How do scientists measure the length of a day on Mars?

Because Mars has visible surface features, researchers can measure its rotation by tracking the same landmarks over time with orbiters, landers, and rovers. Radio signals from missions like Viking, Curiosity, and Perseverance also help pinpoint the planet’s spin very precisely. This is how scientists know that a Martian solar day, called a sol, is about 24 hours 39 minutes long. That is one reason Mars is often considered the most Earth-like planet in day length.