(6) The Calendar
Part of a high school course on astronomy, Newtonian mechanics and spaceflight
by David P. Stern
This lesson plan supplements: "The Calendar," section #6: on disk Scalend.htm, on the web |
Goals: The student will learn|
Terms: solar day, star day (sidereal day), solar year, Julian calendar, leap year, Gregorian calendar, true (sidereal) and apparent (synodic) period of the Moon, Metonic calendar, Moslem calendar.
Stories and extras: How the transition from Julian to Gregorian calendar affected the dates of George Washington's birthday and the anniversaries of the October Revolution. Tidbits about the holidays of the Russian church and of Islam.
This lesson has particular significance. Not only does it transmit useful information about time-keeping, but it also underscores the diversity of human culture. It shows how cultures in different countries (and at different times) addressed the same problem of time-keeping and developed interesting variations of the same basic solution.
Starting the lesson:
Icebreaker riddle: "Do the British have a 4th of July?"
Some that may be mentioned by students or by the teacher:
What are calendars used for?
The questions below may be used to guide the lesson and also in the review afterwards. Items in brackets [ ] are optional.
Projects: Before this class, students with web access could be given assignments to prepare 5-minute reports on various non-western calendars, using encyclopaedias, web links cited by "Stargazers" and other calendars they may find, e.g. the ancient Roman one.
-- How is "one day" usually defined??
-- How would you define noon for this purpose?
(For points south of the equator, the time when the Sun passes exactly to the north.)
Actually the time from noon to noon varies slightly, because of the uneven motion of Earth in its orbit. What clocks measure is the average day.
-- How is the average day divided?
-- The "average solar day" of 24 hours is not exactly the duration of one full rotation of the Earth. Why the difference?
-- How would you measure the rotation period of the Earth?
Astronomers actually do just that, timing the passage of a star through the cross-hairs of a telescope constrained to move in a north-south direction only (pivoted around an east-west axis). In old times the astronomer would lie on his back and watch the star drift along the field of view, pressing a button at the right moment. Today electronics provide greater accuracy.
(Optional addition: Comparing those times to very accurate clocks (nowadays these are "atomic clocks", whose frequency is set by the natural vibration of certain molecules) has shown that the rotation of the Earth is slowing down, at an extremely slow rate. The cause is a loss of rotational energy due to the tides in the oceans, raised by the pull of the Moon. Because of this slowing down, astronomers every few years declare a "leap second" by which all accurate timekeeping must be shifted.)
-- What is the rotation period of the Earth?
-- What is a "solar year" ?
-- What happens (as in the Moslem calendar) when the calendar year is shorter than the solar year?
-- The Julian Calendar, instituted in Rome by Julius Caesar in 46 BC, assumed that the length of the solar year was 365.25 days. How was that fraction taken into account?
--We still add a day every 4th year. Are we using the Julian calendar?
The "Gregorian Calendar", introduced in 1582 Pope Gregory the 13th, took care of most of this discrepancy. By that time, holidays etc. had slipped by a total of about 10 days. What was the change it introduced, and what was its connection with the year 2000?
--Does anyone still use the Julian calendar?
-- George Washington's birthday is usually observed February 22, but Washington himself often noted he was born February 11th "old style. " What did he mean?
-- Our calendar is tied to the annual journey of the Earth around the Sun--or else, to the annual journey of the Sun's apparent position around the sky. Are some calendars based on other celestial phenomena? Which ones?
--What is a "new Moon"?
-- What is the basic unit of lunar calendars and how is it defined?
-- How long is that?
--Is that the orbital period of the Moon? If not, why the difference?
--What is the Moon's orbital period?
--Jews use a lunar calendar--their months always start at the new Moon. How do they keep their calendar lined up with the solar year, and avoid getting holidays that drift through the seasons of the year?
-- What is a "Metonic calendar"?
--The traditional Persian (or Iranian) calendar follows a solar year. Its months have 30 or 31 days (like ours) and an adjustable month can have 28 or (in leap years) 29. The year differs in several ways from ours, but especially in the time of the new year. When does the Persian year begin?
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Author and Curator: Dr. David P. Stern
Mail to Dr.Stern: stargaze("at" symbol)phy6.org .
Last updated: 10 September 2004