Jul 11, 2007 - ..Children, Science & Technology    No Comments    3,782 views

Why is there only 30 or 31 days in a month?

Kids ASK! about Science & Technology

The month is a measure of time corresponding or nearly corresponding to the length of time required by the Moon to revolve once around the Earth.

It is determined by the Moon’s passage around the Earth, and, as in the case of the day, there are several ways in which it can be defined.

In essence, these are of two kinds: first, the period taken by the Moon to complete an orbit of the Earth and, second, the time taken by the Moon to complete a cycle of phases.

Among primitive societies, the month was determined from the phases. This interval, the synodic month, is now known to be around 29.5 days. When we say “synodic”, we are talking about “the period an orbiting body takes to move between successive conjunctions relative to an observer on Earth” (Reference: “synodic.” A Dictionary of Weights, Measures, and Units. Oxford University Press, 2002, 2004. Answers.com 11 Jul. 2007. http://www.answers.com/topic/synodic)

Because of perturbations in the Moon’s orbit, the lengths of all astronomical months vary slightly. The synodic month grew to be the basis of the calendar month.

The year, on the other hand, is the period taken by the Earth to complete an orbit around the Sun

Sources :
1. Calendar. (2007). In Encyclopædia Britannica. Retrieved March 30, 2007, from Encyclopædia Britannica Online: http://search.eb.com/eb/article-59344
2. Month. (2007). In Encyclopædia Britannica. Retrieved March 30, 2007, from Encyclopædia Britannica Online: http://search.eb.com/eb/article-9053549

If you want to find out more information about calendars and the phases of the moon, you may refer to the following books :

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Calendars by Patricia K. Kummer
Publisher : New York : Franklin Watts, 2005
Call No. : J English 529 KUM
Click here to check our catalogue for item availability.

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Phases of the moon 2007 by Gillia Olson
Publisher : Mankato, Minn. : Capstone Press, c2007
Call No. : JP English 523.3 OLS
Click here to check our catalogue for item availability.

Calendars by Brian Williams
Publisher : London : Cherrytree Books, 2002
Call No. : J English 529 WIL
Click here to check our catalogue for item availability.

Time by John Farndon
Publisher : Malaysia : Federal Publications Sdn Bhd, 2003
Call No. : J English 529 FAR
Click here to check our catalogue for item availability.

Some websites you can check out:

Calendars Through the Ages

MSN Encarta: Calendar

All websites are last accessed on 11 July 2007.

Answered by Ms Krist Chan, Librarian, Children’s Services

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