10-02-2007 Moon phases (Article 72)

[Simon Kenny]

Moon phases (Article 72)
As the moon travels round the Earth different parts of its surface become visible to us due to reflection of the Sun’s light. These phases vary cyclically, according to the relative positions of the Earth, Moon and the Sun.
Lunar phases are the result of us seeing an illuminated segment of the Moon at different angles. When the Sun and Moon are on opposite sides of the Earth, the Moon appears full. New Moon occurs when the Moon is on the same side as the Sun, and the time between two Full Moons is 29.53 days. It is longer than the time it takes the Moon to orbit the Earth since the Earth-Moon system is also orbiting the Sun.
Each phase of the Moon has a different name. Between New and Full phases, the Moon moves through Crescent Moon, First-Quarter, and Gibbous, before cycling back to New Moon.
It would seem that every time the Moon passes between Earth and Sun its shadow should fall on Earth, causing a solar eclipse. Similarly, at Full Moon we should see a lunar eclipse. This is actually how eclipses arise but eclipses don’t occur with every Full/New Moon. This is because the plane of the Moon's orbit around the Earth is tilted by 5 degrees with respect to the Earth's orbit around the Sun, meaning that at the New/Full Moons the Moon usually lies to the north or south of a direct line through the Earth and Sun. An eclipse only transpires when the Moon is located at the intersection of the Moon's orbital path and the ecliptic.
While the movement of the moon and its phases was intimately known in times gone by this knowledge has dwindled as we have more precise methods to measure seasonal changes. Despite this most people will still recognise the term “Harvest Moon” which traditionally signified the beginning of the harvest in the Northern Hemisphere. This event was celebrated with festivals and rituals. At this point the moon would rise at a point which is opposite to the Sun, and is sited close to the eastern point of the horizon. Indeed every full moon was given a name depending on its significance of the seasonal actions. However terms such as Storm, Mead and Corn moon are rarely used let alone recognised in today’s society. Perhaps we should all try to understand what our nearest celestial neighbour tells us!