you have already made your own earth in "Make
an Earth," you can use that for your Earth
Clock. If you skipped that activity or have misplaced
your globe, you can use any regular globe instead.
The view on
the right is looking down on the North Pole of the earth showing
day side (light) on the right and the night side (dark) on the
looking down on the North Pole, Earth always rotates counter
It is actually a simplified view of the earth's rotation because
the tilt of the earth's axis with respect to the plane of the
earth's orbit means that sometimes sunrise occurs slightly earlier
or later along the same longitude.
This view is actually accurate enough, however, to get a general
picture of whether or not something will rise with the sun, or
set around the time of sunrise, etc. The model you have made
is also accurate enough for deciding when something will transit.
Transit means to cross the meridian for a given observer.
Remember you are
still looking down on the North Pole. The red disk in the
inset picture is the extension of the Prime
Meridian out to the celestial sphere.
Suppose you want
to figure out the true difference between Rome (15 deg east time
zone) and Ames (90 deg west time zone, or CST), and you want to
what time it is in Rome when it is 7 PM here.
First you need
to count off 6 hours from Ames to Greenwich (90 deg in Ames, divided
by 15 deg time zone segments = 6 hours) and then go past Greenwich
one time zone (15 deg East) to Rome. That's a total of 7
Is Rome 7 hours ahead or behind the time in Ames, Iowa? ("Behind" means
that when it is noon in Ames, it is earlier in the day in Rome,
while "ahead" means it would be early evening).
If you travel eastward, you will "lose one hour" (set your clock
ahead one hour) for every time zone you pass through. If you
go all around the Earth traveling east, you will have to set your
clock ahead 24 times, for a total of 24 hours.
This would put
you one day ahead of your friends when you got home, and that would
mess things up very badly. We solve this problem by having
an International Date Line. The International
Date Line is approximately the half circle on the far side
of the earth, opposite the Prime Meridian. When you cross
the International Date Line going eastward, you set your clock
back by 24 hours, this cancelling the 24 hours you set the clock
In this way, you
have traveled around the world, setting your clock ahead one hour
at a time, for 24 times. When you cross the date line heading
eastward, you automatically set your clock back by 24 hours, or
one day. So you and your friends, and all the countries of
the world for that matter, will agree with you on what day it is.
you are traveling westward, how will you change your clock at
each time zone? What happens when you cross the International
Date Line going west?
Most areas of the United States, Canada and Europe use Daylight
Saving Time during the Spring and Summer months. This
is simply a shift of the clocks by one hour between clock time
and standard time. This is done so that during the summer the
clock time is an hour later than the standard time. This
has the advantage of putting one more of the daylight hours into
evening when most people can take better advantage of it. We
don't do this in the winter because we need some daylight in
the morning to get to work or school. And, there aren't
as many daylight hours in winter (Unit 4 will give more details).
In the Spring,
you "spring ahead" one hour by turning your clocks forward. And
in the Fall, you "fall back" one hour by turning your clocks back