Re: Sundial Info, and the Heinrich Harrer Sundial
Tony Moss wrote:- snipHappy New True Millennium to you all snip Just to keep the thing continuing for as long as possibe - if all this millennium hype is to celebrate 2000 years since the birth of Christ - why are we celebrating on the 1st January? Shouldn't we celebrate the event on 25th December 2001 Mike Shaw Wirral, UK 53' 22 N 03' 02 W
Re: Cast Away
[EMAIL PROTECTED] wrote: I saw Cast Away last week, and had a question about sundials and the movie. After Tom Hanks has spent several years on a desolate island, he constructs an impressive analemma from a thin beam of light that enters his cave, complete with days of the months. This would not be hard, if he had a working watch, but I don't think he did. So, my question is, is this just Hollywood chicanery, or is it really a possible thing to do? Bill G. (I need to know, in case my plane goes down in the south Pacific some day). I haven't seen the film, but if I had a watch which was accurate to within, say, a few seconds per day (not too ambitious), I would use the beam of light as a transit to sight a star every night. I would use this to set the watch, having calculated the length of a siderial day as 24 hours times 365.25/366.25. To ensure my eye was always in the same position, I would place a stone as a backsight or a headrest. I could use the same star for several months, then choose another. Thus, I would always know the time of midday to within a fraction of a minute, and could use this to create a pretty good analemma. or if I knew the eccentricity of the earth's orbit, date of perihelion, inclination of the ecliptic (I could measure this from the sun's altitude changes) and date of the vernal equinox, which I might know was March 21st, I could do the whole thing graphically or mathematically. That way, I could get to sleep at night. Without all that information, and lacking a watch, I know of no way to draw an analemma. By the way, on what surface did he draw the analemma? Happy New Year / Millennium to all Chris Lusby Taylor Newbury, England 51.4N, 1.3W
Re: Cast Away
Thanks, Bill, for once again politely answering a dumb question. You probably won't believe that for our new house in Brattleboro, which has a wall facing 46.56 degrees east of south, I'm just completing a combination noon dial, featuring a local apparent noon vertical line as well as a full analemma for Eastern standard noon, plus appropriate curves for the solstices and a straight line for the equinoxes. (As a result of an earlier question, the indicating spot of sunlight will pass through a circular hole installed parallel to the dial plate. :-) ) A couple more senior moments and I'll go out and purchase stock in a long-term care facility! Best wishes, Mac In a message dated 1/1/01 9:02:52 PM Eastern Standard Time, [EMAIL PROTECTED] writes: If my plane goes down in the south Pacific, I float around for some time in a raft, and then live for a couple of years in a cave which boasts a thin beam of sunlight, how, even with a working watch, would I draw an accurate analemma? You would mark the position of the sunbeam at the same watch time each day, and if the watch remains accurate, in a year you would have a full analemma. This is how the famous cover photograph from Sky and Telescope was made (about 20 years ago?). Bill G.
sunspots
Hello all: We watched the Christmas partial eclipse using a telescope to project an image of the sun onto a white piece of stiff paper. I fitted the large end my 4 refractor with an 18 square piece of cardboard with a 4 hole cut into it so that the cardboard would shade the viewing screen at the other end from unwanted sunlight. Even more amazing than the eclipse were the large number of sunspots. I've never seen so many. We counted 24 of them! This is because the sun is at the peak of its 11 year solar cycle. I checked, and found out that the sun will be at perihelion on January 4. I wonder if the closeness of the earth to the sun at this time of year will make the sun's disk appear larger and therefore permit closer inspection of the sunspots, or if the increase in size of the sun is too small to be significant. Does anybody know the difference in the apparent diameter of the sun, in degrees, between perihelion and aphelion? Is this significant? Thanks John Carmichael Tucson Arizona p.s. Some of the sunspots are so large that they can be viewed without a telescope with the eye, using a welders smoked glass.
suns angular diameter
From the web page: http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/SEgeometry.html Eclipse geometry is complicated by the fact that Earth's orbit around the Sun is elliptical. As a result, the Sun's apparent semi-diameter varies from 944 arc-seconds at aphelion to 976 arc-seconds at perihelion. This 3% range in apparent size is, of course, quite indistinguishable to the naked eye.
Re: suns angular diameter
It is true, however, that the difference is observable in page-size photographs that lie side by side on a table. There is an old project physics activity that has the student plot the distance to the sun based on changes in the apparent size of the sun; and from this data you can computer the shape of the earth's elliptical orbit to some degree of accuracy. You can also get the perihelion and aphelion distances and dates from this sort of data. Jeff Adkins Richard M. Koolish wrote: From the web page: http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/SEgeometry.html Eclipse geometry is complicated by the fact that Earth's orbit around the Sun is elliptical. As a result, the Sun's apparent semi-diameter varies from 944 arc-seconds at aphelion to 976 arc-seconds at perihelion. This 3% range in apparent size is, of course, quite indistinguishable to the naked eye. Content-Type: text/x-vcard; charset=us-ascii; name=Astronomer.vcf Content-Transfer-Encoding: 7bit Content-Description: Card for Jeff Adkins Content-Disposition: attachment; filename=Astronomer.vcf Attachment converted: Macintosh HD:Astronomer.vcf 1 (TEXT/ttxt) (00020FD9)
Re: sunspots
Hi John, Does anybody know the difference in the apparent diameter of the sun, in degrees, between perihelion and aphelion? Is this significant? The info you want is in the BSS Glossary (plug, plug!) under semidiameter. The answers are 15.76 arcmins in July (aphelion) and 16.29 arcmins in January (perihelion). For the purposes of looking at sunspots, I'd say this was insignificant. If the differences were much larger, it would have a noticeble affect on the seasonal temperatures! Best regards, John Dr J R Davis Flowton, UK 52.08N, 1.043E email: [EMAIL PROTECTED]
Re: Cast Away
I suspect that the analemma, being familiar to the public on globes, sundials, etc., was used solely for that reason, call it artistic license. Sun position lines (corresponding to the latitude dimension of an analemma) were marked on the floors of some cathedrals. See J. L. Heilbron's 1999 book The sun in the church: cathedrals as solar observatories. Prior to the availability of accurate clocks, the Equation of Time was determined by measuring the longitude of the sun. This was done by determining the position of the sun relative to the stars. Since the sun was observable only during the day, and the stars only at night, a mutually observable body was used to facilitate the measurement: the moon by the ancient astronomers and Venus by Tycho Brahe. The maximum 16-minute value of the Equation of Time corresponds to about 4 degrees of arc, so that observing instruments should be much more accurate than this. Gordon Gordon Uber [EMAIL PROTECTED] San Diego, California USA Webmaster: Clocks and Time: http://www.ubr.com/clocks
Re: suns angular diameter
I haven't tried to measure the variation of sub-tended arc of the Sun's disk but have read (URL below) of it being done for the Moon, an approx. 14% variation. However, with an enlarged solar image, via a Heliostat, perhaps the 3% variation (mentioned below) could be be teased out. H... http://www.fourmilab.ch/earthview/moon_ap_per.html -Luke Jeff Adkins wrote: It is true, however, that the difference is observable in page-size photographs that lie side by side on a table. There is an old project physics activity that has the student plot the distance to the sun based on changes in the apparent size of the sun; and from this data you can computer the shape of the earth's elliptical orbit to some degree of accuracy. You can also get the perihelion and aphelion distances and dates from this sort of data. Jeff Adkins Richard M. Koolish wrote: From the web page: http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/SEgeometry.html Eclipse geometry is complicated by the fact that Earth's orbit around the Sun is elliptical. As a result, the Sun's apparent semi-diameter varies from 944 arc-seconds at aphelion to 976 arc-seconds at perihelion. This 3% range in apparent size is, of course, quite indistinguishable to the naked eye.
Re: Cast Away
Warren Thom contributed: (2) In the cave the shape of the hole went over his face. But light rays come into the cave in parallel lines -- so the shadow should be as large as the hole not as small as his eye/nose. I am glad I am not the only one who notices such things. By no means! I'm often amused when the skies in a film change from a convective day with fluffy cumulus clouds to 8/8 cirrus within seconds... but perhaps only a glider pilot would pick such nits? Tony Moss
