The duration of Earth's rotation is commonly referred to as what?

• A. A month
• B. A day
• C. A year
• D. A decade

Answer: (B)

A day is the period of Earth's rotation that we use to measure time, and it’s about 24 hours. In everyday use, a day means the time it takes for the Sun to return to roughly the same position in the sky, giving us the familiar day-night cycle. There’s a slightly shorter measure called a sidereal day, about 23 hours 56 minutes, which is based on Earth's rotation relative to distant stars; the extra few minutes in the solar day come from the fact Earth travels along its orbit around the Sun while it’s rotating. The other durations—month, year, decade—are tied to orbital motions (the Moon’s cycle, Earth’s orbit around the Sun, and a span of ten years), not the planet’s rotation.