Local Time and Standard Time Class 8 Geography Notes Maharashtra State Board
We get up early in the morning, brush our teeth, and take a bath. Then we have breakfast and go to school. We study in our classrooms. We return home. We go to the playground to play in the evening. We have dinner; brush our teeth and go to sleep. We keep doing similar activities throughout the day. Considering our daily routine, we need to decide the time for all our activities. In the olden days, people used to take the help of various tools and also depended on various natural events for the measurement of time. Based on observation and experience, they divided the day into the following parts: from sunrise to sunset, daytime, and from sunset to the next sunrise to nighttime. A whole day meant the duration from one sunrise to the next. Earlier, natural events and instruments like Ghatikapaatra (a bowl with a minute hole at its base that would float in a large water-filled vessel); sand timer, etc. were used to tell time.
The Earth takes 24 hours i.e. one day to complete one rotation. We consider the direction where the sun rises to be the east. Thus, the Earth rotates from west to east. As a result, we experience, sunrise, noon, sunset, and midnight. During rotation, the longitudes in the western part face the sun gradually while those in the east experience darkness. The longitude that faces the sun experiences sunrise while, on the other hand, the one in the darkness experiences sunset. While traveling in a bus, we often see trees, electric poles, buildings, etc outside the window. They appear to be moving in the opposite direction. They are stationary and our bus is moving forward. Similarly, because of the rotation of the earth, we feel that the sun is changing its position from east to west daily.
You would have realized by observation that the shadows in the early morning and evening were the longest while those at noon were the shortest. Because of the change in the apparent location of the sun in the sky, the length and direction of the shadow of the pole kept changing. This is because a specific part of the earth faces the sun during rotation and moves forward. Incidentally, we also experience that it is cooler in the morning and evening and warmer in the afternoon.
As soon as the sun starts moving up in the sky after sunrise, the length of our shadow reduces. Normally, the length of the shadow is shortest at noon. As the sun moves towards the horizon in the afternoon, the length of our shadow increases again by evening. The noon time is the same at any given longitude across the earth i.e. from the North Pole to the South Pole. The time of a place as decided by the location of the sun in the sky, is known as its local time.
In the regions lying in between the polar circles and the poles, the daytime could be more than 24 hours depending upon the season. As a result, understanding the timings of sunrise, noon, sunset, and midnight at these places becomes important. At the poles, the daytime and the duration of the day lasts around 6 months. To tell the time of sunrise or sunset at the poles, one has to consider the date. When the sun rises on a specific day, it moves around the horizon and therefore here, the length of the shadow cannot be considered for telling the noon time.
Different longitudes have different times for sunrise, noon, and sunset. When it is noon in Mumbai, it wouldn’t be the same in Kolkata. Because Kolkata lies to the east of Mumbai, it would already be afternoon in Kolkata. The local time of a place on the earth’s surface is determined by its noon time. This implies that places lying on the same longitude have the same local time. There is no difficulty when local time is used for a small area. When people living in a larger area across different longitudes have to interact with each other, then using the local time could lead to chaos and confusion. In such cases, using the local time is not convenient.
- The earth takes almost 24 hours to complete one rotation (360°).
- The earth rotates by 360°/24 hours = 15° in one hour around its axis.
- The earth takes 60 minutes/15 degrees = 4 minutes to cover 1°
- This means that for each degree of longitude, the local time differs by 4 minutes.
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 between noon and midnight is denoted by p.m. means Post Meridiem.
Longitudes lying to the east of any longitude are ahead of the time at that longitude while those lying to the west are behind. As the distance between two longitudes increases, their local times also start differing. If we multiply the difference between longitudes in degrees by 4 minutes, then the difference in the respective local times can be calculated. The difference in longitudes can be known with the help of a globe or map.
Both Mumbai and Kolkata are located within India but on different longitudes. Their local times differ by an hour. If there are different local times within a country because of longitudinal differences, then there will be a lack of synchronization in the routine activities in the country. If each one follows their local time, discrepancies emerge in carrying out daily business in the country. Therefore, generally, the local time at the longitude which generally passes through the middle of the country is deemed to be the standard time for that country. This standard time is used all over the country.
Concerning businesses at a global level, there should be compatibility between the standard times of various countries. To facilitate this, the world has been divided into 24 time zones. These time zones have been created concerning the Prime Meridian itself. Normally, if the difference between the longitudinal extent of the country is less than one or two hours, only one standard time is considered for the country. But if the longitudinal extent (east-west extent) is more than that, then, one standard time is not enough and in such countries, more than one standard time zones are considered.
Indian Standard Time:
The Indian Standard Time (IST) has been decided according to the 82°30’E longitude which passes through Mirzapur (near Allahabad, Uttar Pradesh). This longitude passes through the middle of the country concerning its longitudinal extent. The local time at this longitude has been selected as the standard time of the whole country. When the sun is directly overhead on this longitude, it is assumed that it is noon everywhere in India. There is no difference of more than one hour between the local time at 82°30′ and other places in the country.
Universal Standard Time:
For international coordination between countries, the local time at Greenwich (Greenwich Mean Time) in England is considered to be the international standard time. The difference in standard times of various countries is calculated concerning GMT. The Indian Standard Time is 5 hours 30 minutes ahead of GMT. If it is 5 p.m. in Greenwich then in India it would be 10.30 p.m.
The National Institute of Standards and Technology (NIST), an institute in the United States of America has developed the most accurate clocks in the world. Clocks have to be adjusted by 1 second (added or subtracted) once in 20 million years. In India, the National Physical Laboratory of India, (NPL) New Delhi gives services of accurate time-keeping. The clocks used here are accurate up to 1 lakh part of a second. As accurate time is required for space research, artificial satellite launching, etc., these clocks are used in these activities.
Jantar-Mantar: Astronomical Observatories
Maharaja Sawai Jai Singh II, the king of Jaipur, Rajasthan, was a great astronomer, mathematician, and architect. He built five astronomical observatories called Jantar-Mantar at Ujjain, Varanasi, Jaipur, Delhi, and Mathura. The one at Mathura doesn’t exist today but one can visit the other four.
Even today, one can know the exact time upto seconds through shadows. Jantar Mantar does not only house sundials, but they are complete astronomical observatories. One can observe the sky from here too. With the help of instruments at Jantar-Mantar, it is still possible to see astronomical observations. After the advent of modern instruments, now these instruments are more of a ‘cultural heritage’.
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