Location and time
FOCUS QUESTIONS
What is the time difference between meridians of longitude? What is the earth’s rotation? What is the time of a place using the line longitude?
TIME DIFFERENCE BETWEEN MERIDIANS OF LONGITUDE
We know that the earth revolves about its axis once every 24 hours.
In other words, the sun completes its apparent revolution of 360° in 24 hours.
This means that the sun crosses each of the 360 meridians of longitude once every 24 hours. So, in 1 hour, the sun appears to move 15’, in 4 minutes, it appears to move 1°, in 1 minute it appears to move 15°, in 4 seconds it appears to move 1’.
From this, it becomes obvious that there is a direct relationship between arc and time, such that 1 minute of time equals 15 minutes of arc.
CALCULATING LONGITUDE BY COMPARING TIME DIFFERENCE
If we have two accurate clocks, one calibrated to GMT and the other calibrated to local time, then it is an easy matter to calculate our longitude from the difference between the two times. For example, if the difference between GMT and local time is three hours, then the difference in longitude must be 3 x 15°. If local time is ahead of GMT, then the local longitude must be east of the Greenwich Meridian; and if local time is behind GMT, the longitude must be west.
Example: If it is 18 GMT when it is 09:20 local time on the same day, then local time must be 8 hours and 40 minutes behind GMT. Therefore, Long = - [(8 x 15°) + (40 ÷ 60 x 15°)] = [120° + 10°] = -130° = 130° West
HOW TO CALCULATE LOCAL TIME
1. Work out the longitude difference. 2. Convert this to a time difference. 3. Adjust the time according to the direction of movement (east or west).
Example: What is the time in Calcutta (longitude 96° east) when it is 9 a.m. in Munich (longitude 11° east)? 1. Longitude difference = 85 degrees 2. Time difference 85/15 = 5 hours 40 minutes 3. Calcutta is east of Munich; therefore, the time is ahead. Thus, 9 a.m. plus 5 hours and 40 minutes = 2:40 p.m. There are many ways of determining the longitude of a place. The simplest way is to compare the local time with GMT by listening to BBC radio.
For example, the captain of a ship in the midst of the ocean wants to find out in which longitude his ship lies. If GMT is 8 a.m. and it is noon in the local region, it means that he is four hours ahead of Greenwich, and must be east of Greenwich. His longitude is 4 x 15° or 60° east.
EARTH ROTATION
By definition, the earth’s rotation is the amount of time that it takes to rotate once on its axis. This is, apparently, accomplished once a day – i.e., every 24 hours. The earth rotates about an imaginary line that passes through the North and South Poles of the planet.
This line is called the axis of rotation. Earth rotates about this axis once each day (approximately 24 hours). There are actually two different kinds of rotation that need to be
considered here. I. The amount of time it takes for the earth to turn once on its axis so that it returns to the same orientation compared to the rest of the universe. II. How long it takes for the earth to turn so that the sun returns to the same spot in the sky.
The amount of time it takes for the earth to rotate once on its axis is known as a sidereal day – which is 23.9344696 hours. Because this type of day-measurement is based on the earth’s position relative to the stars, astronomers use it as a time-keeping system to keep track of where stars will appear in the night sky, mainly so they will know which direction to point their telescopes in.
The amount of time it takes for the sun to return to the same spot in the sky is called a solar day, which is 24 hours. However, this varies through the year, and the accumulated effect produces seasonal deviations of up to 16 minutes from the average. This is caused by two factors, which include the earth’s elliptical orbit around the sun and its axial tilt.
SUMMARY
A day is the time it takes the earth to spin around once on its axis.
An hour is 1/24th a day, so the earth is divided into 24 standard time zones, each covering approximately 15 degrees of longitude.
The time of day depends on where you are on the earth. Hours are numbered from the prime meridian, or 0 degree longitude, which passes through Greenwich, England.
Time zones east of the Greenwich zone are ahead of Greenwich time.
This is because the earth spins toward the east, so that lands in the east get the sun earlier than Greenwich does. Time zones west of Greenwich are behind Greenwich time.
They get the sun later than Greenwich. The boundaries of the time zones usually follow straight lines over the oceans.
Every 15th degree of longitude marks a different time zone. On the continents, time zone boundaries usually zigzag.
Some countries are so large that they cover more than one time zone. Russia stretches across 11 time zones.
From the state of Maine to the state of Hawaii, the United States spreads across seven time zones. To determine what time it is in another time zone, you must add or subtract, depending on whether you are going east or west. First, determine the time for the time zone in which you live. Next, find the time zone you want and count how many time zones away it is from yours.
Now, determine if the new time zone is east or west of where you live.
If it is east, add one hour for each time zone away it is from your time.
If it is west, subtract one hour for each time zone away it is from your time.
For example, if it is 8 a.m. where you are and you go three time zones east, you add 3 to 8 and get 11 a.m. in the new time zone. If it is west, it would be 8 subtract 3 for a time of 5 a.m. Using this simple formula, will enable you to always know what time it is. Answer the following questions: 1. The world is divided into how many times zones? 2. How many degrees of longitude is each time zone? 3. When going east, do you add or subtract one hour for each time zone through which you travel?
4. When going west, do you add or subtract one hour for each time zone through which you travel?
5. You are in Time Zone I. What time is it according to the map?
6. You live in Time Zone G. Your friend lives in Time Zone K. How many hours difference is there?
7. You live in Time Zone I. You want to call Time Zone D. How many hours difference is there? 8. If it is 6 p.m. in Time Zone F, what time is it in Time Zone M? 9. If it is 9 p.m. in Time Zone U, what time is it in Time Zone P? 10. You live in Time Zone H. Your friend lives in Time Zone O. If it is 2 p.m. in Time Zone H, what time is it at your friend’s house? Next class, we will look at coral reef.