A Dazzling Sight at Dawn


Last updated 2/15/2024 at 10:52am

Though these all look about the same size in the sky, they are actually very different distances from Earth.

Even as a child, I enjoyed heading outdoors after sunset from time to time to watch as the thin crescent moon paired up with the brilliant planet Venus in the waning light of dusk. It all seemed so magical to me.

But Venus is nowhere to be found in our evening sky right now, so where must we look to see this beautiful scene? In the early morning sky, of course.

On Saturday, Dec. 9, head outdoors about 45 minutes before sunrise to gaze into the eastern sky. Sure, it's the same moon and Venus, and the same dark blue sky, but something is ... well, different. The two just seem to be much brighter and more dazzling than when we see them at dusk.

The difference is that it's dawn, not dusk. Why should that matter? First of all, during the nighttime hours, much of the dust we've stirred up during daylight has settled, so the dawn sky often appears cleaner and brighter. In addition, since we've been asleep most of the night, our eyes are already adjusted to the darkness so when we step outdoors after awakening, the sky and everything in it just seem to glow more brilliantly. And that includes the moon and Venus.

As you admire the pair, be sure to check it out with binoculars, since few celestial sights appear more three-dimensional. Part of the moon's beauty that morning comes from seeing the eerie glow of the entire moon's disk along with the bright crescent. For centuries, this "old moon in the new moon's arms," as it was called, puzzled everyone until the 15th-century artist and inventor Leonardo da Vinci explained that it is caused by sunlight reflecting off of the Earth and shining back onto the moon's dark side.

Today we know this as "Earthshine," and you can imagine it like this: If you were standing on the moon's night side at this time looking into your sky, you'd see the nearly full Earth, significantly larger and brighter than the full moon appears to us. And, just as a bright moon illuminates the dark night on Earth, a bright Earth would illuminate even more the darkness of the moon. Though the moon and Venus appear close together that morning, this is, of course, an optical illusion caused by them lying nearly in our same line of sight. On that morning, the moon lies about 240,700 miles from us while Venus lies 400 times farther – about 96.2 million miles. To their upper right, the star Spica is even farther – about 260 light-years from Venus, or about 1,500 trillion miles away.

To put it another way, the light we see from the moon on that morning has taken about 1.3 seconds to travel from the lunar surface to our eyes; that from Venus takes about 8.6 minutes to get here. The light from Spica, on the other hand, began its journey toward our eyes years before the American Revolution!

I hope you enjoy the sight, and getting up early, because next week I'm going to suggest you do it again ... this time to watch the amazing Geminid meteor shower!

Visit Dennis Mammana at dennismammana.com.