On four days of the year the sun is right and your mark may be set at 12 on these days, but you may use an almanac and look in the column marked "mean time at noon" or "sun on meridian." For example, suppose on the bright day when you are ready to place your noon mark you read in this column 11:50, then when your watch shows 11:50 make your noon mark to the shadow and it will be right for all time to come. Owing to the fact that there are not an even number of days in a year, it follows that on any given yearly date at noon the earth is not at the same place in its elliptical orbit and the correction of this by the leap years causes the equation table to vary in periods of four years. The centennial leap years cause another variation of 400 years, etc., but these variations are less than the error in reading a dial. SUN ON NOON MARK, 1909 ------------------------------------------------------- Clock Clock Clock Date Time Date Time Date Time ------------------------------------------------------- Jan. 2 12:04 May 1 11:57 Sep. 30 11:50 " 4 12:05 " 15 11:56 Oct. 3 11:49 " 7 12:06 " 28 11:57 " 6 11:48 " 9 12:07 June 4 11:58 " 10 11:47 " 11 12:08 " 10 11:59 " 14 11:46 " 14 12:09 " 14 12:00 " 19 11:45 " 17 12:10 " 19 12:01 " 26 11:44 " 20 12:11 " 24 12:02 Nov. 17 11:45 " 23 12:12 " 29 12:03 " 22 11:46 " 28 12:13 July 4 12:04 " 25 11:47 Feb. 3 12:14 " 10 12:05 " 29 11:48 " 26 12:13 " 19 12:06 Dec. 1 11:49 Mar. 3 12:12 Aug. 11 12:05 " 4 11:50 " 8 12:11 " 16 12:04 " 6 11:51 " 11 12:10 " 21 12:03 " 9 11:52 " 15 12:09 " 25 12:02 " 11 11:53 " 18 12:08 " 28 12:01 " 13 11:54 " 22 12:07 " 31 12:00 " 15 11:55 " 25 12:06 Sep. 4 11:59 " 17 11:56 " 28 12:05 " 7 11:58 " 19 11:57 Apr. 1 12:04 " 10 11:57 " 21 11:58 " 4 12:03 " 12 11:56 " 23 11:59 " 7 12:02 " 15 11:55 " 25 12:00 " 11 12:01 " 18 11:54 " 27 12:01 " 15 12:00 " 21 11:53 " 29 12:02 " 19 11:59 " 24 11:52 " 31 12:03 " 24 11:58 " 27 11:51 ------------------------------------------------------- The above table shows the variation of the sun from "mean" or clock time, by even minutes. [Illustration: Fig. 4--12-Inch Modern Horizontal Sundial for Latitude 40°-43´] [Illustration: Fig. 5--The Earth, Showing Relation of Dial Styles to Axis]