When the sun is due south (in the northern hemisphere), it is 
always 12 o’clock sun time. 
However, for practical purposes it is desirable that days 
should be of equal length. So uniform time is not sun time, but 
is measured by an imaginary or meati sun which corresponds to 
the real sun only four times a year. The mean sun gives mean 
or local time. 
Since the earth rotates on its axis from West to East, mean 
time varies, going East and West. As one travels West his 
watch becomes too fast since he is travelling ahead of the sun. 
He must either stop his watch until the sun catches up, or else 
set his watch back. Standard time is the mean time of a certain 
longtitude, or standard meridian ; it is the actual time shown by 
clocks. In the Eastern United States the standard meridian, 
which is 75 degrees west of Greenwich, passes just east of 
Philadelphia. Now Brooklyn lies on 74 degrees west longitude. 
Therefore the sun is south in Brooklyn four minutes earlier than 
in Philadelphia, since the sun requires four minutes to travel 
westward over one degree of longitude. 
Finally, daylight saving time , when used, is one hour faster 
than standard time. Under daylight saving time 12 o’clock 
standard time is called 1 o’clock. It is determined arbitrarily, 
by law. 
The diagram (Fig. 2) indicates the differences between the 
four kinds of time: Sun time, Mean time, Standard time, and 
Daylight Saving time. It will be seen that in Brooklyn, during 
July and most of August, the sundial shows standard time 
within two minutes, that is, it is practically “correct.” 
With an understanding of the relation of sun time to standard 
time, a sundial properly made and set up should show time 
within two minutes. This is the width of the penumbra, or half- 
shadow, two minutes being the time required for the sun to pass 
over its own diameter. Once securely set up a sundial indicates 
the time from year to year unchanged. 
A. Gundersen. 
