Moon’s Constellations

As the Moon orbits the Earth, we can actually watch it travel through a total of 17 constellations.  13 of these are the signs of the Zodiac, which are:

Aries  Taurus  Gemini  Cancer  Leo
Virgo  Libra  Scorpio Ophiuchus Sagittarius
Capricornus Aquarius Pisces

In the 1920’s astronomers of the International Astronomical Union redrew the uncertain boundaries of the constellations, including those of the Zodiac.  At that time, they made sure that each place in the sky belonged to only one constellation (no gaps and no overlapping). They also insured the Sun’s path would always lie within a Zodiac constellation.  Along the way, they corrected an incorrect popular legacy.  You see, the Sun actually spends almost a month in the large constellation of Ophiuchus, a constellation thousands of years old.  And, the Sun spends only 5 days in the constellation of Scorpio (the scorpion), which really lies fairly far below the Sun’s path.  However, many more people are comfortable with the number 12 than the number 13.  Also, Scorpio actually looks a lot like a scorpion in the sky!   With most constellations you just have to accept that a random pattern of stars actually represents some familiar person or object.  Not so for Scorpio!

Only appeals to tradition by some astronomers kept Scorpio in the “official” Zodiac.  But, astronomers also didn’t sweep Ophiuchus under the rug, and it was also voted into the Zodiac (although it had been there for thousands of years anyway!)  Thus, since the late 1920’s we have OFFICIALLY had 13 signs of the Zodiac!

OK, then, in addition the Moon’s path carries it inside 4 other non-Zodiac constellations.

Just as the Earth orbits the Sun, the Moon orbits the Earth.  The apparent path of the Sun in the sky is known as the Ecliptic. And the constellations through which we see the Sun move are known as the Zodiac.  As the Earth revolves around the Sun, the Sun moves slowly across approximately one definable Zodiac constellation per month.  So, the Ecliptic (Earth’s orbit) defines the constellations of the Zodiac.  Each year, the Sun moves through exactly the same path in the sky, and through the same signs of the Zodiac.

The path of the Moon in the sky has no such specific name.  But, it is a remarkable path in any case.  Firstly, the Moon does not orbit the Earth exactly in the plane of the Earth’s orbit (nor does it orbit directly above the Earth’s Equator).  The Moon’s orbit is tilted by about a 5-degree angle (about 1/18th of a right angle) with respect to the Earth’s orbit. As a result, if you watch the Moon move through the sky during a complete month, you will see it move above the Ecliptic by about 5 degrees of arc and then down below the Ecliptic by about 5 degrees.  (In between it crosses the Ecliptic on the way “up” and then on the way “down”.)  The extra 5 degrees of travel above and below the Ecliptic means the Moon does not necessarily travel at all times in Zodiac constellations!

Also, the Moon’s orbit is not quite as regular as the Earth’s orbit. Because the Moon is much lighter than the Earth, the Moon is pulled and moved around by the Sun’s and Earth’s gravity.  These gravitational tugs have the effect of having the Moon move on slightly different paths each year around the sky (within similar north and south limits).

About once each year, the zodiac constellations where the Moon’s orbit crosses the Ecliptic (going up and going down) change over by one constellation.  Thus, the place in the Moon’s orbit where it is highest and lowest in the sky by 5 “extra” degrees change as well.  This causes the Moon to move through 4 more constellations than just the 13 of the Zodiac.  When astronomers redrew the constellation boundaries in the 1920s, 4 other constellations near the Zodiac now occasionally host the Moon.  These are: Cetus the Whale, Orion the Hunter, Sextans the Sextant, and Corvus the Crow.  During June, 2005, both the Moon and Mars travel through Cetus as they make their ways around the sky.  Over the next few years the Moon will also occasionally be seen in Orion, Sextans, and Corvus as well!