Square Root Day September 9, 1981 (9/9/81). Gordon remains the holiday's publicist, sending news releases to world media outlets. Gordon's daughter set up a Facebook group where people can share how they were celebrating the day. One suggested way of celebrating the holiday is by eating square radishes, or other root vegetables cut into shapes with square cross sections (thus creating a "square root"). Square Root Day occurs on the following dates each century: The number of years between consecutive Square Root Days in a century are consecutive odd numbers: 3, 5, 7, 9, 11, 13, 15, 17. This illustrates the fact that
Square root of the original estimate and its square. Furthermore, ("x" + "c") ≈ "x" + 2"xc" when "c" is close to 0, because the tangent line to the graph of "x" + 2"xc" + "c" at "c" = 0, as a function of "c" alone, is "y" = 2"xc" + "x". Thus, small adjustments to "x" can be planned out by setting 2"xc" to "a", or "c" = "a"/(2"x"). The most common iterative method of square root calculation by hand is known as the "Babylonian method" or "Heron's method" after the first-century Greek philosopher Heron of Alexandria, who first described it.