Seasons

Seasons are climatic variations caused by the angle of a planet's rotational axis. Calendars are aligned with the seasons, such that the same date each year identifies the same point in the seasonal cycle. This enables the calendar to be used as a means of knowing when to expect changes in climate, and therefore, when to plant, when to harvest, when to plan holidays, and so on.

Mars' rotational axis is tilted like Earth's and therefore also has seasons, however, because of its elliptical orbit, Mars' seasons are not of equal length.

Note that the different season lengths means that the northern hemisphere of Mars has a longer summer and spring than the southern hemisphere. This is somewhat inconvenient, as Mars, if terraformed, will eventually have a large, planet-circling northern ocean (called 'Oceanus Borealis') - thus the majority of the population will live in the colder south.

Although the Gregorian calendar does not begin on a seasonal marker (a solstice or equinox), the convention in both astronomy and astrology is to mark the beginning of a year with the Northern Vernal Equinox. As a result, this approach has also been used in many Martian calendars, and since it makes sense to re-use existing systems wherever possible, the Utopian Calendar also begins each calendar mir at this marker.