Locating Places on the Earth — NCERT Solutions

Local Time vs. Standard Time

Local Time is the time at a specific place based on the position of the Sun directly above that location. Since the Earth rotates from west to east, the Sun appears to be at different positions at different longitudes at the same moment. Every place has its own local time, which changes by 4 minutes for every $1°$ of longitude.

Standard Time is the uniform time adopted by a country or a region for practical purposes. Instead of every city following its own local time (which would cause great confusion in railways, communication, and daily life), a country selects one central meridian and all places in that country follow the time of that meridian.

For example, India has adopted $82.5°\text{E}$ (passing through Mirzapur, Uttar Pradesh) as its Standard Meridian. The time at this meridian is called Indian Standard Time (IST), and the entire country follows it.

Key Difference:

• Local time varies from place to place (even within the same country).
• Standard time is the same for all places within a country or time zone.

For instance, the local time of Mumbai and Kolkata differ by about 1 hour and 20 minutes, but both cities follow IST (UTC + 5:30).

Given:

• Delhi: Latitude $29°\text{N}$, Longitude $77°\text{E}$
• Bengaluru: Latitude $13°\text{N}$, Longitude $77°\text{E}$

Concept: Local time depends on longitude, not latitude. The Sun moves from east to west, and time differences arise due to differences in longitude. Latitude tells us how far north or south a place is from the Equator, but it does not affect local time.

Step 1: Find the difference in longitude between Delhi and Bengaluru.
$$\text{Difference in longitude} = 77°\text{E} - 77°\text{E} = 0°$$

Step 2: Calculate the time difference.
$$\text{Time difference} = 0° \times 4 \text{ minutes} = 0 \text{ minutes}$$

Final Answer: Since both Delhi and Bengaluru are located on almost the same longitude ($77°\text{E}$), there is no difference in their local time. The difference in their latitudes ($29°\text{N}$ and $13°\text{N}$) does not affect local time at all.

(a) All parallels of latitude have the same length.

False. Parallels of latitude are circles that become smaller as we move away from the Equator towards the poles. The Equator ($0°$) is the longest parallel, and the circles get progressively smaller towards $90°\text{N}$ and $90°\text{S}$, where they reduce to a single point (the poles).

(b) The length of a meridian of longitude is half of that of the Equator.

True. The Equator is a full circle going all the way around the Earth. A meridian of longitude is a semicircle — it goes from the North Pole to the South Pole on one side. Therefore, a meridian is exactly half the length of the Equator.

(c) The South Pole has a latitude of $90°\text{S}$.

True. Latitudes are measured from $0°$ at the Equator to $90°$ at the poles. The South Pole is the southernmost point on Earth, located exactly at $90°\text{S}$. Similarly, the North Pole is at $90°\text{N}$.

(d) In Assam, the local time and the IST are identical.

False. IST is based on the Standard Meridian of India at $82.5°\text{E}$. Assam is located further east (around $90°–96°\text{E}$). Since Assam is to the east of $82.5°\text{E}$, its local time is ahead of IST by approximately 30 to 54 minutes. Therefore, local time in Assam is not identical to IST.

(e) Lines separating the time zones are identical with meridians of longitude.

False. Ideally, time zone boundaries follow meridians of longitude (every $15°$). However, in practice, time zone boundaries are often modified to follow political boundaries, national borders, or administrative divisions so that a country or region can follow a single standard time. Therefore, actual time zone boundaries are not always identical to meridians of longitude.

(f) The Equator is also a parallel of latitude.

True. The Equator is the $0°$ parallel of latitude. It is the largest and most important parallel, dividing the Earth into the Northern Hemisphere and the Southern Hemisphere. All parallels of latitude, including the Equator, run east–west and are parallel to each other.

Across:

1. SCALE — A scale lets you squeeze (represent) a huge real-world area into a small map.

4. GLOBE — A globe is a convenient sphere that represents the Earth.

5. EQUATOR — The Equator ($0°$ latitude) is the longest parallel of latitude.

6. GREENWICH — The Prime Meridian ($0°$ longitude) passes through Greenwich, England.

8. COMPASS — A compass is a very convenient tool to find your way (direction).

10. LATITUDE — Latitude is a measure of the angular distance of a place from the Equator.

Down:

2. LONGITUDE — Longitude is a measure of the angular distance of a place from the Prime Meridian.

3. COORDINATES — Latitude and longitude together are called coordinates, which allow us to locate any place on Earth.

6. GRID — Latitudes and longitudes together create a grid on the map.

7. IST — IST (Indian Standard Time) is the time we all follow in India.

9. NORTH — The North Pole is on top of the world.

11. IDL — IDL (International Date Line) is the line across which the day and date change.