# Earth Clock

Since 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 the day side (light) on the right and the night side (dark) on the left.

Note:  When looking down on the North Pole, Earth always rotates counter clockwise.

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 determine 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 hours.

1) 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).

7 hours behind.

International Date Line

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 ahead.

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.

2) If 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?

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Daylight Saving Time

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 the after-work 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 an hour.