~ Useful Link ~



~Photo Credits~

clip art:









~ Rotation and Revolution ~

( and calendars too )


Rotate: Spin on an axis (one day)

Revolve: Move around another object (one year)

* Planets REVOLVE around stars.

* Moons REVOLVE around planets.

* Moons, planets, and stars ROTATE around their axis.

FYI: Planet Earth ROTATES in a counter-clockwise direction.

Earth ROTATES around its axis once in a day.

( 24 hours )



Earth's ROTATION (spinning) causes night and day.

Above: Model of Night and Day

Above: Night and Day (satellite image)

The Earth REVOLVES around the sun in a counter-clockwise direction.

The Earth REVOLVES around the sun once in a year.

( 365.2422 days )

So...every four years we have a "Leap Year"

by adding one extra day to the month of February (29th).

BUT...this is still not totally accurate.

This system is off by 11 minutes and 14 seconds per year...

...so a century year cannot be a Leap Year...

...unless it is divisible by 400.


~ Song of the Months ~

30 days in September,

April, June and November,

All the rest have 31,

Except for February.

...with 28...

...but on a Leap Year, 29.



Definition of Orbit:

the curved path of one object around another object in space.

FYI: Earth's orbit around the Sun is shaped like an ellipse.

A circle has one focus. An ellipse has two foci.

Aphelion = Earth's farthest point from the sun during its revolution.

Perihelion = Earth's closest point to the sun during its revolution.


On what date is the sun farthest away from the Earth (aphelion)?

On what date is the sun closest to the Earth (perihelion)?


FYI: Aphelion is pronounced "uh-fee-lee-on"

( aphelion means far away )


Johannes Kepler: Three Laws of Planetary Motion (1609)


~ Period of Rotation (Length of Day) ~

>>> Which planet has the shortest day?

>>> Which planet has the longest day?




"So you say you want a revolution..." - The Beatles



~ Calendars ~


The English word "calendar" is derived from the Latin word, kalendae, which was the Latin name for the first day of every month.

The ancient Egyptians created one of the first calendars. They counted the number of days between each first appearance of the star Sirius in the morning. In that way, they found that there are about 365 days in a year.

Dividing the year into smaller parts was difficult. Early people used moon cycles to divide the year. The time from one full moon to the next is 29 1/2 days. A year of 12 of these "moonths" adds up to only 354 days.

The ancient Egyptian calendar had 12 months of 30 days each, with an extra 5 days at the end. (360 + 5 = 365 days total)

The Romans borrowed the Egyptian calendar of 365 days. But the Earth orbits the sun in about 365 1/4 days, so they adjusted the Egyptian calendar by adding one day every four years. You know the fourth year as a "leap year." During a leap year, February is given 29 days instead of the usual 28. This was known as the "Julian Calendar" named after Julius Caesar.

The Roman calendar was off by a little more than 11 minutes a year. Over the centuries, these minutes added up. By the 1500's, the beginning of spring was about ten days too early. To straighten things out, Pope Gregory XIII dropped ten days from the year 1582. He also made some other minor changes to the Roman system to form the calendar that we use today, called the "Gregorian Calendar."

Credit: Prentice Hall Astronomy, Wikipedia


~ Cultural Calendars ~

( Lunar vs. Solar )

Lunar = Moon / Solar = Sun

Above: Hindu Lunar Calendar

Lunar calendars are based on cycles of the moon.


Above: Egyptian Pyramids were used as Solar Calendars

Solar calendars are based on cycles of the sun.

Above: Egyptian Sun God Ra


"Time is running." - Migmar Tseten

Click here: Time / Clocks / History of Clocks / Timeline

Click here: Sundials / Make a Sundial / Pendulum / Swing


Above and Below: Chinese Zodiac

Click here: Chinese Han/Tibetan Lunisolar Calendar


A Chinese Lunar New Year Celebration

Above: Moon Cakes for the Chinese Harvest Moon Festival

What was all the hype about December 21, 2012?

Our sun was alligned with the plane of our galaxy on that date.

A Mayan calendar cycle ended on that date.

( Myth Busted )


Above: Mayan Solar Calendar

Above: Mayan Pyramid (Temple to the Sun)

Above: Aztec Solar Calendar

Above and Below: Stonehenge, England (solar calendar)

Click here: America's Stonehenge