Once In A Blue Moon


One of the fundamental aspects of our daily lives is the cycle of day and night. It’s a natural rhythm that we often take for granted, but it’s rooted in the Earth’s rotation on its axis. The question that many might ponder is whether the Earth truly completes a full rotation in precisely 24 hours. In this article, we will explore the science behind this phenomenon and understand why the Earth’s rotation period isn’t exactly 24 hours.

The Basics of Earth’s Rotation

The Earth rotates on its axis, an imaginary line that runs from the North Pole to the South Pole. This rotation is responsible for the alternating cycle of day and night. As the Earth spins, different parts of its surface are exposed to the Sun’s rays, creating daylight and darkness.

Sidereal Day vs. Solar Day

To answer the question accurately, we need to distinguish between two essential concepts: the sidereal day and the solar day.

  1. Sidereal Day: A sidereal day is the time it takes for the Earth to complete one full rotation concerning distant stars. This period is approximately 23 hours, 56 minutes, and 4 seconds. It is slightly shorter than 24 hours because, as the Earth rotates, it also orbits the Sun. This orbital motion means that the Earth must rotate a bit more than 360 degrees to return to the same position concerning distant stars.
  2. Solar Day: The solar day is the time it takes for the Earth to complete one full rotation concerning the Sun. This period is what we commonly refer to as a “day” and is about 24 hours long. It is the time it takes for the Sun to appear in the same position in the sky, such as when the Sun is directly overhead (solar noon).

Why the Solar Day Is Slightly Longer

The Earth’s orbit around the Sun is an elliptical (slightly oval) shape, which means that the speed at which it orbits is not constant throughout the year. When the Earth is closer to the Sun in its orbit (perihelion), it moves slightly faster, and when it’s farther away (aphelion), it moves slightly slower.

Because of this elliptical orbit, the Earth’s rotation period required to return to the same position concerning the Sun (solar day) can vary slightly throughout the year. This variation is why the solar day is not precisely 24 hours every day but averages out to about 24 hours over the course of a year.

Leap Seconds

To keep our timekeeping in sync with the Earth’s rotation, scientists occasionally add or subtract a “leap second” to or from Coordinated Universal Time (UTC). This adjustment ensures that the solar day remains close to 24 hours. Leap seconds are added or subtracted at irregular intervals to account for the Earth’s changing rotation rate due to factors like tides and geological processes.


In conclusion, while the Earth’s rotation period concerning distant stars (sidereal day) is approximately 23 hours, 56 minutes, and 4 seconds, the more familiar solar day, which is based on the Earth’s rotation concerning the Sun, averages out to about 24 hours. However, due to the Earth’s elliptical orbit and other factors affecting its rotation, the solar day can vary slightly from day to day. To maintain accuracy in timekeeping, leap seconds are occasionally added or subtracted from Coordinated Universal Time (UTC). So, while the Earth doesn’t complete a full rotation in precisely 24 hours, our timekeeping systems are designed to keep our days in sync with the Sun’s position in the sky.

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