Look in the west, shortly after sunset on Saturday evening – April 13, 2013 – for the waxing crescent moon below a dazzling planet, Jupiter. Tomorrow evening – on Sunday, April 14 – you’ll see a wider lunar crescent closer to Jupiter.
As evening darkens, stars will begin to pop into view. Look in the vicinity of the moon and Jupiter for the star Aldebaran and the Pleiades star cluster, also called the Seven Sisters. If you have binoculars, bring them along with you. There is nothing quite as spectacular as viewing the Pleiades through binoculars. The waxing crescent moon will be moving among these objects for the next few evenings.
The moon was new, or more or less between the Earth and sun, several days ago on April 10. So tonight’s moon has pulled away from the Earth/sun line, and its lighted half is still facing mostly away from us. As the line of sunset moves westward across Earth’s surface tonight, the moon will be inching slowly away from our line-of-sight to the sun. Thus from the west coast of the U.S. this evening, the moon will be a somewhat fatter crescent, somewhat easier to see in a very clear western twilight sky.
We often hear from people who wonder if, when the moon is a crescent, part of the moon is dark because Earth’s shadow is falling on it. It’s true that Earth’s shadow extends approximately 3 times as far as the moon in space. Earth’s shadow does sometimes fall on the moon, and when that happens, we see a lunar eclipse.
But that can only happen at full moon, when the Earth lies in between the sun and moon. You are seeing a shadow on the moon tonight – but it’s the moon’s own shadow. All worlds in the vicinity of our sun have a “day” side and a “night” side, just as Earth does. Earthly night happens when we move into the shadow of our own planet. The moon appears as a crescent tonight because we can now see only a portion of its lighted day or illuminated side.
If you could fly to the moon, and go around to the back side of it, you’d find a nearly full face of the moon shining brightly. So the darkened part of the moon this evening isn’t an eclipse. It’s the moon’s own shadow. But why does the shadowed side of the moon glow dimly, as though illuminated by a fainter light. It’s because, when we see the moon as a crescent, any beings on the moon would see a nearly full Earth in their sky.
Just as the light of a full moon illuminates the earthly landscape at night, so the light of the nearly full Earth illuminates the nighttime lunar landscape. That’s why the darkened portion of a crescent moon sometimes glows. It’s shining with reflected earthlight, called earthshine.
Bottom line: As soon as evening twilight falls on April 13, you will see the waxing crescent moon in the west. The bright object near it is the planet Jupiter. Today’s sky chart shows a few bright stars in the vicinity of the moon and Jupiter on April 13.