1. Local Time and Standard Time
Complete solutions to the textbook exercise. Maharashtra State Board (Balbharati), Grade Eight Geography.
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Exercise
(a) The earth requires 24 hours for one rotation. In one hour,
- (i) 5 longitudes will face the sun
- (ii) 10 longitudes will face the sun
- ✓ (iii) 15 longitudes will face the sun
- (iv) 20 longitudes will face the sun
Answer: 15 longitudes will face the sun
(b) To calculate the difference between the local times of any two places on the earth,
- (i) the noon time at both the places should be known
- ✓ (ii) the difference in degrees of their longitudes should be known
- (iii) the difference in standard times of both the places should be known
- (iv) Changes need to be made according to International Date Line
Answer: the difference in degrees of their longitudes should be known
(c) The difference between the local time of any two consecutive longitudes is
- (i) 15 minutes
- ✓ (ii) 04 minutes
- (iii) 30 minutes
- (iv) 60 minutes
Answer: 04 minutes
(a) The local time is decided by 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 exactly noon. As the sun moves towards the horizon in the afternoon, the length of our shadow increases again. The noon time is the same at any given longitude across the earth (from the North Pole to the South Pole). Therefore, the time of a place as decided by the location of the sun in the sky is known as its local time, making noon the most accurate natural reference point.
(b) The local time at Greenwich is considered to be the international standard time.
For smooth international coordination and compatibility between different countries, it is essential to have a single, globally accepted reference point for time. The local time at Greenwich in England, which lies exactly on the Prime Meridian (0° longitude), is considered the Greenwich Mean Time (GMT) or international standard time. The difference in standard times of various countries is calculated with reference to this GMT to avoid chaos and confusion in global interactions.
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(c) The standard time of India has been decided by the local time at 82.5° E longitude.
The 82°30' E longitude passes through Mirzapur (near Allahabad, Uttar Pradesh) and roughly divides India into two equal halves geographically. When the sun is directly overhead on this longitude, it is assumed to be 12 noon everywhere in India. By choosing this central longitude, the time difference across India's extreme east and west ends does not exceed more than one hour relative to the central meridian, which maintains synchronization in the routine activities across the country.
(d) Canada has 6 different standard times.
Normally, if the difference between the longitudinal extent of a country is less than one or two hours, only one standard time is considered for the entire country. However, Canada has a massive longitudinal (east-west) extent. If a single standard time were used, the local times at the eastern and western extremes would be drastically out of sync with the actual position of the sun. Because one standard time is not enough for such a large area, Canada is divided into 6 different standard time zones.
(a) If it is 12 noon at 60°E longitude, then explain what would be the time at 30° W longitude?
From 60° E to the Prime Meridian (0°) is 60°.
From the Prime Meridian (0°) to 30° W is another 30°.
Total difference in longitudes = 60° + 30° = 90°.
Since 1° of longitude corresponds to a time difference of 4 minutes:
Total time difference = 90 × 4 minutes = 360 minutes.
360 minutes ÷ 60 = 6 hours.
Since 30° W is to the west of 60° E, the time will be behind the time at 60° E.
Subtracting 6 hours from 12 noon gives 6 am.
Therefore, the time at 30° W longitude would be 6 am.
(b) How is the standard time of a place determined?
If people living in a larger area across different longitudes use their own specific local times, it would lead to a lack of synchronization, chaos, and confusion in carrying out daily business. To resolve this, a central longitude that passes through the middle of the country (or region) is selected. The local time at this specific central longitude is deemed to be the standard time for that entire country or region.
(c) A football match being played at Sao Paulo, Brazil started in India at 6 am IST. Explain what would be the local time at Sao Paulo?
The Indian Standard Time (IST) is 5 hours 30 minutes ahead of Greenwich Mean Time (GMT).
Brazil (Sao Paulo standard time) is generally 3 hours behind GMT (GMT -3).
The total time difference between India and Sao Paulo is 5 hours 30 mins + 3 hours = 8 hours 30 minutes.
Since Sao Paulo is located to the west of India, its time runs behind India's time by 8 hours 30 minutes.
If the match started in India at 6:00 am, we subtract 8 hours 30 minutes from this time.
6:00 am minus 8 hours 30 minutes is 9:30 pm of the previous day.
| Place | Longitude | Date | Time |
|---|---|---|---|
| A | 120° E | 22nd June | 6:00 am |
| B | 160° W | 21st June | 11:20 am |
| C | 60° E | 22nd June | 2:00 am |
Place A (120° E): 120 × 4 mins = 480 mins = 8 hours. Since it is East, the time is ahead of the Prime Meridian. 10:00 pm + 8 hours = 6:00 am on the next day (22nd June).
Place B (160° W): 160 × 4 mins = 640 mins = 10 hours 40 minutes. Since it is West, the time is behind the Prime Meridian. 10:00 pm - 10 hours 40 mins = 11:20 am on the same day (21st June).
Place C (60° E): 60 × 4 mins = 240 mins = 4 hours. Since it is East, the time is ahead of the Prime Meridian. 10:00 pm + 4 hours = 2:00 am on the next day (22nd June).
Look at where place A is in each textbook diagram (page 8) and use this rule. The earth rotates from west to east.
- A is facing the sun directly: Noon
- A is exactly opposite the sun, in the middle of the dark half: Midnight
- A is on the edge, just moving from the dark half into the lit half: Sunrise
- A is on the edge, just moving from the lit half into the dark half: Sunset