Many students (and adults!) are unaware of the origin of our calendar
system. The calendar we use today was developed in 525 AD. The
previous calendar, developed by the Romans, used the founding of the
city of Rome (some 1200 years earlier) as its “zero date”.
A monk named Dionysius Exiguus proposed using the birth of Jesus Christ
as the “zero date” of the calendar (although most historians
agree Dionysius got the date of that event wrong by a few years). The
calendar was adjusted by Pope Gregory XIII in 1582 to eliminate errors
that had accumulated over the previous century. Gregory also ordered
that only centuries not divisible by 400 would be leap years (therefore,
for example, 2000 was a leap year, but 1900 was not). This
“Gregorian calendar” has become the standard used the world
over, though not everyone adopted it at the same time. Britain and her
colonies did not adopt it until 1752 (skipping 11 days between 3
September and 14 September, and resetting the beginning of the year from
25 March to 1 January). Russia did not adapt the new calendar until
1917, so the October Revolution is now celebrated on 7 November.
Other than the day/night cycle, the most obvious cycle that can track
the passage of time is the cycle of the phases of the moon. It should
come as no surprise, therefore, that all early calendars (with the
exception of the ancient Egyptian calendar) were lunar calendars. Even
the word “month” derives from the word “moon”.
Originally there were ten months: January (from Janus, a Roman god),
February (from Februar, a Roman festival of purification), March (from
Mars, a Roman god), April (from the Latin word aperire, 'to open'), (May
from Maia, a Roman goddess), June (from Juno, a Roman goddess), and
September, October, November, December (from the Latin words meaning
seventh, eighth, ninth, and tenth, respectively). Later, Roman emperors
Julius Caesar and Caesar Augustus invented months to honor themselves,
giving us the months of July and August.
The days of our weeks are, with a few exceptions, named for Norse gods:
Monday (named for the Moon), Tuesday (named for Tiu, an Anglo-Saxon god
or Tyr, a Norse god), Wednesday (named for Woden or Odin, a Norse god),
Thursday (named for Thor a Norse god), Friday (named for Freya, a Norse
goddess), Saturday (named for Saturn, a Roman god), and Sunday (named
for the Sun). Under this system, the days of the week belonged to the
god for whom they were named. Monday was literally “Moon’s
Day”, Thursday was “Thor’s Day”, etc. Not all
calendars have weeks. Historically, weeks are strictly religious cycles
– unlike days, months, and years, they have no ties to celestial
The Gregorian calendar is the most widely used calendar today, but it is
not the only one in use, even now. Calendars of other cultures which are
still in use today include the Jewish, Chinese, and Muslim calendars.
All three are lunar calendars, based upon cycles of the Moon, though the
Jewish calendar is a complex mix of lunar and solar cycles.
Grade Levels: K-12
Time Frame: 30-45 minutes
Students will learn how the physical and orbital characteristics of Mars
are similar to and are different from those of the Earth. Additionally,
students will gain insight into the history of time-keeping on Earth and
how the various calendars of Earth have evolved.
Real World Application:
Students will be given a handout that contains some information about
how Martian “time” differs from Earth “time” and
will be asked to create a calendar that represents these differences.
This project will help students begin to understand how our own
calendars came into being.
National Science Education Standards:
Standard D: Earth in the Solar System
Introduce the idea of calendars and the history behind them. Discuss
with your students why we have the time divisions (days, weeks, months,
seasons, and years) that we do. Have the students do research to
discover the equivalent values for Mars (they are listed here for your
reference, but let your students discover them for themselves). It is
up to the students to decide if these same principles will work for
Martian time, and come up with a way to incorporate them. Here are some
facts about Earth and Mars in the Solar System.
- Mars has a rotation rate such that one day or sol (Martian day)
equals 24.6 hours, or 24 hours, 37 minutes. The Earth rotates once
every 24 hours.
- Mars orbits the Sun in 687 Earth days. The Earth orbits the Sun in
- The rotational axis of Mars tilts towards the Sun at an angle of 25
degrees. Earth has a similar tilt of 23.5 degrees. This tilt causes
seasons on both planets.
- Mars has two moons. Phobos travels around the planet twice in one
sol. Deimos travels around Mars once a sol. The Earth’s moon
orbits the earth once every 28 days.
From this information, the groups will design the Martian calendar. It
is important to take into account such questions as:
- Are you going to have days/weeks/months for your calendar?
- If you have weeks, how many days will you have per week? What will
the names of the days of the week be?
- If you have months, how many days will you have per month? What
will the names of the months be?
- When will your New Year occur?
- Will “leap years” be necessary? How will you handle
“leap years”? Into which month/months will you put the
- Do you think it will be necessary to coordinate with Earth
- When is your “zero year” to be determined? We have
established AD (also called “CE” or “Common
Era”) on Earth.
- How did you come up with the names? Do they have meaning?
Have the student groups present their calendars to the class. Depending
upon your focus, you could evaluate the calendars (for utility or
design), the names assigned to days, weeks, or months (for historical
significance or other appropriateness), or on the accuracy of their
calculations and conversions between Earth and Mars time. The quality
of the presentation could be factored in as well.
- For K - 5, a creative project would be to draw a calendar for one
- For 6 - 12, a creative project would be a paper or PowerPoint©
presentation that describes their calendar in full detail. Research on
the origins of Earth’s calendars would be appropriate prior to the
start of creating Martian calendars.