The rotation of planets across the Solar System creates a remarkable range of time scales, where the concept of a “day” can vary from just a few hours to hundreds of Earth days. These spinning speeds are a direct outcome of each planet’s formation and history, establishing vastly different environments and cycles of light and darkness.
Defining a Planetary Day
The length of a planetary day must be defined by two distinct measurements: the sidereal day and the solar day. The sidereal day is the time it takes for a planet to complete one full 360-degree rotation on its axis relative to the distant, fixed stars. For Earth, the sidereal day is approximately 23 hours, 56 minutes, and 4 seconds.
The solar day is the more familiar measurement because it dictates the cycle of daylight and darkness; it is the time it takes for the Sun to return to the same position in the sky. As a planet rotates, it is also simultaneously moving in its orbit around the Sun. The planet must rotate slightly more than 360 degrees to bring the Sun back to the starting point, making the solar day slightly longer than the sidereal day on most planets. For Earth, this difference is only about four minutes, resulting in a familiar 24-hour solar day.
The Record Holder: Venus
The planet with the longest solar day is Venus, where a single day-night cycle lasts an astonishing 116.75 Earth days. This extreme length is the result of the planet’s unique rotation and its orbital motion.
The planet’s sidereal day, the time for one complete 360-degree spin, is 243.02 Earth days, which is the longest rotation period of any planet in the Solar System. The slow rotation means that a Venusian year, the time it takes to orbit the Sun, is only about 225 Earth days. Remarkably, Venus’s year is shorter than its sidereal day, meaning the planet completes an orbit before it can complete one rotation on its axis.
The second, equally important factor is Venus’s retrograde rotation, meaning it spins backward relative to its orbit and most other planets. This backward spin effectively works against the planet’s orbital motion, reducing the length of the solar day significantly compared to the sidereal day. The solar day of 116.75 Earth days is less than half the length of its 243-day sidereal rotation.
Planetary Day Length Extremes
Venus’s massive solar day stands in stark contrast to the rapid rotation found elsewhere in the Solar System, particularly with the gas giants. Jupiter, the largest planet, holds the record for the shortest day, completing a full rotation in just under 10 hours.
Among the inner rocky planets, Earth and Mars have day lengths that are quite similar, with Mars taking 24 hours and 37 minutes to complete its rotation. Mercury, the innermost planet, presents another unusual case. Mercury’s sidereal day is about 59 Earth days, but due to a unique 3:2 spin-orbit resonance, its solar day is 176 Earth days long. This means the sun rises and sets only once every two Mercury years.