jay@npois.UUCP (Anton Winteroak) (02/11/85)
In the early twelfth century, al-Zarqali (the blue eyed one) measured the rate of precession of "solar perigee" as 1 degree of arc per 299 years. He had some tools to make astronomical measurements, including quadrant, astrolabe, water clocks of his own devise, recent translations of the works of the greeks, and large empty churches in Spain, where pinhole images might be measured. One of his documents from the greeks, gave a position in the sky for the solar perigee (in those days), and an approxomate rate of change. Since the suns apparent size only changes 2 percent in its annual cycle, and since perihelion is not a time when it changes quickly, I am finding it hard to think of ways that someone could have made this measurement. Possibilites include studiously watching the sun go behind a tower of such a size that on only one day of the year could you see the edge of the sun on both sides of the tower at the same time, and figuring out the suns position on this basis. Another possibility is measuring the time between mean time and solar time for noon, and figuring that when difference is greatest, that is perihelion. I don't know if his clocks were able to measure small increments of time. If anyone actually knows how he did it, or knows of a clever way to determine the position of solar perigee that is accurate, using medieval tools, please post it on the net Thanks in advance, for any help
lew@ihlpa.UUCP (Lew Mammel, Jr.) (02/25/85)
I believe that the time of perihelion is most simply measured by recording the time of occurrence of the equinoxes and solstices. For example, if the winter solstice occurs at perihelion there will be less time between autumnal and vernal equinoxes than vice versa. I think Aristarchus measured the eccentricity of the earth's orbit in this way. Measuring the change in the duration of the intervals would be the next step, I suppose. Lew Mammel, Jr. ihnp4!ihlpa!lew ( I'm back )