Jennie Worden's List: Final frontier

• The principal astronomical cycles are the day (based on the rotation of the Earth on its axis), the year (based on the revolution of the Earth around the Sun), and the month (based on the revolution of the Moon around the Earth).
• The Gregorian Calendar
• cycle of 400 years
• resulted from a perceived need to reform the method of calculating dates of Easter.
• As it exists today, the Hebrew calendar is a lunisolar calendar that is based  on calculation rather than observation. This calendar is the official calendar of Israel and is the liturgical calendar of the Jewish faith.
• he beginning of each month is determined by a tabular New Moon (molad) that is based on an adopted mean value of the lunation cycle. To ensure that religious festivals occur in appropriate seasons, months are intercalated according to the Metonic cycle, in which 235 lunations occur in nineteen years.
• The earliest evidence indicates a calendar based on observations of Moon phases
• Islamic calendar is a purely lunar calendar in which months correspond to the lunar phase cycle. As a result, the cycle of twelve lunar months regresses through the seasons over a period of about 33 years. For religious purposes, Muslims begin the months with the first visibility of the lunar crescent after conjunction. For civil purposes a tabulated calendar that approximates the lunar phase cycle is often used.
• For religious purposes, each month begins in principle with the first sighting of the lunar crescent after the New Moon. This is particularly important for establishing the beginning and end of Ramadan. Because of uncertainties due to weather, however, a new month may be declared thirty days after the beginning  of the preceding month.

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