Maharashtra Board Class 8 Geography Chapter 1 Local Time And Standard Time PDF Download

Local Time And Standard Time

Let's Recall

Why does the duration of day and night keep changing?

How many longitudes can be drawn on a world map keeping an interval of 1° each?

The apparent movement of the sun from east to west is a result of what?

What is the direction of the rotation of the earth?

While the earth rotates, how many longitudes face the sun daily?

At which longitude does the date change?

How was time measured in olden days?

In present times, what are the instruments used for time measurement?

Geographical Explanation

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 olden days, people used to take the help of various tools and also depend on various natural events for the measurement of time. On the basis of observation and experience, they divided the day into the following parts: from sunrise to sunset, daytime and from the sunset to the next sunrise 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 which 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 which faces the sun experiences sunrise while, on the other hand, the one in the darkness experiences sunset.

While travelling in a bus, we often see trees, electric poles, buildings, etc outside the window. They appear to be moving in the opposite direction. Actually, 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.

Try This

Perform this activity in the kho-kho ground on a bright sunny day. Consider the following points:

Choose one pole on the ground that stays under the sun for the whole day for this activity.

Observe the directions in which the shadow of this pole falls at different times of the day.

Measure the shadows and record in your notebooks.

Note the relative directions of the shadows and the sun in your notebook.

Where is the sun located in the sky when the shadow is the shortest?

At what times are the shadows longest during the day?

Teacher's Note

This activity shows us how the sun moves across the sky during the day. In India, you can do this activity in your school ground on any clear day.

Exam Trick

Remember: The shortest shadow comes at noon when the sun is highest in the sky. Long shadows come in the morning and evening when the sun is low.

Points to Remember

The earth takes 24 hours to complete one full rotation.
The earth rotates from west to east.
Different places have different local times based on their longitude.
The shadow of an object changes length and direction throughout the day.
Noon time is when the sun is highest in the sky and shadows are shortest.

Geographical Explanation

You would have realized by observation that the shadows in the early morning and evening were the longest while those at the 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.

Local Time

The earth takes almost 24 hours to complete one rotation (360°).

The earth rotates by 360° / 24 hours = 15° in one hour around its own 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.

Different longitudes have different timing for sunrise, noon and sunset. When it is noon at Mumbai, it wouldn't be the same at 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 with reference to 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.

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.

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.

Think About It

At the poles, sunrise occurs on one equinox and sun sets on the next equinox. If you happen to be at any of the poles during this time, then what would be the route of the sun in the daytime?

On which day, would the sun appear at the highest point in the sky?

Teacher's Note

At the poles, the sun behaves very differently from what we see in India. For six months, the sun never sets, and for six months, it never rises.

Exam Trick

Remember: Local time = time based on the sun's position at that place. Every 1° longitude = 4 minutes difference in time.

Points to Remember

Local time is decided by where the sun is in the sky at that place.
Places on the same longitude have the same local time.
Each degree of longitude differs by 4 minutes in time.
Noon is when the sun is highest and shadows are shortest.
Local time cannot be used when large areas need to work together.

Try This

The local time of a place can be calculated with reference to the time at the Prime Meridian.

Examine the examples given below.

Example 1:

Mashad, a town in Iran, is located on the 60° E longitude. When it is 12 noon at Greenwich, calculate the local time of Mashad town.

Statement: As we move towards the east of the Prime Meridian, the local time increases by 4 minutes for every longitude.

The difference between the longitudes of Greenwich and Mashad = 60°

Difference in local time = 60° × 4 = 240 minutes = 240 ÷ 60 minutes = 4 hours

This implies that the time at Mashad would be _________ in the _________.

Example 2:

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?

Statement: _____________________________________________________

The difference in longitudes between the two places: = _________

Total difference in time = _________ × _________ = _________ minutes = _________ ÷ 60 minutes = _________ hours

As Manaus is located to the ______________ of Greenwich, the local time of Greenwich is _________________________ of Greenwich time by ________________ hours. And, therefore, when it is noon at Greenwich, it would be _________________________ at Manaus.

Can You Tell?

We have studied that the local time is different in different parts of the world. The daily routine of the people there, is determined according to the local time in those places. Figure 1.3 shows the local times of different longitudes. Study this map and answer the following questions. Use the relation between degrees and time for this.

Between which longitudes does the region experience daytime?

Which longitudes experience noon and midnight respectively?

Edward from New Orleans is on which longitude?

What is the time at Accra city?

At the same time, what is Sharad from Patna and Yakaito from Japan doing? What time is it in these cities?

Select any one longitude. Calculate the local time of the longitudes lying 1° to the west and east of this longitude.

Do You Know?

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.

What is the maximum number of local times that can there be in the world?

How many longitudes pass the sun in one hour?

Teacher's Note

Local time calculations help us understand why different cities have different times. In India, Mumbai and Kolkata have different local times because they are on different longitudes.

Exam Trick

Remember: East = ahead in time. West = behind in time. Always multiply the longitude difference by 4 minutes to find the time difference.

Points to Remember

The Prime Meridian (0°) passes through Greenwich, England.
Places east of any longitude are ahead in time.
Places west of any longitude are behind in time.
To find time difference, multiply longitude difference by 4 minutes.
There are 360 degrees of longitude, which means 360 different local times.

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