Notes
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Topics to be learn :
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Measurement of time :
- In olden days, the time was measured with the help of the natural events of sunrise and sunset and the instruments like Ghatikapaatra, sand timer, etc.
- In present times, the instruments like watches, calendars, etc. are used for time measurement.
Daytime, nighttime and day :
- Daytime : The duration from the sunrise to the sunset is called a daytime.
- Nighttime : The duration from the sunset to the next sunrise is called a nighttime.
- Day : The duration from one sunrise to the next is called a day
Rotation of the earth :
- The earth takes nearly 24 hours (one day) to complete one rotation (360°). The earth takes 4 minutes to rotate by 1° around its axis.
- During the rotation of the earth, the 180° longitudes in the west face the sun gradually, while those 180° longitudes in the east face the darkness.
- As the direction of the rotation of the earth is from west to east, we experience the apparent movement of the sun from east to west.
- Due to the rotation of the earth, we experience sunrise, noon, sunset and midnight.
- Due to the rotation of the earth, the length and direction of shadows change.
- In the early morning and evening, we experience long shadows falling in west and east respectively. We experience the shortest shadows in the noon.
Local time :
Measurement of the time on the poles :
- The regions located between the polar circles and the poles may have the duration of daytime more than 24 hours depending upon the seasons. At the poles, the duration of daytime lasts around 6 months.
- At the poles, sun rises on one equinox and sets on the next equinox. At the poles, the sun appears above the horizon and keeps moving around the horizon nearly for 6 months. For the next six months, sun is below the horizon leading to darkness for nearly six months.
- At the poles, the length of the shadow cannot be considered for telling the noontime.
Longitudes and the local time :
- For each degree of longitude, the local time differs by 4 minutes. Longitude lying to the east of any longitude will be ahead by 4 minutes in local time. Longitude lying to the west of any longitude will be behind by 4 minutes in local time.
- By multiplying the difference between longitudes in degrees by 4 minutes, the difference between the local times of any two longitudes can be calculated. For example, if the difference between two longitudes is of 10 degrees, the difference in their local times will be of 10 x 4 = 40 minutes.
Q. Manaus city in Brazil is located on 60° W longitude. What would be the local time at Manaus when it is 12 noon at Greenwich?
(1) Statement : As we move towards the west of the Prime Meridian, the local time decrease by four minutes for every longitude. (Manaus is located to the west of Greenwich.) (2) The difference in longitudes between the two places = 60°. (3) Total difference in time = 60 x 4 = 240 minutes. = 240 minutes / 60 minutes = 4 hours. (4) As Manaus is located to the west of Greenwich, the local time is behind of Greenwich mean Time by 4 hours. And, therefore, when it is noon at Greenwich, it would be 8 a.m. at Manaus.
Longitudinal extent and the use of local times : Standard Time :
Indian Standard Time (IST) :
Universal Standard Time : As the standard time is essential for synchronizing routine activities in a country with comparatively more longitudinal extent, the universal standard time is essential for an international coordinating between the countries in the world.
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Remember :
When the sun is directly overhead at a place on the earth, it is noon over there. While telling the time between midnight and noon, we put a.m. in front of the time. This means Ante Meridiem. When the longitudes cross the noon time, then the time is past midday or afternoon. The time in between noon and midnight is denoted by p.m. means Post Meridiem. |