Dear John, Let me try it this way. Take the Earth's orbit as it is and change the tilt from 23 degrees to 10 degrees, but still pointing in the same direction. Does this change affect where the Earth is at any particular moment? No. Does this change affect the positions on the orbit that correspond to the solstices and equinoxes? No. Therefore it does not change the time (measured not with a sundial but in seconds since the Big Bang) that the solstices and equinoxes occur. The answer are the same if we change to tilt to 1 degree, or 0.1 degree. The tilt is needed to define the seasons, but the amount of the tilt makes no difference at all in the lengths of the seasons.
The tilt does affect the Equation of Time due to something that I like to think of as a coordinate transformation. The trick is that the coordinate systems for any degree of tilt happen to coincide at the solstices and equinoxes, which is why this part of the Equation of Time is zero on these four days. You wrote: > I'm sorry but I have to disagree. BETWEEN the Vernal Equinox and the > Summer solstice the correction due to the tilt is NOT zero. Every day > EXCEPT at the equinox and solstice the day is a bit shorter (as the > sun is early) due to this tilt contribution. Summing up these days > (Solar days which the Civil calendar uses and not Sidereal days which > astronomers use) leads to a shorter Spring than the summer where the > days are now a bit longer. The sundial is fast compared to the clock for every day from April 16 to June 14, but that doesn't mean that the solar day is always less than 24 hours during this period. Take the beginning of June, for instance. Looking at the Equation of Time, we see that one each successive day, the sundial is about 9 seconds less fast, compared to a clock, than the day before. That means the solar day is 24 hr 0 min 9 sec long. (If you think I made a sign error, the length of the solar day around May 1 calculated this way is 23 hr 59 min 52 sec.) Servus, Art Carlson -
