Michigan Test for Teacher Certification (MTTC) Secondary Integrated Science Practice test

What happens when a traveler crosses the International Date Line heading west?

1. They gain a day.

2. They lose a day.

3. They change their time zone.

4. They remain in the same calendar day.

The correct answer is: They lose a day.

When a traveler crosses the International Date Line heading west, they lose a day. This phenomenon occurs because the International Date Line is set approximately along the 180-degree meridian in the Pacific Ocean and serves as a marker for calendar days. As one travels west across this line, they move from a zone that is behind Coordinated Universal Time (UTC) to a zone that is ahead. Specifically, crossing this line means that when they enter the new time zone, it is already the next calendar day. Thus, in a practical sense, if a traveler leaves a location on a specific day, after crossing the line heading west, it will feel as though they have 'missed' a day, landing them in the day that follows the one they left behind. This understanding is crucial for travelers making plans that involve time, such as flight itineraries or appointments, as the loss of a day can impact scheduling significantly. The other options do not accurately represent the effect of crossing the International Date Line in a westward direction.