and Time Zones
As the earth turns, the sun
appears to move across the sky. This apparent
motion gives us day and night. A sundial uses
the position of the sun in the sky to tell the time. The
time measured by a sundial is solar
time. Solar time is 12 noon when the sun
is on the (observer's local) meridian. Note
that we don't say "when the sun is directly overhead" --
Kinds of Time
do not keep time at a uniform rate the way mechanical clocks
do. If you set a sundial to be on time at one time of the
year then it will run fast or slow at other times of the year. When
people started depending more on mechanical clocks than sundials
it became convenient to switch to using time that, on average,
matched sundial time, but ran at a constant rate. This
solar time. The difference between mean solar time
and solar time is called the "equation
Here's something to remember:
Mean Solar Time is directly related to longitude. Your
longitude is equal to [(your Mean Solar Time) minus (Greenwich Mean Solar
Time) ] x 15 degrees. You need to be
sure to express the mean solar time in hours (and fractions
of an hour).
is a location on Earth that has a mean solar time 8 hours
different than Greenwich's mean solar time. What is
this location's longitude?
Iowa State University
is located in Ames, Iowa. There is a town nearby called Boone. Because
they are located at different longitudes, the two towns have slightly
different mean solar times. It would certainly be inconvenient
if we still used mean solar time for each town!
When railroads came in and allowed
people to travel across the country quickly, mean solar time was
replaced by zone
time (or standard time) for everyone to use. With
zone time we all agree to use the mean solar time of the nearest
longitude divisible by 15 deg. Thus, for Central Standard
Time used at Iowa State, we use the mean solar time for longitude
90 deg W.
A quick side note: Standard
time for longitude 0 deg is called Greenwich Mean Time or
Universal Time, or just UT for short.
Standard time for the time zone we are in,
centered around 90 deg West, is Central Standard Time, or CST.
is the time called in the zones centered at 75 deg W, 105
deg W, and 120
earth is the fundamental "clock" we use in defining time,
because we are sensitive to the difference between day and night. However,
the earth is not a perfect clock and there are now atomic clocks
that are more accurate than the earth.
In order to keep the time displayed
by these highly accurate clocks in agreement with the Earth's rotation we
have to insert "leap seconds" at midnight on December
31, every now and then. However, for astronomical purposes
it is useful to have a kind of time that isn't complicated by leap-seconds. This
kind of time is called ephemeris
time. Ephemeris time once agreed with Universal
Time (UT) at the beginning of the 20th century; now they differ
by about half a minute.
In the United States, we usually
use A.M. to mean the morning. "AM" is from "ante meridiem",
the sun crosses the meridian, or before noon. P.M., post
meridiem, helps, of course, to distinguish between morning and
afternoon hours. In Europe and in the U.S. military, it is
common to use a 24-hour
clock, starting at midnight. So, 1 AM just after
midnight, is 0100 hours in military language, while 1 P.M. is 1300
hours and so on. Your computer and most digital watches have
an option to display 24-hour time instead of the usual ante meridiem
and post merdiem time. It is actually easier for calculations
to use the 24-hour clock, so you will find 24
hour time scattered througout this course.
On a 24-hour clock, we might
eat dinner at 1800. What
time is this in terms of AM / PM?
is the 24-hour clock time when it is 3 AM? 11 PM?
The time we use,
(mean) solar time, is related to the position of the sun in the
sky, because most of us prefer to be awake or to work when the
sun is up. It is also possible to define time according to
the positions of the stars in the sky. This kind of time
is called sidereal
time. Instead of the sun, sidereal time uses a special
spot in the sky called the Vernal
Equinox to mark the hours. The Vernal Equinox is the
point in the sky where the sun crosses the celestial equator heading
north. The sun does this around March 21st each year. The
term "Vernal Equinox" can refer to the date, the time and the event
Sidereal time is zero hours when
the Vernal Equinox is on the meridian. Sidereal time is more
uniform than solar time so there is no need for a mean sidereal
time. In fact, one way to determine the equation
of time is to keep track of the
exact difference between the apparent solar time and sidereal time
over the year.