From the December 2004 issue

Bob Berman’s strange universe: Moon orbit oddities

December 2004: We who love the night sky often regard the Moon with mixed feelings.
By | Published: December 1, 2004
Bob Berman
We who love the night sky often regard the Moon with mixed feelings. When full, it ruins almost everything else. Galaxies, nebulae — gone, washed out. The sky actually turns a deep blue during the three nights surrounding the Full Moon, if you’re not under the amber canopy of urban glow. Yet even after committing felonies against us, the Moon doesn’t provoke loathing. It gets a pass because it is a celestial body and does offer more juicy telescopic detail than anything else in the universe. It’s love-hate.

The situation was different a mere century ago, when civilization was more agrarian and had few streetlights. Back then, a bright Moon permitted safe nocturnal travel while a moonless night was just plain dangerous. The Moon mattered. And it still was mysterious.

The best telescopic resolution couldn’t determine what lurked among those craters and mountains. Nineteenth-century astronomers knew that, by coincidence, 1″ of resolution revealed 1 mile of lunar real estate. Even now, a good telescope and a steady night with 1″ of seeing let you see lunar detail as small as a mile across.

… our Moon ignores Earth’s tilt and circles us in the same ecliptic plane in which planets orbit the Sun.

Throughout the ages, one aspect of the Moon could be explored by anyone with eyes: its motion. This single natural body that revolves around our world remained fascinating for that reason alone.

You’d think that nearly everyone, in the past and present, would be aware of the Moon’s orbit. Yet in today’s educated society, not one person in 100 knows even the basics about lunar activity. Of course, I’m bringing the subject up now because the Moon reaches a sort of orbital milestone this month.

So, let’s start with those fundamentals. First, the Moon doesn’t orbit our planet’s center, but a spot just a thousand miles beneath Earth’s surface. Where is that magic place? When the Moon is almost overhead, as it will be at midnight December 25/26, the exact spot it orbits lies a thousand miles beneath your feet.

Next, the Moon’s orbit changes shape all the time. Around New and Full Moon, it becomes stretched out and eccentric. At other times of the month, it’s more circular. Moreover, its oval shape keeps changing direction, advancing completely around Earth every 9 years. As a result, the Moon never returns to its starting spot or actually completes its orbit. Its real motion is similar to a rosette, a spirograph pattern, as if tracing out the petals of a daisy.

The Moon is the only known body that moves through space its own diameter each hour. With a 2,160 mile (3,476 kilometers) width and an average speed of 2,287 mph (3,680 km/h), it’s easy for an observer to predict where it will be throughout its nightly journey.

This size-speed coincidence also explains why both lunar and solar eclipses take about an hour from start to totality.

The Full Moon is 450,000 times dimmer than the Sun. It grows brighter, however, when it’s higher because it’s then closer to you by a few percent, and also because its light then passes through up to 13 times less air. It’s brighter when the air is dry also. All these conditions come together late this month.

I’ve saved the best for last. The Moon does not orbit around Earth’s equator. Unlike how every other major moon circles every other planet, our Moon ignores Earth’s tilt and circles us in the same ecliptic plane in which planets orbit the Sun. Doing so, its path slowly wobbles like a dropped dish rattling on the floor, taking 18.61 years to perform a complete wobble. The consequences are dramatic.

This wobble causes the Moon’s 5° tilt relative to the Earth-Sun plane to change alignment during that 18.61-year period. Result: Some years, its monthly path around the sky carries it under where the zodiac is highest and above where the zodiac’s the lowest. Those years, like during the mid-1990s, the Moon is in middling positions and Full Moons never get as high as early summer Suns.

Now, however, we’ve entered the 3-year period when winter Full Moons (always the highest of the year) get loftier than usual. You’ll see this December 25 — the highest Full Moon of the year. (If it’s cloudy, look the next night.) Check it out around midnight. It’s the highest Moon since the 1980s.

Will anyone else notice? Not likely. Despite our expensive plans to return there someday, the truth is few bother with the Moon. Oh well. Love-hate.