Monday, July 18, 2011

Movements of the Moon - Part V

Links to Part I, Part IIPart III, and Part IV of this series.

Far, far, far away from our little solar system are the visible stars of our local neighborhood in the Milky Way galaxy.  As you go outside at night, and look up at the starry sky (to be fully human, you should do this periodically), you'll see random scatterings of stars and your mind will automatically want to arrange them in patterns.  Humans have done this since time immemorial and created what we today know as constellations.  Modern astronomy recognizes 88 constellations (some we can't see here in the Northern Hemisphere), most named from ancient Greek mythology.

All of the planets basically orbit the Sun in the same plane which is called the ecliptic.

When we look up at the night sky, any planets, if they're visible, will always lie along the ecliptic.  Here's what you would have seen in Ulster County, for example, in early May, 2002 just after sunset.

The red line is the ecliptic and the five naked-eye planets were visible in the sky that evening - Mercury, Venus, Mars, Jupiter, and Saturn.  You always look for the planets in the region of the sky where the ecliptic runs; you'd never look for a planet in the constellation of Orion, for example, because the ecliptic doesn't run through Orion.

The part of the sky where the ecliptic occurs was long ago (we're talking ancient cultures of Meospotamia thousands of years ago) divided up into roughly equal regions signified by 12 distinct constellations - Aries, Taurus, Gemini, Cancer, Leo, Virgo, Libra Scorpio, Sagittarius, Capricorn, Aquarius, and Pisces.  The zodiac (modern astronomers recognize these constellations today even though they totally reject their astrological roots.

We call this the zodiac, derived from the Greek ζῳδιακὸς κύκλος (zōdiakos kuklos) or "circle of animals".  During different parts of the year, we see different zodiacal constellations in the night sky because we're in different parts of the Earth's orbit.

OK, what does this have to do with the Moon?  Well the Moon orbits the Earth a few degrees off the ecliptic (we'll talk more about this in another post) so the Moon also appears each night in one of the zodiacal constellations.

As an example, the following figures (click to enlarge) show the position (and phase) of the Moon on July 10, 12, 14, 16, and 18 of 2011 in the southern sky as seen from Ulster County, NY at midnight.  The red line is the ecliptic, the plane in which all the planets orbit the Sun (almost).  Note that the Moon moves from slightly below the ecliptic to slightly above the ecliptic for reasons we'll talk about another day.


Over the course of about a week the Moon moves through the zodiacal constellations of Libra, Scorpius, Sagittarius, Capricornus, and Aquarius.  Over the course of a sidereal month (one orbit of the Moon around the Earth with respect to the distant stars), the Moon will move through all 12 of the zodiacal constellations.  We can't see it in some of them because when the Moon is out during the day (near its New Moon phase), we can't see the stars.  On August 1, 2011, for example, the Moon will be in Leo but rising and setting with the Sun so completely invisible.

Some ancient cultures kept calendar time by the Moon's 27.32 day sidereal movement through the zodiac instead of the by the 29.52 day synodic cycle of phases.  Ancient Chinese astronomers, for example, divided the zodiac into four regions - azure dragon, black tortoise, white tiger, and vermilion bird.  Each of these was then subdivided into 7 "mansions" giving a total of 28 "mansions".  The Moon basically passes through a lunar mansion each day.

Next time, we'll talk about the rising and setting times of the Moon.

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