2 full moons in October

It was a dark and stormy night. The full moon stared through the scudding clouds and the bare tree branches.

It seems  in almost all the spooky movies on TV around Halloween, the moon is always full. Interestingly ,the moon is full on that night only once every 19 or so years, and when it does happen, it is always the second full moon that month.

Two full moons in the same month comes up every two or three years. This second full moon of the month is often referred to as a Blue Moon.

This is where the expression "Once in a Blue Moon" came from, referring to something that does not happen very often. However, this definition of a Blue Moon might not be correct. Apparently the 13th full moon in any given year is referred to as a Blue Moon.

The phases of the moon are caused by the changing arrangement of Sun, Earth and moon as the moon orbits the Earth.

A full moon happens when the Earth lies between the moon and Sun, and we see the moon lit with the light coming from behind us.

The complications in the timing of these events arise because Mother Nature does not seem to like whole numbers.

The interval between two consecutive full moons is 29.5306 days, and a year, the time the Earth takes to go around the Sun, is 365.25635 days.

This is why we have to have leap years, leap seconds and other adjustments to keep the date in step with the seasons. There are therefore 12.3687 lunar phase cycles in a year. 

Just as 13 fence posts in a line have 12 gaps between them, the time interval covered by 13 full moons is 354.3672 days. Therefore, if the first full moon of the year turns up early enough in the year, there will time for a 13th full moon before the year ends.

This can lead to a confusing situation. If the first full moon of the year occurs at 1 a.m. on Jan. 1 Eastern Standard Time, then that year will be a Blue Moon year for inhabitants of Ontario and points east.

However, in B.C. that moment will happen at 10 p.m. Pacific Standard Time on Dec. 31 the preceding year, so that year would have been a Blue Moon year for those in British Columbia.

This sort of confusion is why in science we use just one time zone. By international agreement it is Universal Time, the standard time at the Old Greenwich Observatory, in the U.K., located on the zero degrees of longitude meridian.

Compared with the size of the Earth, the moon is unusually large. This has often led to astronomers referring to the Earth-Moon combination as a double planet.

For example, Mars has two moons: Phobos and Deimos, both of which are tiny. One day, when we are standing on the surface of the red planet, looking up, we will see those moons as small, star-like objects.

The fact that the moon looms so large in our skies, and its phases are so obvious, led to it becoming the basis of our calendar, which is where the word month (moonth) comes from.

The problem of managing a lunar calendar based on 12.3687 lunar months for each orbit of the Earth around the Sun has led to abandonment of the moon as a calendar basis over much of the world.

However, the moon's presence in our daily and cultural life is more intimate than just something associated with the date. It has been associated with religious events for thousands of years.

Easter is celebrated on the first Sunday after the first full moon falling on or after the spring equinox.

In addition, think of the amount of classical, jazz and popular music where the moon is featured, or its widespread presence in poetry and art.

This helps explain why, on those stormy nights in those old, spooky movies, the moon is always there.

• After dark, Saturn and Jupiter lie low in the south
• Mars is rising in the east.
• Venus rises in the early hours.
• Mercury lies low in the dawn glow.
• The moon reaches Last Quarter on the 8th. 

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.