Here's how long one day is on other planets, according to NASA. A day is one cycle from daytime to nighttime, called a solar day. ⁣ ⁣ NASA says: ⁣ "Mercury's highly eccentric, egg-shaped orbit takes the planet as close as 29 million miles (47 million kilometers) and as far as 43 million miles (70 million kilometers) from the Sun. Mercury spins slowly on its axis and completes one rotation every 59 Earth days. One Mercury solar day (one full day-night cycle) equals 176 Earth days – just over two years on Mercury.⁣ ⁣ A day on Venus is 243 Earth days long – longer even than a Venus year (one trip around the Sun), which takes only 225 Earth days. Because of the planet's extremely slow rotation, sunrise to sunset would take 117 Earth days. On Venus, the sun would rise in the west and set in the east, because Venus spins backward compared to Earth.⁣ ⁣ As Mars orbits the Sun, it completes one rotation every 24.6 hours, which is very similar to one day on Earth. Martian days are called sols – ...