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Mars time on a mission

Space and time · level 4, lesson 3 of 6

A question to start

It is noon at Mars's prime meridian. Perseverance is about a fifth of the way round the planet to the east. What time is it there?

The idea

On Earth, longitude 0° runs through Greenwich in England. On Mars it runs through a tiny crater called Airy-0, named after George Airy, the astronomer who built the Greenwich telescope that marks Earth's line.

Mean solar time at Airy-0 is called Airy Mean Time (AMT). Every 15° of longitude to the east puts you one Mars hour later, because \(360° \div 24 = 15°\). So the time at a landing site, its Local Mean Solar Time, is

\[\text{local time} = \text{AMT} + \frac{\text{east longitude}}{15°} \text{ Mars hours}.\]
Mars Earth
Line of longitude 0° Airy-0 crater Greenwich
Mean time there Airy Mean Time Greenwich Mean Time
One hour of time zone 15° of longitude 15° of longitude

Missions also count sols from landing day. Perseverance landed on Sol 0, 18 February 2021. Some earlier rovers, such as Spirit and Opportunity, called their landing day Sol 1.

Airy-0, 0°Perseverance,about 77° east:AMT + 5 Mars hoursseen from above the north polespin

Worked examples

1. Perseverance. It landed at about 77.45° east. \(77.45 \div 15 \approx 5.16\) Mars hours, about 5 hours 10 minutes. At noon AMT it is about 5:10 p.m. at Perseverance.

2. Curiosity. It landed at about 137.44° east. \(137.44 \div 15 \approx 9.16\) Mars hours. At noon AMT it is about 9:10 p.m. at Curiosity.

3. A sol count. On 1 January 2026, 1,778 Earth days had passed since Perseverance landed. A sol is about 1.0275 Earth days: \(1{,}778 \div 1.0275 \approx 1{,}730\). Perseverance was near sol 1,730.

Try it

  1. Where is Mars's line of longitude 0°?
  2. How many degrees of longitude make one Mars hour of time zone?
  3. It is 8 a.m. AMT. A lander sits at 45° east. What is its local time?
  4. A rover landed 1,028 Earth days ago, on its Sol 0. About what sol is it now?
Answers
  1. At the crater Airy-0.
  2. 15°.
  3. \(45 \div 15 = 3\) Mars hours later: 11 a.m.
  4. \(1{,}028 \div 1.0275 \approx 1{,}000\): about sol 1,000.

Common mistake

Using Earth hours in the time-zone sum. The 15° rule gives Mars hours. Each is about 61.6 Earth minutes.

Where it leads