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A calendar for Mars

Space and time · level 4, lesson 6 of 6

A question to start

Our months come, roughly, from the Moon. Mars has two moons. Why do they not give Mars months?

The idea

Mars's moons are tiny and fast. Phobos goes round about three times a sol, and Deimos in about a sol and a quarter. Neither gives anything like a month. Many calendars have been suggested for Mars, but none is in common use.

Instead, scientists give a Mars year and an Ls, such as "Mars Year 39, Ls 30°". For counting sols over long spans, there is the Mars Sol Date (MSD): a plain count of sols since 29 December 1873. Earth has the same idea, the Julian Date, a count of days used by astronomers.

A plain day count Mars Earth
Name Mars Sol Date Julian Date
Counts from 29 December 1873 noon, 1 January 4713 BC
Value on 1 January 2026 about 54,034 2,461,041.5

The Mars Sol Date was 44,796 on 6 January 2000. From there, divide the Earth days since by 1.0275 and add.

0December 187344,796 in January 2000about 54,034 in January 2026

Worked examples

1. From 2000 to 2026. From 6 January 2000 to 1 January 2026 is 9,492 Earth days. \(9{,}492 \div 1.0275 \approx 9{,}238\) sols. \(44{,}796 + 9{,}238 = 54{,}034\).

2. Sols between two dates. Two events have Mars Sol Dates 54,000 and 54,365. They are 365 sols apart: about \(365 \times 1.0275 \approx 375\) Earth days.

3. Writing a date. Mars Year 39 began on 30 September 2026, at Ls = 0°. A date halfway through northern spring is "Mars Year 39, Ls 45°".

Try it

  1. Why do Mars's moons not give it months?
  2. What does the Mars Sol Date count?
  3. How many Earth days are 100 sols?
  4. Write the start of northern summer in Mars Year 39 using Ls.
Answers
  1. They are tiny and go round far too quickly: Phobos about three times a sol.
  2. Sols since 29 December 1873.
  3. \(100 \times 1.0275 \approx 103\) Earth days.
  4. Mars Year 39, Ls 90°.

Common mistake

Dividing the wrong way. To turn Earth days into sols, divide by 1.0275. A sol is longer, so there are fewer of them.

Where it leads