Chapter 1 - Local Time and Standard Time
Q 1. Complete the sentence by selecting the correct option:
(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
Ans: Option (iii) – 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
Ans: Option (ii) – 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
Ans: Option (ii) – 04 minutes
Q 2. Give geographical reasons:
(a) The local time is decided by the noon time.
Ans:
(i) The local time of a place is fixed with reference to the apparent movement of the sun.
(ii) When the sun is overhead in the sky, it is midday or noon.
(iii) The noon time is the same at any given longitude across the earth, i.e., from the North Pole to the South Pole.
(b) The local time at Greenwich is considered to be the international standard time.
Ans:
(i) Greenwich is at 0° longitude in London.
(ii) When the Sun is at its highest point exactly above the Prime meridian, it is noon at Greenwich.
(iii) The local time increases by 4 minutes for every longitude towards the west of Greenwich and decreases by 4 minutes for every Longitude towards the east of Greenwich.
(iv) People living in large areas across different longitudes interact with each other. To avoid chaos and confusion, International Standard Time is convenient.
(c) The standard time of India has been decided by the local time at 82.5° E longitude.
Ans:
(i) 82.5⁰ longitude passes through the middle of the country with reference to its longitudinal extent.
(ii) When the sun is directly overhead on this longitude, then it is assumed that the time is 12 noon across India.
(iii) There is no difference of more than one hour between the local time at 82.5° longitude and other places in the country.
(d) Canada has 6 different standard times.
Ans:
(i) If the difference between the longitudinal extent of the country is less than one or two hours, then the country has only one standard time.
(ii) The longitudinal extent (East-West extent) of Canada is four and a half hours.
(iii) From West to East, the time zones are: Pacific, Mountain, Central, Eastern, Atlantic, and Newfoundland.
Q 3. Answer in brief:
(a) If it is 12 noon at 60°E longitude, then explain what would be the time at 30° W longitude?
Ans:
(i) As we move towards the east of the prime meridian, the local time increases by 4 minutes for every longitude.
(ii) As we move towards the west of the prime meridian, the local time decreases by 4 minutes for every longitude. The difference between 60°E longitude and 30° W longitude = 90⁰
Difference in local time = 90 × 4
∴ Difference in local time = 360 minutes
Converting from minutes to hours
\(\large \frac {360}{60}\) = 6 hours
∴ Difference in local time = 6 hours
(iii) This shows the time at 30⁰ W would be 6 a.m. in the morning.
(b) How is the standard time of a place determined?
Ans:
(i) Standard time is determined on the basis of time zones, into which the earth is divided.
(ii) Standard time is the same for a particular country as it is established by the law of that country.
(iii) Standard time is determined by the distance, east or west, from the Prime Meridian at Greenwich of the line of longitude lying within the 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 in Sao Paulo?
Ans:
India lies on 82⁰ East Longitude.
Brazil lies on 45⁰ West Longitude.
The difference in longitudes between the two places is 127⁰
Total difference in time = 127 × 4
Total difference in time = 508 mins
Converting from minutes to hours
\(\large \frac {508}{60}\) = 8.28 hours
Approximately 8 hours 30 mins
∴ India is 8 hours and 30 mins ahead of Brazil
It is is 6 a.m. in India
Local time at Sao Paulo Brazil = 6 a.m. – 8 hours 30 mins = 9.30 p.m. the previous day.
Ans: Local time at Sao Paulo Brazil will be 9.30 p.m. the previous day.
Q 4. If it is 10 pm on 21st June at Prime Meridian, write the dates and time at A, B and C in the table.
| Place | Longitude | Date | Time |
|---|---|---|---|
A | 120⁰ E | ||
B | 160⁰ W | ||
C | 60⁰ E |
Ans:
| Place | Longitude | Date | Time |
|---|---|---|---|
A | 120⁰ E | June 22 | 6 a.m. |
B | 160⁰ W | June 21 | 11.20 a.m. |
C | 60⁰ E | June 22 | 2 a.m. |
Q 5. Write the situations of place A shown in these diagrams in the boxes below them :
(i) sunrise
(ii) midnight
(iii) noon
(iv) sunset
Ans:
(i) midnight
(ii) sunset
(iii) sunrise
(iv) noon