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Seasons of unequal length

Space and time · level 4, lesson 5 of 6

A question to start

On Earth the four seasons are all about three months long. On Mars, northern spring is about 50 sols longer than northern autumn. Why?

The idea

Mars has seasons for the same reason Earth does: its axis is tilted, by about 25.2° (Earth's is about 23.4°). But Mars's orbit is more stretched. Its distance from the Sun swings from about 207 million km to about 249 million km, and it moves faster when it is close to the Sun.

Mars is farthest from the Sun, and slowest, near the start of northern summer. So the northern spring and summer drag on, and the autumn and winter rush by.

Scientists mark the time of a Mars year by Ls, the angle Mars has gone round the Sun since the start of northern spring. Ls = 0° starts spring, 90° summer, 180° autumn and 270° winter.

Northern season (Ls) Mars Earth
Spring (0° to 90°) about 193 sols about 93 days
Summer (90° to 180°) about 179 sols about 94 days
Autumn (180° to 270°) about 143 sols about 90 days
Winter (270° to 360°) about 154 sols about 89 days

The south gets the opposite seasons: its summer is short and hot, close to the Sun. The dust storm season begins just after Mars passes closest to the Sun, at about Ls = 260°.

Ls 0°Ls 90°Ls 180°Ls 270° springsummerautumnwinter

Worked examples

1. Spring's share. \(193 \div 668.6 \approx 0.29\): northern spring is about 29% of the Mars year. With four equal seasons it would be 25%.

2. Longest minus shortest. \(193 - 143 = 50\) sols.

3. Reading Ls. Mars is at Ls = 135°. That is halfway between 90° and 180°: the middle of northern summer.

Try it

  1. Why does Mars have seasons?
  2. Which northern season on Mars is longest? Which is shortest?
  3. What does Ls = 270° mean?
  4. Why is the southern summer on Mars short and hot?
Answers
  1. Its axis is tilted, like Earth's.
  2. Spring is longest (about 193 sols); autumn is shortest (about 143 sols).
  3. The start of northern winter.
  4. It happens when Mars is closest to the Sun, moving fastest and getting the most sunlight.

Common mistake

"Seasons come from being nearer the Sun." The tilt makes the seasons. The changing distance only makes them unequal, and stronger in one hemisphere.

Where it leads