Shortest day and latest sunrise
A friend of mine asked me the following question: "According to the popular proverbs, the erliest sunset happens in Dec-08th and the latest sunrise in Jan-03th but, as we all know, the shortest day of the year is Dec-21st (in summer it happens the same)... I checked the dates and they seem to be correct, but when I tried to find an explanation on this I found long documents dealing on equations of time, analemmae, sidereal times and lots of things I don't understand. Can you give me an easier explanation in plain language?" Then I tried to explain him this curious fact by saying "Imagine you could see the Earth from the Sun: then, the Earth spins around itself not every 24h00m but every 23h56m, blah, blah, blah As the Earth's axis is tilted towards the North Star, you do not see perfect halves of the Parallel Circles, but sometimes more and some others less than this, blah, blah, blah" . I'm afraid I wasn't quite successful and I still wonder if there is a better way to explain these things to common people, maybe by drawing circles in a bowl as Fer de Vries did or some other way. Any suggestion? Anselmo Perez Serrada -
Shortest day and latest sunrise
Message text written by INTERNET:sundial@rrz.uni-koeln.de >Can you give me an easier explanation in plain language?"< Others will have a better explanation I am sure but my simple (non-technical) one is this: The motion of the planets around the sun whilst perhaps seeming simple is actually very complex when looked at accurately. Even the sun is not stationary - it moves around its barycentre as a result of the effects of the (larger) planets. The effects that make a planetary body's motion differ from what we might simplistically expect can sometimes be very significant. The moon's orbit is one such. It is effectively a three body problem. These are renowned for their erratic nature. Remember the exeutive toy of a small steel ball suspended above a base on a string and moving between magnets in the base? It takes over a hundred mathematical terms to be able accurately predict the moon's motion. A lesser example is the Equation of Time (the difference between the actual sun and the movement of a 'mean' hypothetical sun). This arises from the accumulation of very many small daily effects (primarily caused by the earth's tilt) over the year. These differences from the 'simply expected figure' are present in almost all measurements relating to the solar system and we should expect them rather than be surprised about them. As well as the ones you mention one also sees longer summers in the Northern hemisphere than the Southern. Some of the effects we see are of course made very much worse by the fact that with our calendar we do not keep track of the actual position of the sun but we make (leap year) corrections, or not, only every 4 years, 100 years and 400 years. This means that we can easily be out by a day or so for the onset of any one expected Solar System occurrence. We should really be surprised that simplistic formulae give the accuracy they do, rather than the other way round. Does this help? Patrick - E-Mail: [EMAIL PROTECTED] Web: http://ourworld.compuserve.com/homepages/Patrick_Powers/ Lat: N 51d. 49m. 09s: Long: W 00d. 21m. 53s -
Re: Shortest day and latest sunrise
Regarding the attempt to learn why the latest sunrise and sunsets don't fall on the solstices, I think analemma.com has a pretty good explanation. I suppose some background info and lingo is necessary to understand any subject, and that site does a good job of giving the background for the beginners. -
Re: Shortest day and latest sunrise
Regarding the attempt to learn why the latest sunrise and sunsets don't fall on the solstices, I think analemma.com has a pretty good explanation. I suppose some background info and lingo is necessary to understand any subject, and that site does a good job of giving the background for the beginners. - Yes, it's really fantastic! -
