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# Time: Day Lengths and Time Zones (page 2)

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Author: Janice VanCleave

### Try New Approaches

1. Demonstrate a solar day by repeating steps 7 through 9, but continue rotating the clay sphere until the toothpick points toward the flashlight. Science Fair Hint: Take a photograph of the position of the clay sphere at the end of step 8 with the sphere on line A and the toothpick pointing toward the flashlight. This photograph represents the position of a meridian at noon, the starting of a time period. Take a second photo after step 9 of the original experiment, when the toothpick is parallel with line B. This second photograph represents the position of the meridian at the end of one sidereal day. Take a third photo after step 9 of the repeated experiment, when the toothpick points toward the flashlight. This photo represents the position of the meridian at the end of one solar day. Use these photos to compare the positions of a meridian during one sidereal day and one solar day.
2. The clay model greatly exaggerates the angular difference between the position of a meridian at noon on the end of a sidereal day and on a solar day. Calculate this difference (x) using the following facts:
• The time difference between a sidereal day and a solar day is about 4 minutes.
• Earth rotates 360° in about 23 hours 56 minutes.
3. First change sidereal time to minutes (23 hours × 60 minutes/hour + 56 minutes = 1,436 minutes), then use the following equation:

x/4 minutes = 360°/1,436 minutes
x = 360°/1,436 minutes x 4 minutes
= ? degrees

Science Fair Hint: Make a diagram representing the angular difference between a sidereal day and a solar day, similar to Figure 7.2, replacing X° with your calculation for the angular measurement.

Earth is divided into 24 internationally agreed time zones. Each time zone is about 15° of longitude wide, and local time is the same throughout a given time zone. Each zone is centered on a meridian called the time meridian, with about 7.5° of longitude on each side of the meridian. The clock time within each time zone is called standard time. Standard time for each time zone is based on the mean solar time of the time meridian of the zone. The prime meridian (0°) is the reference line for standard time. Time at the prime meridian is called Greenwich Mean Time, or GMT. GMT applies 7.5° east and west of the prime meridian. Each time meridian 15° east or west of the prime meridian marks a time difference of 1 hour. Time zones west of the prime meridian are earlier than GMT, and those east of it are later than GMT.

Design a map showing time zones of the Earth. Find out what and where the international date line is. For more information about time zones, see Janice VanCleave's Geography for Every Kid (New York: Wiley, 1993), pp. 135–144.

### Get the Facts

1. The Earth is not a perfect sphere, so it wobbles as it rotates on its axis. This wobble is called precession, and the length of time it takes the Earth to make one complete precession is called a platonic year. How long is a platonic year? For information about the Earth's precession, see H. A. Rey, The Stars (Boston: Houghton Mifllin, 1976), pp. 127–129.
2. The Sun appears to move fastest during January and slowest during July. Thus, noon as determined by sun time is faster than standard clock time on some days and slower on other days. What causes this change in the Sun's motion? What is True Sun? Average Sun or Mean Sun? What is an analemma? How can you determine the difference between sun time and standard time where you live? For information, see Philip Harrington and Edward Pascuzzi, Astronomy for All Ages (Old Saybrook, Conn.: Globe Pequot Press, 1994), pp. 89–95.
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