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Days, years and seasons

Gravity and orbits · level 2, lesson 6 of 6

A question to start

Why is there a day and a night? Why is a year as long as it is? And why is it colder in winter?

The idea

A day is one spin. The Earth spins like a top. When your side faces the Sun, it is day; when it faces away, it is night. One full spin takes 24 hours. Mars spins too, a little more slowly: one Mars day, called a sol, is 24 hours and 39 minutes.

A year is one trip round the Sun. Gravity keeps the Earth in orbit round the Sun, and one trip takes 365 days. Mars is farther out and moves more slowly, so its trip is much longer: 687 Earth days, nearly two of our years.

Seasons come from a tilt. The Earth's spin is tipped over a little, by about 23°. As the Earth goes round the Sun, the tilt always points the same way in space. For half the year your half of the Earth leans towards the Sun: the Sun climbs high, days are long, and it is summer. For the other half it leans away: winter.

top half: wintertop half: summer

Mars is tilted too, by about 25°, very close to the Earth's tilt. So Mars has seasons like ours: spring, summer, autumn and winter. But each season lasts about twice as long, because the Mars year is nearly twice as long.

Mars Earth
A day (one spin, Sun to Sun) 24 h 39 min 24 h
A year (one trip round the Sun) 687 Earth days 365 days
Tilt 25° 23°
Seasons about 6 months each about 3 months each

Worked examples

1. A child's birthday on Mars. If you had a birthday party every Mars year, you would have one party about every 23 months. A 10-year-old on Earth would be only about 5 in Mars years.

2. A longer day. In one week of Earth days, a sol clock falls behind by \(7 \times 39\) minutes, about 4.5 hours. Mission teams who work on Mars time shift their work hours later every day.

Try it

  1. What causes day and night?
  2. Why is a year on Mars longer than a year on Earth?
  3. What causes the seasons?
  4. Does Mars have seasons? How long does each one last?
Answers
  1. The planet spinning, so your side faces towards or away from the Sun.
  2. Mars is farther from the Sun, so its trip round is longer, and it also moves more slowly.
  3. The tilt of the planet's spin: your half leans towards the Sun in summer and away in winter.
  4. Yes, because Mars is tilted about 25°. Each season lasts about twice as long as on Earth, about 6 months.

Common mistake

"Summer is when the Earth is closest to the Sun." The Earth is actually closest to the Sun in early January, during winter in the north. Seasons come from the tilt. (On Mars, whose orbit is more stretched, the distance does matter as well: the southern summer is hotter and shorter than the northern one.)

Where it leads