Astro Bob: 'Old' moon hangs by fingernail
See the super-thin crescent pair up with Mercury at dawn Monday morning, Oct. 24.
Before the moon rose Sunday morning, Oct. 23, I hunted for a newly discovered, faint supernova and a modestly-bright comet, both located in Ursa Major the Great Bear. Smeary cirrus clouds moved off overnight, leaving a crisp, clean sky. All of winter's finest jewelry was on display neatly arranged in an enormous necklace called the Winter Hexagon.
Off to the east I noticed a flicker of light through the tangle of branches. Ah, the moon! Thin as a twig and balancing on its bright edge, the sickle slowly climbed the sky as morning twilight bloomed. Usually, a crescent's bright, sunlit edge catches your attention first. This time the earth-lit portion stole the show. It was so bright and distinct.
Earth's clouds and surface reflect sunlight into space, some of which the moon reflects back to our eyes. Since the light is twice-reflected it's much weaker than the sunshine part, the reason earthshine looks gray and mysterious compared to the crescent. It's best visible shortly before and after the new moon. At these times, the Earth appears full (Full Earth) from the moon and reflects the maximum amount of light.
Like me, the moon was getting old — it had aged all of 27.5 days. While that sounds young, it's ancient by lunar standards. The oldest the moon can get is 29.5 days. Then it's reborn as a New Moon and begins the aging cycle all over again. On Oct. 26 at dusk, it will be a tender waxing crescent just 1.5 days old, practically a newborn.
Some people believe in reincarnation and multiple lives. I wish I was one of them. I'm pretty sure that my odometer will reach a finite number and not roll over. But at least the lunar cycle offers a hint of what eternity is about.
On Monday morning, Oct. 24, you can spot an even older moon. Aged 28.5 days, it will be just a day or less before new depending on your location. Look low above the eastern horizon about 40 minutes before sunrise for a crescent fragile enough to crumble in the breeze. Mercury will shine directly below the waning moon, giving us one last, easy chance to to spot the innermost planet before it disappears in the sun's glare.
Use binoculars. The two will be a degree or less apart and 3°-4° above the horizon. Once you've seen them in binos, try to spot the pair without optical aid. This sunrise calculator will help plan your outing. Once you know when the sun comes up, back up an hour and arrive at your observing site at that time.
The farther west you live, the closer the moon and Mercury approach one another. From Denver they'll be just 1/2° or one moon-diameter apart, while from coastal cities like San Francisco and Los Angeles, the crescent occults (covers) the planet shortly after the duo rises.
To better know exactly where to look and when, especially if you hope to see the occultation, download the free Star Chart app or another stargazing app of your choice. To find it, Google "Star Map app Android" or "Star Chart app iPhone." I hope your forecast is better than mine. I'm pretty sure we'll be clouded out. Let us know what you see — clear skies!