagonist is our helper. -- Edmund Burke
Re: GMT and UTC
Dear John, That's an excellent point, but of course time is used for more than determing when the sun will be visible. A sundial is actually a poor indicator even of what it does best, as the time of sunset varies by as much as 6 hours from solstice to solstice (here in Michigan). So really what good does it do someone to determine the length of rotation besides as a yard stick to measure other temporal phenomena? Dear Troy, Do you think it would be safe to say that a sundial is the ONLY clock which automatically keeps up with the earth's slowing rotation rate because it will always divide the day into 24 hours, no matter how long the day is? As the earth's rotation slows and the days become longer, won't seconds, minutes and hours become longer also? Surely, in the distant future there won't be 25 hour days, but longer 24 hr. days. Right? John Carmichael Tucson Az
Re: GMT and UTC
Some of the recent postings on GMT and UTC contain inaccuracies and half-truths. It was to set the record straight that I wrote the article on GMT and UTC in the first place. It's enough to drive a professor to drink (feel free to send some 18 yr. Macallan single malt!). For those who haven't yet had a chance to read my article: 1) UTC is not a French acronym. They used to use TUC as an acronym but they now use UTC along with everyone else. Before UTC, we had UT0, UT1, and UT2. So it was obvious that Coordinated Universal Time should be UTC and not CUT. 2) When the Gregorian calendar was introduced in 1582, 10 days were dropped. For a good on-line reference to calendar issues see Claus Tonderings's Frequently Asked Questions about Calendars: http://www.pip.dknet.dk/~pip10160/cal/calendar20.html. 3) UT1 does not vary due to polar motion but only due to changes in the earth's spin. The effect of polar motion is, however, in UT0. 4) GMT, if U.K. standard time is meant (which is the usual case when the term GMT is used these days), is EXACTLY the same as, and therefore equally as accurate as, UTC. (Different timing labs around the world maintain different versions of UTC and through clock comparisons, chiefly using GPS, a definitive UTC is maintained by the Bureau International des Poids et Mesures). 5) Perhaps the use of GMT as a synonym for UT1 should be deprecated. 6) UTC is actually adjusted for leap seconds, not TAI. A plot of UTC vs. TAI over the years shows the leap second jumps. -- Richard Langley Professor of Geodesy and Precision Navigation === Richard B. LangleyE-mail: [EMAIL PROTECTED] Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics EngineeringPhone:+1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ ===
GMT and UTC
Dear All, Thank you for the many responses regarding the difference between GMT and UTC. Let me see if I've got this right... To all intents and purposes the two are the same. UTC is more acurate (or should I say regular) because it's based on the frequency of some arbitrary atom that has nothing to do with rates of rotation of the earth or any other astronomical rhythm. It's used by the scientific community because of its acuracy, but not by anyone else, and whenever it strays too far from GMT, leap seconds are used to bring it back in line. Hmm... So who's the master here, and who the slave?! David Higgon London, not too far from Greenwich ;-)
GMT and UTC
Dear Dialists... Regarding the assertion by David Higgon that the Earth makes a better clock than the frequency of an arbitrarily chosen atom. Unfortunately Earth's rotation is slowing down, so the atom is preferable, though admittedly less romantic. Presumably, if the human timekeepers endure long enough on the Earth there would be a noticeable difference in the day of the equinoxes and solstices. (the Earth's revolution IS NOT slowing down, which makes the atomic measurement necessary...please correct me if this is wrong) Troy Heck Ortonville, Michigan
