Earth-centered Confusions—Part 2

The third night sky object—the most charming and dramatic one—is the moon. When it’s overhead on a clear night it commands our attention like nothing else. We’re all familiar with the moon’s various phases during the lunar month. Paeans have been written and sung about the full moon and its influence on the moods of people—as well as wolves.

But understanding the moon’s journey—its location in and path across the sky—is far more complicated than either the sun or stars. Not only does the Earth rotate beneath it, but it is the only heavenly body that circles us. The Earth may have been demoted, as far as its place in the cosmos goes, but we retain the moon as our exclusive satellite. So when we follow the moon’s journey across our sky, we’re seeing the composite pattern of its monthly revolution around us and our daily rotation under it.

And the moon’s cyclical periods are neither simple nor match those of the Earth. The lunar month is some 29.5 Earth days long and there are 12.4 lunar months in an Earth year. They’re out of sync with each other, so events do not repeat themselves in a regular manner. As a result, you may get a straightforward track of the moon across the sky during a given night, but its location shifts quite a bit the next night. For example, tonight at 9:00 it may be directly overhead, but each subsequent night it shifts a substantial amount eastward. In a week it’ll be near the eastern horizon at 9:00, and then completely gone from the night sky the following week.

But there’s more. The moon’s orbital plane is at a slight angle to the ecliptic and it wobbles up and down a bit. That waver completes a cycle every 18.61 years. One needs a thorough knowledge of these wobbles in order to predict when lunar and solar eclipses will occur. There’s still more: The moon’s orbit about the Earth is not a perfect circle; it is egg shaped. So it’s closer to us at times (at perigee, when it’s also a little larger) and farther at other times (at apogee, when it’s a little smaller).

It makes my mind swoon to try to comprehend all these variations. I am humbled by the ancients who had the time and inclination to take note of all these lunar complexities and accurately predict their periodic occurrences in such exquisite observatories as Stonehenge and similar monuments.

The last and most complex of all the heavenly objects are the planets. Their behaviors are truly bizarre. If you mentally elevate yourself to a place high above the plane of the solar system, the orbits of the planets around the sun can be seen to be simple near-circles. From an Earth-centered perspective, however, the planets (Greek word for wanderer) trace weird paths across our sky. Although they may behave themselves nightly—arcing across the sky like the stars—their positions relative to those background stars wander in an almost inebriated manner, over a period of weeks or months.

Jupiter may be seen to slowly migrate westward, over a period of weeks, and then abruptly swing back to the east. It may next head west again and then dive below the western horizon, disappearing from the night sky, to reappear in the pre-dawn sky, now towards the east! In fact, these pop-up appearances of the planets—now at night in the west, now at dawn in the east—were so unaccountable that the ancients did not even recognize their reappearance as the same planet.

Only with a mixture of modern astronomy (handily found on web sites) and a little prehistoric sky monitoring can I follow the planets in their mysterious travels across the sky.

I am doing my best to comprehend the paths taken by heavenly bodies across my sky. It’s almost beyond me. I try to soak it all up from a combination of direct observation and reading books. I could do this for another few decades and still not quite approach the proficiency of my forebears. I often wish I had one of them by my side, passing on her clan’s wisdom.