sundial a simple way to get the hour marks is by timing the 
sun’s shadow with a watch. 
To set up a sundial, or to read it accurately one must know 
the difference between sun time and clock time. The amount 
and direction of this difference varies through the year as shown 
in Fig. 2. If, for example, on May 15 the sundial reads say 10:30, 
standard time is 9 minutes slower, or 10:21, daylight saving time 
would be 11:21. In the middle of February the sundial is about 
10 minutes slow (in Brooklyn), in the beginning of November 
20 minutes fast. 
To apply this correction it is not necessary to understand the 
reason for the varying of sun time. The reason is interesting, 
though somewhat complicated. We must consider first the daily 
then the more difficult annual motion of the earth. 
Due to the earth’s rotation from West to East, the sun 
appears to move from East to West. The shadow of any object 
travels from West to East. The daily motion of the earth is 
uniform, but the position of the shadow depends not only on the 
earth’s daily rotation but also on its yearly motion (revolution) 
around the sun. Since there are 365 days approximately in a 
year, the earth must turn on its axis every day 1/365 part of a 
circle more than one rotation to “catch up” with the sun. In 
fact, one rotation takes only about 23 hours 56 minutes, the 
additional four minutes are required for any given place to reach 
the same position it had the day before with respect to the sun. 
Now, the earth’s motion around the sun is slightly irregular, 
for two reasons with rather long names. The first is the 
eccentricity of the orbit , which means that the earth moves, not in 
a circle, but in an ellipse, wdth the sun at one of the foci. When 
nearest the sun (which happens to be in January), the earth 
moves faster in its orbit. So it takes a few seconds longer 
before the sun is directly south again, casting a shadow due 
north. Thus the sundial is slow, and this difference increases 
(i. e. is cumulative) from day to day. When the earth is at an 
average (or mean) distance from the sun, in spring and fall this 
difference is zero. 
The second irregularity is caused by the obliquity of the 
ecliptic , or plane of the earth’s orbit. Only in June, when the 
days are longest, and in December when they are shortest 
(summer and winter solstice) is the line of the earth’s motion 
perpendicular to the earth’s axis. During spring and fall the 
earth moves at an angle to the plane of the equator. Thus its 
motion perpendicular to the earth’s axis is slower. That is, 
considering this cause alone, the actual duration of a day, or 
the interval between the time the sun is in the south two succes- 
sive days, is long in summer and winter, short in spring and 
fall, and the sundial movement is correspondingly affected. 
Four times a year there is no correction from this cause. 
These two irregularities combine to make the curve of Fig. 2. 
Sun time, also called apparent time, is the real time by the sun. 
