In world geography and timekeeping, what is meant by the International Date Line with reference to the Earth longitude system?

Introduction / Context:
The International Date Line is an important concept in world geography and timekeeping. It explains why the calendar date is different on opposite sides of the Earth at the same moment. Competitive exams often ask at which longitude this line lies and how it relates to the system of longitudes and time zones. This question checks your understanding of the basic definition of the International Date Line.

Given Data / Assumptions:

• We are dealing with the Earth longitude system.
• The International Date Line is being compared with other reference lines such as the Equator and the Prime Meridian.
• We assume a simplified model where the line is described by a main longitude, even though in reality it deviates around some islands.
• The options list several possible longitudes and interpretations.

Concept / Approach:
Longitudes are imaginary lines running from the North Pole to the South Pole, measured east or west from the Prime Meridian at 0 degrees. The International Date Line is an imaginary line on the opposite side of the globe, roughly following the 180 degree longitude in the Pacific Ocean. When you cross this line, the calendar date changes by one day: moving east you subtract a day, and moving west you add a day. It is not the Equator, which is a latitude line, and it is not the Prime Meridian at 0 degrees longitude.

Step-by-Step Solution:
Step 1: Recall that the Prime Meridian at 0 degrees longitude passes through Greenwich and is used as the reference for time zones. Step 2: Understand that the International Date Line is located roughly 180 degrees away from the Prime Meridian, on the other side of the Earth. Step 3: Remember that the Equator is a circle of latitude at 0 degrees, not a line of longitude, so it cannot be the date line. Step 4: Recognise that the 90 degree east longitude is simply one meridian in Asia and has no special significance in date change. Step 5: The 180 degree meridian running mostly through the Pacific Ocean is taken as the basis for the International Date Line, with small deviations to keep island groups in the same calendar date. Step 6: Therefore, the best description is that it is the 180 degree longitude where the calendar date changes.

Verification / Alternative check:
Maps showing time zones and date lines consistently draw the International Date Line near the 180 degree meridian. While the exact line zigzags to avoid splitting countries and island groups, exam questions normally treat 180 degrees longitude as the defining reference. This is why many textbooks state that the International Date Line roughly follows the 180 degree meridian opposite the Prime Meridian, confirming the correct option.

Why Other Options Are Wrong:

• It is the Equator dividing the Earth into northern and southern hemispheres: The Equator is a latitude line, unrelated to date change.
• It is the 0 degree longitude or Prime Meridian: The Prime Meridian is used for Greenwich Mean Time, not for changing the calendar date.
• It is the 90 degree east longitude: This meridian has no special global role in dates and is simply one longitude among many.

Common Pitfalls:
A common confusion is between the Prime Meridian and the International Date Line because they are both important reference lines in the longitude system. Some students also confuse latitude and longitude, mistakenly thinking that the Equator might be the date line. To avoid such errors, remember this pair clearly: 0 degrees longitude is the Prime Meridian and 180 degrees longitude on the opposite side of the world is used as the basis for the International Date Line where dates change.

Final Answer:
The International Date Line is the 180 degree longitude where the calendar date changes.