Skip to content

The sol

Space and time · level 4, lesson 1 of 6

A question to start

After Perseverance landed in 2021, about 350 people at NASA's Jet Propulsion Laboratory worked on Mars time. Their first shift started at 2 p.m. When did it start a week later?

The idea

A Mars solar day, from noon to noon, is called a sol. On average it lasts 24 hours 39 minutes 35 seconds.

Length of a day Mars Earth
Noon to noon, on average 24 h 39 min 35 s 24 h

Since the Viking landers of 1976, mission clocks split the sol into 24 Mars hours, each of 60 Mars minutes. So a Mars hour is a little longer than an Earth hour: about 61.6 Earth minutes.

The extra 39 and a half minutes adds up. If you follow Mars time, your day starts about 40 minutes later on the Earth clock each day. After about 36 days the drift has gone all the way round the clock, and you are back where you started.

Earth day: 24 hsol: 24 h 39.6 minextra 39.6 min

Worked examples

1. A Mars hour. \(24 \text{ h } 39.6 \text{ min} = 1{,}479.6\) minutes. \(1{,}479.6 \div 24 \approx 61.6\) Earth minutes.

2. A week of drift. Each day the start time slips about 40 minutes. After 7 days: \(7 \times 40 = 280\) minutes, about 4 hours 40 minutes. A shift that started at 2 p.m. starts at about 6:40 p.m. a week later.

3. Round the clock. A full day is 1,440 minutes. \(1{,}440 \div 39.6 \approx 36\) days.

Try it

  1. What is a sol?
  2. How much longer is a sol than an Earth day?
  3. Is a Mars hour longer or shorter than an Earth hour?
  4. You follow Mars time for 3 days. About how much later does your day start on the Earth clock?
Answers
  1. A solar day on Mars, noon to noon.
  2. About 39 and a half minutes.
  3. Longer: about 61.6 Earth minutes.
  4. \(3 \times 40 = 120\) minutes, about 2 hours later.

Common mistake

"A sol is 24 hours 37 minutes." That is how long Mars takes to turn once against the stars. A sol, noon to noon, is a couple of minutes longer. The next lesson shows why.

Where it leads