Why is this the 21st century and not the 46th?

The 21st century is the 21st century because it is the period of time from January 1, 2001 to December 31, 2100. This period of time is based on the Gregorian calendar, which was first introduced in 1582. The Gregorian calendar is a solar calendar, meaning it is based on the rotation of the Earth around the Sun. It is the most widely used calendar in the world today and is used for most international and national holidays and events.

The Gregorian calendar is based on a solar year, which is the amount of time it takes for the Earth to complete one revolution around the Sun. This is 365 days, 5 hours, 48 minutes, and 46 seconds. To account for the extra time, a leap year is added every four years, which adds an extra day to the calendar. This is why the 21st century is the 21st century and not the 46th century.

In addition to the Gregorian calendar, there are other calendars in use around the world. For example, the Chinese calendar is a lunisolar calendar, meaning it is based on both the rotation of the Earth around the Sun and the phases of the Moon. In the Chinese calendar, the 21st century is the 79th century.