Unit 2 : Activity 2 : The Terrestrial Coordinate System : Longitude and Time Zones : Earth Clock : Unit Exam

Unit 1: How to Point to a Star
Unit 2: Where on Earth Are You?
Unit 3: Earth's Rotation and the Sun's Apparent Motion
Unit 4: Yearly Changes in the Sky
Unit 5: Seasons and Climate
Unit 6: Sundials
Unit 7: Navigation
Unit 8: Ancient Astronomy
Unit 9: Constellations

Longitude 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" -- why not? 

Various Kinds of Time

Sundials 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 is mean solar time.  The difference between mean solar time and solar time is called the "equation of time."

Sundial Graphic

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).

1) There 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?

It's impossible to tell without knowing exactly what time it is.
It's 120 West
There are two places, 120 East and 120 West.

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. 

2) What is the time called in the zones centered at 75 deg W, 105 deg W, and 120
deg W? 


The 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 timeEphemeris 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", meaning before 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.

3) On a 24-hour clock, we might eat dinner at 1800.  What time is this in terms of AM / PM?


4) What 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 itself.

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.

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