The variation in time between the easternmost and westernmost parts of a country is due to the concept of time zones. Time zones are imaginary lines that run vertically on the globe, dividing the world into different regions where the time is the same.

Heading: Time zones and their purpose
Time zones were created to standardize time across the world, making it easier for people to coordinate travel, communication, and business across different regions. The earth rotates 360 degrees in 24 hours, which means that each hour corresponds to 15 degrees of longitude. Therefore, each time zone is approximately 15 degrees wide.

Heading: The reason behind the time difference
The time difference between the easternmost and westernmost parts of a country is due to the fact that the earth rotates on its axis from west to east. This means that places further east will experience sunrise and sunset before places further west. When the sun is directly overhead at one location, it is noon at that location. However, it will be earlier in the day further west and later in the day further east.

Heading: Time difference and longitude
The time difference between two locations is directly related to the difference in longitude between them. For every 15 degrees of longitude, there is a one-hour time difference. Therefore, if two locations are separated by 30 degrees of longitude, there will be a two-hour time difference between them.

Heading: Example of time difference in the US
In the United States, the easternmost point is West Quoddy Head, Maine, which is located at approximately 67 degrees west longitude. The westernmost point is Amatignak Island, Alaska, which is located at approximately 179 degrees east longitude. The difference between these two points is approximately 246 degrees of longitude, which results in a time difference of approximately 16 hours.

In conclusion, the variation in time between the easternmost and westernmost parts of a country is due to the concept of time zones and the difference in longitude between the two locations. This time difference can be significant, with a variation of nearly 30 degrees resulting in a time difference of nearly two hours.