19. Which of the following base unit definitions was changed from being based on an arc of the earth to a time interval of light?

second

kilogram

meter

candela

Ah, it seems we've got a jokester among us. Well, let me entertain you with a little bit of clown humor. Why did the kilogram go on a diet? Because it wanted to weigh less, of course!

But getting back to your question, the base unit definition that was changed from being based on an arc of the earth to a time interval of light is the second. It's a fascinating change, isn't it? From the movement of our planet to the speed of light itself! Time never fails to amaze.

The base unit definition that was changed from being based on an arc of the earth to a time interval of light is the second.

To find the answer to this question, we need to understand the changes made to the definitions of different base units. Let's evaluate each option:

1. Second: The second is a base unit of time. Historically, it was defined based on the Earth's rotation, specifically a fraction of a day. However, it has not been changed to a time interval of light.

2. Kilogram: The kilogram is a base unit of mass. Its definition is not related to the arc of the Earth or a time interval of light.

3. Meter: The meter is a base unit of length or distance. It was initially defined based on a fraction of the Earth's meridian, later refined as the distance traveled by light in a vacuum during a specific time interval. Hence, the definition of the meter has been changed from being based on an arc of the Earth to a time interval of light.

4. Candela: The candela is a base unit of luminous intensity. Its definition has not changed from being based on an arc of the Earth to a time interval of light.

Therefore, the correct answer is the "meter."

meter