The duration of a solar day varies throughout the year. There are two main reasons why this occurs:
The Earth moves in an
elliptical orbit around the sun at variable speeds and introduces small
variations in the length of the solar day.
The 23½ degree tilt of the earth's axis means that the sun's eastern motion along the ecliptic is not constant. This also introduces small variations in the length of the solar day.
These corrections have been calculated and summarized on a chart, which is called the EQUATION OF TIME. The table below shows you the time adjustment between mean solar time (your wall clock) and local apparent time (the readings on a sundial). Remember there are other adjustments needed, namely, how far you are from the center of your time zone and daylight saving time.
+ means that the clocks are behind the sun (or the sun is ahead the clocks)
- means that the clocks are ahead of the sun (or the sun is behind the clocks)
For example, if it is February 4th, your clock will read 12:14 pm when your sundial reads noon. You will also have to make one more adjustment if you are not at the center of your time zone. There is no daylight saving time in effect on 2/4.
Date Time Adjustment
JAN
2 -4
5 -5
7 -6
9 -7
12 -8
14 -9
17 -10
20 -11
24 -12
28 -13
FEB
4 -14
20 -14
27 -13
MAR
4 -12
8 -11
12 -10
16 -9
19 -8
23 -7
26 -6
29 -5
APR
2 -4
5 -3
8 -2
12 -1
16 0
20 +1
28 +2
MAY
2 +3
15 +4
28 +3
JUNE
4 +2
10 +1
15 0
20 -1
24 -2
29 -3
JULY
4 -4
10 -5
19 -6
AUG
12 -5
17 -4
22 -3
26 -2
29 -1
SEPT
2 0
5 +1
8 +2
11 +3
13 +4
16 +5
19 +6
22 +7
25 +8
28 +9
OCT
1 +10
4 +11
7 +12
11 +13
15 +14
20 +15
28 +16
NOV
10 +16
17 +15
22 +14
25 +13
29 +12
DEC
1 +11
4 +10
6 +9
9 +8
11 +7
13 +6
15 +5
17 +4
19 +3
21 +2
23 +1
25 0
27 -1
29 -2
31 -3