and 1987. The third is not readily apparent, but is crucial to the appearance of Figure 1 - 1. It is the calendar that determines how the two data sets align. The chart shows the first day of trading in 1929 aligned above the first day of trading in 1997, etc. By aligning both charts at the beginning of the year, we am using the calendar as the yardstick of comparison. The fact that the similarities in the charts occurred at the same time of yew is what makes it most interesting. If one crash had occurred in March and the other in October, the shape of the charts may still be similar, but the comparison would no longer be as striking. The similarity in the two charts is greatly a function of the calendar. The examination of the exact dater, of the similarities also uses the yardstick of the calendar as the absolute against which conclusions am drawn. The questions I wanted answered were, "Is this calendar the proper yardstick?" and "What would happen if the two years of wading data are compared with some other method of counting time?"

The calendar we use is the Gregorian calendar. It is an improved version of the Julian calendar named for Julius Caesar. Our calendar, with its Roman origin, does a very good job of keeping time. The time it keeps is solar time, the rotation of the earth around the sun once every 365.25 days. Them am other units of time a calendar could use. Ancient calendars usually measured the time it takes for the moon to circle the earth relative to the sun. Most calendars measure one type of time accurately and then approximate the other type of time. Our solar calendar approximates lunar time with its month, but this is an inaccurate measurement of lunar time. To distinguish between the modern month, which is 1/12th of a solar year, and the ancient month, which is one moon. I will use the ten, "moon" for the ancient month and "month" in its present context as a fraction of a solar year.