“O, swear not by the moon, the fickle moon, the inconstant moon, that monthly changes in her circle orb.” — William Shakespeare, “Romeo and Juliet”

A full moon on Halloween is rare, but when it happens it is always a “blue moon.”

A blue moon has nothing to do with color, but refers to the second full moon in the same calendar month.

A blue moon happens every 2-1/2 years or so, but a Halloween blue moon happens only three or four times each century.

We tend to take the moon for granted, but even when we don’t notice, it is always there repeating its succession of phases every 29-1/2 days.

If the moon were not there, it is unlikely that any of us would be here to notice.

We are part of a unique dual-planet, earth-moon system. The moon is much larger compared with Earth than any other satellite in the solar system compared with its planet, the result of an improbable accident early in the history of the solar system.

The planets were formed by countless impacts of particles of dust that coalesced into progressively larger bodies. As the planets formed, the amount of interplanetary debris decreased as each planet’s gravity swept out debris from the vicinity of its orbit.

During the early bombardment phase, a protoplanet the size of Mars collided with proto-Earth with just the right velocity and at just the right angle to form the earth-moon system.

The probability of such a collision with just the right speed, size and angle is close to zero; it is highly unlikely to have happened elsewhere to a different planet.

When the moon formed it was only about 15,000 miles from Earth. Since tidal forces vary with the third power of distance, tides would have been 4,000 times greater than today, producing an extreme drag on both Earth and moon and slowing the rotation of both bodies.

At the time of its formation, Earth was spinning four times faster than today.

Without the moon’s drag, the six-hour days would have created a different set of environmental pressures that would have altered the evolutionary path that has led to the modern planetary biota that includes us.

The evolution of complex life forms requires long periods of stable conditions. The stability that the moon provides has changed the way life evolved on Earth compared with a planet where drastic climatic change would allow only small, robust organisms to survive.

Since wind velocities and global circulation patterns are highly dependent on rotational speeds, global climates would also be different if there were no moon.

The moon’s gravity stabilizes the tilt of Earth’s axis and keeps it close to 23.5 degrees. Without this stabilizing influence the axis would wobble, and the tilt would vary from 0 to 90 degrees, creating extreme variations in the global climate far beyond what has transpired in the eons of Earth’s history.

Even if you missed the blue moon on Halloween last week, there will be others in 2039, 2058, 2077 and 2096.

It might not be blue, but the moon will still be there tonight, rising late in the evening. If you get a chance, give it a glance and a thanks.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.