Where is the interface between the cosmos and planet Earth? Everywhere. Watching birds fly across the shiny coin of the Harvest Moon last night was but one example. Every September anyone with a telescope magnifying 30x and up who happens to look at the full moon can’t help but notice the occasional silhouettes of migrating birds fluttering across the moon’s face.
Many birds migrate at night both to conserve energy and avoid predators. Identifying the many warblers, blackbirds, sparrows, vireos, orioles and other species that fly across the moon while we sleep may be next to impossible, but seeing them is easy. Just for fun, I counted the birds in the five-minute interval between 10:57 and 11:02 p.m. last night while looking at the moon at 76x through my 10-inch telescope. The total came to 16, which multiplied by 12 yielded an hourly count of 192 birds.
As you might suspect, most of those birds crossed the moon from north to south (about two-thirds) with the other third traveling either east to west or northeast to southwest. Only one little silhouette flapped back up north in the ‘wrong’ direction. Who knows. Maybe it veered off course to pursue a nighttime snack.
According to the Chipper Woods Bird Observatory, located in Indianapolis, most nighttime migrators begin their flight right after sunset and and continue until about 2 a.m. Peak time is between 11 p.m. and and 1 a.m. Bird typically migrate at altitudes ranging from 1,500 to 5,000 feet, but on some nights, altitudes may range from 6,000 and 9,000 feet. I could tell the high ones from the low ones by their size and sharpness. Nearby birds flew by out of focus, while distant ones were very sharply defined and took longer to cross the moon.
Watching birds pass across the moon is a very pleasant activity and reminiscent of meteor shower watching. At first you see nothing, then blip! a bird (meteor) flies by. You wait another minute and then suddenly two more appear in tandem. Both activities give you that delicious sense of anticipation of what the next moment might hold.
The best time to watch the nighttime avian exodus is around full moon, when the big, round disk offers an ideal spotlight on the birds’ behavior. It’s a fine sight to see one of Earth’s creatures streak across an alien landscape, and another instance of how a distant celestial body “touches” Earth in unexpected ways. If you’d like to learn more about birdwatching by moonlight, check out the Chipper Woods webpage.