moon monsters
Dear Roger Baily, Arthur Carlson and Fred Sawyer, Thanks for all of your detailed answers to my inquiry as to whether one should correct sundial moonlight readings with the Equation of Time. You all seem to be in agreement that to make an EXACT suntime/moontime correction requires some monstrous mathematics. As this information is going into my Owner's Manual, I really want to keep it simple for my average sundial customer (and for me!) In my manual I begin by explaining how the moon moves like the sun at night following roughly the same path. Then I point out that when there is a full moon the moon is opposite the sun on the horizon at sunrize and sunset. Everybody's seen this. Since the sun and the moon and the earth are in alignment during a full moon, the moon will act like the sun at night and the sundial becomes a moondial. I state the a good reading may be off by as much as 45 minutes because the moon's orbital plane is tilted 5 degrees away from the earth-sun plane. I provide an easy to read table with the classic 2 min./hr. (48min./day) lunar phase time correction. (my sundials are already longitude corrected). My customers are thrilled when the find out that they can use their sundials at night using the moon, even if it is 45 minutes off. I just thought that I could narrow down the error with the E.O.T. (they understand the E.O.T.) and still keeps it simple for the layman. I am still in doubt whether to use the E.O.T. Would you please reconsider your answers knowing that I'm trying to keep it simple for my sundial customers? Thanks again, John Carmichael p.s. I have noticed that right before, during, and after a partial or total eclipse of the moon that the sundial tells PERFECT time without using the E.O.T. I don't remember the dates so it might have happened on or near those times of the year when there is little or no E.O.T. correction.
Re: Latitude/Longitude
There's a difference between astronomical latitude and longitude and geodetic latitude and longitude. Prof. Charles Merry at the University of Cape Town should be able to help you out with the specifics of geodetic datums used in South Africa: [EMAIL PROTECTED] -- Richard Langley Professor of Geodesy and Precision Navigation On Wed, 13 Jan 1999, Anton Reynecke wrote: >Apologies for this non-sundial question, but I do hope someone can help. > >I've always been under the impression that Latitude/Longitude is a universal >and unambigious method of indicating a position on the earth but now I am >not so sure > >In South-Africa, the National survey system is based on a Gauss conform >system with the Clarke 1880 (Modified) Ellipsoid. > >It is fundamentaly the same as the wordwide UTM system, with a slightly >different scale factor, and the width of a system is only two degrees in >longitude, one on either side of a central meridian, whereas UTM covers six >degrees. > >Now the National system is based on the the same projection but we are using >the WGS 84 ellipsoid. > >That caused the Latitude of a fixed point to increase South by 2,04 arc >seconds, and Longitude West by 1,06 arc seconds (approximately), and the >projected co-ordinates changed by approx. 296 metres South and 27 metres >West (Differences calculated by comparing coordinates from the old system >with the new system, around Pretoria). > >I am under the impession that Lat/Long is astronomically fixed from distant >objects, with the origin being the rotation axis of the earth, so how can >the values be influenced by adopting a new ellipsoid ? > >What fundamentals am I missing? > > >D. Anton Reynecke > > > > > > > === 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/ ===
Latitude/Longitude
Apologies for this non-sundial question, but I do hope someone can help. I've always been under the impression that Latitude/Longitude is a universal and unambigious method of indicating a position on the earth but now I am not so sure In South-Africa, the National survey system is based on a Gauss conform system with the Clarke 1880 (Modified) Ellipsoid. It is fundamentaly the same as the wordwide UTM system, with a slightly different scale factor, and the width of a system is only two degrees in longitude, one on either side of a central meridian, whereas UTM covers six degrees. Now the National system is based on the the same projection but we are using the WGS 84 ellipsoid. That caused the Latitude of a fixed point to increase South by 2,04 arc seconds, and Longitude West by 1,06 arc seconds (approximately), and the projected co-ordinates changed by approx. 296 metres South and 27 metres West (Differences calculated by comparing coordinates from the old system with the new system, around Pretoria). I am under the impession that Lat/Long is astronomically fixed from distant objects, with the origin being the rotation axis of the earth, so how can the values be influenced by adopting a new ellipsoid ? What fundamentals am I missing? D. Anton Reynecke
Re: moonlight readings
John Carmichael writes: > >I have a section which tells how to tell time by using moonlight and a > >sundial. I provide a table of corrections from which the time can be > >estimated if one knows the age (the phase) of the moon. > > > >One question though: Is it nessary to correct moontime with the Equation of > >Time ? > >Since the Equation of time is due to the eccentricty of the earth's orbit > >around the sun and the tilt of the earth's axis, it seems to me that this > >has nothing to do with the moon and should not be considered in the > >corrections. Am I right? Roger Bailey <[EMAIL PROTECTED]> writes: > Hello John, > > My advice is "Don't go there. There be monsters!" * Good advice. > The motion of the moon is quite complicated and the "equation of time" > shortcut will not work. You were right is concluding that the solar > "equation of time" does not apply, and the eccentricity and obliquity of > the ecliptic were the determinants of the equation of time. I wouldn't agree the Equation of Time does not apply, just that other corrections are much larger. John does, after all, want to correct for the phase of the moon, so the position of the sun is relevant. > The major > problem with the moon is the time between new moons (lunation) is 29.53 > days, different from the orbital period of 27.32 days. This means the > declination cycle, connected with the orbital period, is out of phase with > the lunation cycle. This makes it sound like these are two separate orbital parameters. They are simply connected by the length of the year: 1/27.32 - 1/29.53 = 1/365 In fact, the time between any two particular adjacent lunations will have a correction closely related to the Equation of Time. > For night time checks, I use a "nocturnal" and determine the time based on > the rotation of the big dipper around Polaris. The date / sidereal time > correction is easier to build into the instrument. Even easier than correcting a sundial for the Equation of Time. I have been interested for some time in the related problem of finding directions from the moon, possibly given watch time. I haven't formulated the mathematics yet. To quantify the error of various methods I will need some more information on the distribution of the relative positions of the sun and moon. This is certainly known. Is it also readily available in a comprehensible form? Art Carlson
Reproductions
I would appreciate any reference to someone in the UK who is qualified to make a reproduction of a mariner's astrolabe from the 15th century from drawings. Such a device would be made of a solid wood disk with a brass edge and a brass alidade. The reproduction is for a BBC documentary. Best regards, James E. Morrison Astrolabe web pages at: http://myhouse.com/mc/planet/astrodir/astrolab.htm
