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The Moon’s Hidden Face

Its far side is always dark to us

The dark hours at week’s end are still brightened by the glow of the waning Hunter’s Moon, which rises mid-evening and dominates the night sky until daybreak. On clear days this week, you may even see the moon in the west after sunrise.
    Over the weekend, the moon travels with the constellation Taurus. Friday it is 10 degrees to the right of the Pleiades star cluster while the bull’s red eye Aldebaran is a little farther below the moon. The brightests stars of the Pleiades form a small but distinct dipper, which makes up the bull’s back. Saturday the moon is much closer to Aldebaran and the Hyades star cluster, which makes up the bull’s face. If the moonlight is too much to discern these stars, wait a day or two until the waning moon has shifted to the west.
    Just like here on earth, half the moon is always illuminated by the sun and the other half facing away from the sun. But as the angle between the sun, earth and moon changes, so does the portion of the moon’s illuminated face that we can see. With the moon waning, the angle is closing, obscuring more of the lunar surface behind earth’s shadow. This darkened section of the moon still faces earth and should not be confused with the so-called dark side of the moon. Better to think of that as the far side of the moon, which faces away from earth. The far side is still bathed in sunlight — we are just never in a position to witness it.
    The moon rotates on its own axis, with one side facing the sun for about two weeks and then facing away from the sun the next two weeks. Over billions of years, earth’s stronger gravitational pull has slowed the moon’s rotation to the point that it spins in synch with its pace around the earth. As a result, one side of the moon faces earth only during new phase, when it is between us and the sun, obscured by the light of day. So we never see the far side of the moon.
    Mars and Saturn pop into view in the wake of the setting sun. Saturn is sinking fast and is visible for less than an hour. Mars is well to the east of Saturn but not quite as bright. Don’t confuse it for the similarly hued star Antares, the heart of the Scorpion wriggling below.
    Jupiter rises around 2am and is high overhead in the east as morning approaches. Over the next month this gaseous giant climbs higher and grows brighter in our pre-dawn sky.