Moon's shadowplay with Earth sets the stage for a celestial show
The moon this week is marching eastward across the evening sky toward a rendezvous with Earth's shadow, which will engulf the moon and turn it into a dim orange specter for 25 minutes one week from tonight.
This evening the moon looks entirely normal. It shines in the south near its first-quarter phase. First-quarter means, confusingly, that the moon appears half lit. (The name stems from the fact that the moon is a quarter of the way around its monthly orbit of Earth, starting from "new moon" when it was nearly in line with the sun a week ago.)
Bright Mars makes a good reference point for tracking the moon's nightly procession across the sky, as shown here. Mars is still big and bright more than two months after its grand showing last August, and its yellow-orange color also helps to identify it.
You'll notice too that the moon is growing larger (waxing) from night to night, taking on a thicker gibbous shape. This happens because, as the moon moves around Earth, it shows us more and more of its sunlit side. In other words, when you face the moon, the sunlight that illuminates it is coming more and more nearly from behind your back.
Next Saturday the sun will be directly behind your back when you face the moon, so the moon will show its full sunlit face. With the sun behind you, you're also looking in the direction of Earth's shadow. In most months the full moon passes a little above or below the shadow, missing it. But this time the moon will go right through it.
Lunar Eclipse Timetable
Mark Nov. 8 on your calendar to catch this celestial spectacle. Here is a timetable of what to watch for.
5:55 p.m. -- Twilight has faded to darkness, and the full moon is hanging low in the east. For many minutes the moon's left edge has been moving into the pale, outermost fringe of Earth's shadow -- the shadow's "penumbra." So far the change has been too slight to show, but now something is starting to happen. Look for a subtle duskiness stealing onto the moon's left side. It grows stronger and more definite as the minutes tick by.
6:32 p.m. -- The moon's leading edge enters the shadow's dark inner region, the umbra. This is when the partial phase of the eclipse begins. A dark dent begins deforming the moon's round outline. As the minutes pass, the dent widens and moves across the moon's face, swallowing one lunar marking after another until only a thin sunlit sliver remains.
8:06 p.m. -- The last of the moon slips into the umbra; total eclipse begins. But the moon will not disappear. It will probably glow a spooky reddish or ochre color, like a luminous rotten orange in the sky. The ruddy glow you're seeing is golden-red light filtering into Earth's shadow from all the sunrises and sunsets ringing Earth at this moment. At this eclipse the moon skims barely inside the umbra's edge, so watch for one rim of the moon to remain bright. This bright rim swings from the moon's lower-right edge around to the bottom during the 25 minutes of total eclipse.
8:31 p.m. -- The moon's bottom edge emerges into sunlight, and the eclipse's final partial phase begins. For the next 93 minutes the eclipse unwinds in reverse, as more and more of the moon returns to light.
10:04 p.m. -- Partial eclipse ends, leaving just the pale penumbral shading on the moon's upper right side.
10:45 p.m. -- The last, subtle trace of the penumbra exits the moon's right side around now, and the full moon shines high and bright as if nothing had happened.
If the sky is cloudy next Saturday evening, be patient. The next total eclipse of the moon for North America comes on Oct. 27, 2004.
Skyline Telephone News A three-minute recording of the week's astronomy news can be heard by calling 617-497-4168, the "Skyline" service of Sky & Telescope magazine.
Alan M. MacRobert is a senior editor of Sky & Telescope magazine in Cambridge (SkyandTelescope.com). His Star Watch column appears on the Weather page the first Saturday of every month.
