Supermoons on the rise

I'm sure most of you witnessed the brighter-than-average moon in mid-October. Many of you also heard talk of the supermoon phenomenon. I assure you that it isn't a contrivance of DC Comics; it's for real.

Let's start with your basic full moon. The moon orbits the earth in nearly the same plane as the earth's orbit around the sun. Thus, once a month the moon and sun are on opposite sides of the earth. When the moon is on the far side of the earth, the side of the moon facing us will be fully illuminated. If the alignment is close to perfect, the moon passes directly through the earth's shadow and we have a lunar eclipse.

The straight alignment of the sun, moon and earth is called a syzygy. It's not only responsible for our highest tides, it's worth 25 Scrabble points! And it must be noted that exhaustive studies have debunked claims that full moons cause up-ticks in crime, hospital admissions, "lunacy," or other strange behavior.

The moon's orbit (like all orbits) isn't perfectly circular; it's an ellipse. An orbit's non-circularity is called orbital eccentricity. The moon's orbital eccentricity is 0.0549, which is fairly high as Solar System bodies go. This means that the moon is about 50,000 km (30,000 miles) closer to us at perigee than at apogee. This makes a perigee moon 14% larger and 30% brighter than an apogee moon.

It is these perigee-syzygy moons that we call supermoons. The term originated in astrology (not astronomy). Depending on who you ask, it can include full (or new) moons that are within 10% or 50% of perigee.

The upcoming full moon on November 14 is quite a supermoon indeed. Full moon will occur within two hours of perigee. This will be the closest full moon of the 21st century to date. (The last comparable supermoon was in 1948, and the next will be in 2034.) The full moon will occur during morning twilight at 5:52, so the hour before will be the brightest.

Another important phenomenon makes the moon noticeably brighter during a full moon: Retroreflectivity. Most of us have seen the demonstration of a small curved mirror concentrating the sun's rays on a piece of paper and setting it on fire. You have a very similar effect when you shine sunlight into a lunar crater and bounce it to earth near a full moon. The closer to the actual time of the full moon, the greater the effect. That's why it may be possible to read by the light of a full moon, but not the day before or after.

Retroreflectivity was also an important science objective of Project Apollo. The astronauts placed arrays of retroreflectors aimed at earth. We then pointed lasers at them from the earth. By timing how long it took the beam to return, we could precisely determine how far away the retroreflectors – and thus the moon – was. These Lunar Laser Ranging Experiments have helped us precisely establish the distance to the moon (385,000.6 km on average), and the rate at which it's moving away from us (38 mm/yr). (It's also a powerful tool in the fight against those who continue to insist that we never really went to the moon!)

There is another noteworthy phenomenon. Whenever the moon is seen against the horizon, buildings, or other foreground objects, it looks larger than it really is. This is called horizontal illusion. Although this illusion is universal, it's not well understood.

We've all lived our entire lives watching the moon rise, set, and change phases. It lights our way on midnight walks and generates tales of flight and fright. Yet, few of us have ever reflected on how special our moon really is.

Luna is the largest satellite orbiting a terrestrial planet (i.e., Mercury, Venus, Mars, and the earth). No other planet in the Solar System has such a large moon (compared to its planet). Indeed, reasonable arguments have been made for calling our moon a planet in its own right, and ours a two-planet-system.

The moon's affect on human history is incalculable. Who knows -- perhaps we would never have ventured out of our cozy caves at night had there never been any moonlight. Without the moon, everything in the sky would have been nothing more than pinpoints. From here all of the other planets are just dots; but the moon is clearly a world. We see the shadow of our own world projected on the moon during a lunar eclipse, and there would no solar eclipses without the moon.

I believe that the moon has stimulated so much thinking that we couldn't possibly be where we are today without it. After all, President Kennedy could hardly have challenged us to go to Mars by 1970!

The moon has always compelled us to look up at the night sky in wonder and curiosity. It not only lit our way along the dark paths of the earth, it fired our imagination as we began humanity's journey to the stars.

Almanac

Venus is now hard to miss in the western evening sky. Saturn is still visible nearby, but will be lost in the twilight by November's end. Look for Mercury rising out of the twilight in Saturn's vicinity as the month progresses.

Jupiter has emerged from the morning twilight and will be easy to spot in the east by month's end.

Sunrise/Sunset: 7:15 a.m./5:59 p.m. PDT (November 1)

Time Change: Daylight Savings Time ends at 2 a.m. on Sunday, November 6.