Glossary

aphelion
The point in Earth's orbit where it is farthest from the Sun.
autumnal equinox
The point in Earth's orbit where the light part of the day is the same length as the dark, after which the nights get longer.
calendar year
The length of the year according to the calendar, always a whole number of days.
common year
A solar calendar year with 365 days.
day
See solar day.
generation
A period of time equal to 33 years, used as the leap year cycle in the Earthian Calendar.
intercalary
Refers to an extra day, week or month periodically included in a calendar year in order to maintain synchronicity with the seasonal cycle.
leap year
A solar calendar year of 366 days.
lunar calendar
A calendar that uses lunations for months, with 12 lunations per calendar year.
lunation
The average length of a synodic month, equal to about 29.53 days.
lunisolar calendar
A calendar that uses lunations for months, with either 12 or 13 lunations per calendar year.
month
A period equal, or approximately equal, to a lunation, with possible lengths of 28 to 31 whole days.
perihelion
The point in Earth's orbit where it is closest to the Sun.
perpetual calendar
A calendar in which weeks and months are synchronised, and thus each calendar date always occurs on the same day of the week.
season
The time between seasonal markers, equal to about ¼ years.
seasonal marker
A solstice or an equinox.
sol
One martian solar day, equal to about 24 hours and 40 minutes.
solar calendar
A calendar that uses a fraction of a year as a month (usually 1/12), and has year lengths of 365 or 366 days.
solar day
The time taken for Earth to rotate once on its axis and the Sun to return to the same position in the sky, equal to 24 hours.
summer solstice
The point in Earth's orbit where the length of the night has reached a minimum.
synodic month
The time for the Moon to orbit Earth and return to the same position with respect to Earth; the time between new moons.
vernal equinox
The point in Earth's orbit where the light part of the day is the same length as the dark, after which the nights get shorter.
vernal equinox year
The average period between successive northern vernal equinoxes, equal to about 365.2424 days.
week
A period of 7 days.
winter solstice
The point in Earth's orbit where the length of the night has reached a maximum.
year
Usually means calendar year.