Semicolon in the Sky
My 10-and 11-year old grandchildren journeyed outdoors toward evening last Saturday with their uncle to search for milkweed caterpillars, the larvae of monarch butterflies. Milkweed caterpillar collection has been a summer staple of family nature study in our family for three generations. If captured, each caterpillar greedily devours milkweeds in a jar, soon suspends itself to become a beautiful, emerald green, gold bejeweled chrysalis, and finally, after a few days of quiet development, emerges as a fully developed adult monarch.
Twilight was descending on the neighborhood as the threesome returned from their country caterpillar pilgrimage. A short time before, I noted an unusual conjunction of astronomical bodies in the western sky. The thin waxing crescent moon was positioned just below Planet Venus in the fading twilight. The moon’s diameter is about 0.5 degrees. On that night, bright Venus was only one degree distant from the moon. These numbers make for a stunning close meeting, a “conjunction” of sky objects. The moon is second only to the sun in brightness. Venus ranks number three.
When my grandson first saw the configuration, he volunteered that “it looks like a semicolon.” Indeed, it did. On infrequent instances such unusual positioning of celestial bodies offers rare beauty and an occasion for wonder. I could not resist donning my science teacher’s hat: “Tomorrow night the waxing crescent moon will appear about 12º east, leaving Venus by itself,” I exclaimed. I pointed to the approximate locations of the moon for the next few nights at the same clock time—a visual reminder of the moon’s revolution around the earth, I hastened to explain.
In just over 29 days, our lunar companion would return to the same spot in the sky at the same time by the clock. But during early August Venus dips completely below the horizon so another conjunction would not occur. Our moon, meanwhile, would favor us with its monthly phases and stages: waxing crescent, first quarter, waxing gibbous, full moon, waning gibbous, last quarter, and waning crescent. This sequence of events occurs every 29+ days. Earth residents may count on it!
Careful observers notice movements of the moon and Venus as described above. The moon’s speed of revolution, almost 2300 mph, is noticeable because it is so close to Earth. Venus’ speed of 78000 mph is barely noticeable because it is much more distant. When we compare size, speed, distance, and appearance of astronomical bodies in our Solar System, we have a formula for enjoyment of the world as gifted to us by the Creator.
The semicolon in the twilight sky of July 18, 2015 was a “still picture” of startling beauty. Momentary observations did not reveal changes in position or appearance of the sky objects. But with a little patience, in a few minutes we perceived the twilight fading and the descent of the semicolon below the horizon. These observations resulted from the rotation of our planet—once each 24 hours. Many years ago I challenged my students to witness a sunset patiently for a few minutes in an effort to observe movements not otherwise visible. Our classroom discussions of their experience revealed some students claimed to “feel” the Earth’s rotation. Of course, they did not “feel” the rotation except in a visual sense.
The search for Monarch larvae yielded two tiny caterpillars and one tiny Monarch egg, discovered by my granddaughter. Combined with the aforementioned astronomy lesson in fading twilight, the children experienced two diverse lessons without cost, but full of value. Three generations shared the joy of wholesome natural discovery that evening.