Friday, 29 March 2013

The Moon And The Month

[Another post in preparation for writing about Easter]

We were taught at primary school that the word "month" comes from "moon" - that a month is about the time that the moon takes to go round the Earth.

And this is an approximation.

But there are different types of month.

In my previous post (I suggest opening it in a new window or new tab as I will refer to it later) I had a look at the tropical year and the First Point of Aries. In stellar co-ordinate systems there is something called ecliptic longitude and ecliptic latitude. These are both zero at the First Point of Aries.

If you picture a line perpendicular to the ecliptic at the First Point of Aries (go back to the image of Pisces in the previous post) then anything along that line has an ecliptic longitude of zero.

The average time it takes for the Moon to return to an ecliptic longitude of zero is known as the Tropical month and this is 27.321582 days long.

New moon is when the Sun, Moon and Earth line up. You might think that one tropical month after new moon it would be new moon again, but this ignores that the Earth is in orbit around the Sun.

Suppose, for example, we have a new moon at precisely the vernal equinox. The Sun (at the First Point of Aries) and the Moon both have an ecliptic longitude of zero. One tropical month later, the Moon has an ecliptic longitude of zero. But the Sun?

Well, that's moved on, and by then would be just on the border of the neighbouring constellation Aries (all constellation images from Heavens Above:

So, the Moon has to catch up the Sun, which takes it a couple of extra days or so to do. This gives us the Synodic month of 29.530589 days, the average time from new moon to new moon.

These are not the only months. You'll notice I've been talking about the Moon having an ecliptic longitude of zero, rather than being at the First Point of Aries. If you recall the pictures of Pisces and Aquarius you would see five of the planets - Mercury, Venus, Mars, Uranus and Neptune - were near the ecliptic but not on it.

And the same is true for the Moon. For example, the northernmost part of the ecliptic - which will be where the Sun is at the summer solstice - lies in Gemini

Actually, it's only just in Gemini. It's very near the border with Taurus:

I recall recently walking home and seeing the Moon, and later realising it had actually been in Orion:

The ecliptic passes through 13 constellations (not 12) - but Orion isn't one of them! But the northern part of it (traditionally marking his club) is very close to the ecliptic - close enough to have the Moon passing through it briefly.

The full moon nearest to the winter solstice - the full moon that will be highest in the sky and above the horizon longest - will probably be in Gemini or Taurus. But if the Moon is near its furthest below the ecliptic then it could be in Orion. And if the Moon is near its furthest above the ecliptic it can be in Auriga:

We are all aware that north of the Arctic Circle there is the Midnight Sun. Well, in parts of the USA, Canada, Greenland, Iceland, Norway, Sweden, Finland and Russia south of the Arctic Circle you can at times have the Moon above the horizon for over a day - when it is to the north and at its lowest it is still above the horizon.

The Moon moves from below to above the ecliptic at its ascending node and from above to below at its descending node. The average time from ascending node to ascending node is the Draconic month and is 27.21222 days - shorter than both the tropical month and the synodic month. Hence, if there is a new moon at the ascending node (something I'll come back to in a moment), then at the next new moon, the Moon will be above the ecliptic.

After 242 draconic months - 6,585.35724 days - the Moon would again be at the ascending node. What's so special about this? 223 synodic months is 6,585.321347 days - just 52 minutes less than 242 draconic months.

Suppose there is a new moon with the Moon near a node, then the Moon is likely to be seen in front of the Sun - a solar eclipse. This close connection between 242 draconic months and 223 synodic months gives us the Saros cycle of a little over 18 years. If there is an eclipse one day, there will probably be one a Saros cycle later. The American total solar eclipse of August 2017 is one Saros later than the August 1999 one that I saw from Alderney.

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