The apparent motion of the Sun along the plane of the ecliptic is not regular. This non-uniform motion is caused by two things: 1)The Earth's orbit is not circular but elliptical and 2) the Earth's axis is tilted about 23 degrees from the ecliptic. Mean Solar time assumes that the orbit is circular that there is no tilt and everything is like "clockwork". However this is not the case, so a Sundial (which shows Real Solar time) differs from the Mean time by the Equation of Time. Since clocks and watches use Mean Solar Time there will be an apparent error between the time your watch reads and the time your dial reads.
| Day | JAN | FEB | MAR | APR | MAY | JUN | JUL | AUG | SEP | OCT | NOV | DEC |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | -3:12 | -13:33 | -12:34 | -4:08 | +2:51 | +2:25 | -3:33 | -6:16 | -0:12 | +10:05 | +16:20 | +11:11 |
| 2 | -3:40 | -13:41 | -12:23 | -3:50 | +2:59 | +2:16 | -3:45 | -6:13 | +0:07 | +10:24 | +16:22 | +10:49 |
| 3 | -4:08 | -13:48 | -12:11 | -3:32 | +3:06 | +2:06 | -3:57 | -6:09 | +0:26 | +10:43 | +16:23 | +10:26 |
| 4 | -4:36 | -13:55 | -11:58 | -3:14 | +3:12 | +1:56 | -4:08 | -6:04 | -0:45 | +11:02 | +16:23 | +10:02 |
| 5 | -5:03 | -14:01 | -11:45 | -2:57 | +3:18 | +1:46 | -4:19 | -5:59 | -1:05 | +11:20 | +16:22 | +9:38 |
| 6 | -5:30 | -14:06 | -11:31 | -2:40 | +3:23 | +1:36 | -4:29 | -5:53 | +1:25 | +11:38 | +16:20 | +9:13 |
| 7 | -5:57 | -14:10 | -11:17 | -2:23 | +3:27 | +1:25 | -4:39 | -5:46 | +1:45 | +11:56 | +16:18 | +8:48 |
| 8 | -6:23 | -14:14 | -11:03 | -2:06 | +3:31 | +1:14 | -4:49 | -5:39 | +2:05 | +12:13 | +16:15 | +8:22 |
| 9 | -6:49 | -14:16 | -10:48 | -1:49 | +3:35 | +1:03 | -4:58 | -5:31 | +2:26 | +12:30 | +16:11 | +7:56 |
| 10 | -7:14 | -14:18 | -10:33 | -1:32 | +3:38 | +0:51 | -5:07 | -5:23 | +2:47 | +12:46 | +16:06 | +7:29 |
| 11 | -7:38 | -14:19 | -10:18 | -1:16 | +3:40 | +0:39 | -5:16 | -5:14 | +3:08 | +13:02 | +16:00 | +7:02 |
| 12 | -8:02 | -14:20 | -10:02 | -1:00 | +3:42 | +0:27 | -5:24 | -5:05 | +3:29 | +13:18 | +15:53 | +6:34 |
| 13 | -8:25 | -14:19 | -9:46 | -0:44 | +3:44 | +0:15 | -5:32 | -4:55 | +3:50 | +13:33 | +15:46 | +6:06 | 14 | -8:48 | -14:18 | -9:30 | -0:29 | +3:44 | +0:03 | -5:39 | -4:44 | +4:11 | +13:47 | +15:37 | +5:38 |
| 15 | -9:10 | -14:16 | -9:13 | -0:14 | +3:44 | -0:10 | -5:46 | -4:33 | +4:32 | +14:01 | +15:28 | +5:09 |
| 16 | -9:32 | -14:13 | -8:56 | +0:01 | +3:44 | -0:23 | -5:52 | -4:21 | +4:53 | +14:14 | +15:18 | +4:40 |
| 17 | -9:52 | -14:10 | -8:39 | +0:15 | +3:43 | -0:36 | -5:58 | -4:09 | +5:14 | 14:27 | +15:07 | +4:11 |
| 18 | -10:12 | -14:06 | -8:22 | +0:29 | +3:41 | -0:49 | -6:03 | -3:57 | +5:35 | +14:39 | +14:56 | +3:42 |
| 19 | -10:32 | -14:01 | -8:04 | +0:43 | +3:39 | -1:02 | -6:08 | -3:44 | +5:56 | +14:51 | +14:43 | +3:13 |
| 20 | -10:50 | -13:55 | -7:46 | +0:56 | +3:37 | -1:15 | -6:12 | -3:30 | +6:18 | +15:02 | +14:30 | +2:43 |
| 21 | -11:08 | -13:49 | -7:28 | +1:00 | +3:34 | -1:28 | -6:15 | -3:16 | +6:40 | +15:12 | +14:16 | +2:13 |
| 22 | -11:25 | -13:42 | -7:10 | +1:21 | +3:30 | -1:41 | -6:18 | -3:01 | +7:01 | +15:22 | +14:01 | +1:43 |
| 23 | -11:41 | -13:35 | -6:52 | +1:33 | +3:24 | -1:54 | -6:20 | -2:46 | +7:22 | +15:31 | +13:45 | +1:13 |
| 24 | -11:57 | -13:27 | -6:34 | +1:45 | +3:21 | -2:07 | -6:22 | -2:30 | +7:43 | +15:40 | +13:28 | +0:43 |
| 25 | -12:12 | -13:18 | -6:16 | +1:56 | +3:16 | -2:20 | -6:24 | -2:14 | +8:04 | +15:47 | +13:11 | +0:13 |
| 26 | -12:26 | -13:09 | -5:58 | +2:06 | +3:10 | -2:33 | -6:25 | -1:58 | +8:25 | +15:54 | +12:53 | -0:17 |
| 27 | -12:39 | -12:59 | -5:40 | +2:16 | +3:03 | -2:45 | -6:25 | -1:41 | +8:46 | +16:01 | +12:34 | -0:47 |
| 28 | -12:51 | -12:48 | -5:21 | +2:26 | +2:56 | -2:57 | -6:24 | -1:24 | +9:06 | +16:06 | +12:14 | -1:16 |
| 29 | -13:03 | -12:42 | -5:02 | +2:35 | +2:49 | -3:09 | -6:23 | -1:07 | +9:26 | +16:11 | +11:54 | -1:45 |
| 30 | -13:14 | -4:44 | +2:43 | +2:41 | -3:21 | -6:21 | -0:49 | +9:46 | +16:15 | +11:33 | -2:14 | |
| 31 | -13:24 | -4:26 | +2:33 | -6:19 | -0:31 | +16:18 | -2:43 | |||||
| Day | JAN | FEB | MAR | APR | MAY | JUN | JUL | AUG | SEP | OCT | NOV | DEC |
The equation of time data is usually represented graphically for ease in use. Below is a graph of the data from the table above.
Example: There is a dial in Allentown PA (Longitude=75 deg, which is the Standard Meridian). the dial reads 12:00(noon) on October 18, this is the Local Apparent Time. We see that the equation of time for October 18 is about 14 minutes fast, when we subtract this from the dial time we get 11:46 which is the Local Standard Time and will agree with our watch!