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If you step outside on the evening of Wednesday, Oct. 18, and look up, you might notice that the moon looks a little funny.
That's because it will be in the midst of a penumbral lunar eclipse. These are subtle and occur when the full moon passes through the Earth's partial shadow, or penumbra. During this type of eclipse the moon will darken slightly but not as completely and colorfully as it would during a total lunar eclipse.
As Earth orbits the sun, and the moon orbits Earth, sometimes the three bodies align in such a way that causes an eclipse. Solar eclipses, the rarest and arguably the most stunning of these, occur when the moon is positioned between Earth and the sun, causing the moon to blot out the sun.
The orbit of the moon, the smallest of the three celestial bodies, is tipped about five degrees relative to Earth's orbit around the sun. A full moon usually passes above or below Earth's shadow and no eclipse happens. But two to four times a year, that five degrees of difference comes into play and the moon passes through some portion of the Earth's shadow.
Partial lunar eclipses occur when just a portion of the moon passes through Earth's umbral shadow. Total lunar eclipses play out when Earth glides directly between the sun and the full moon, placing all of the full moon completely within Earth's umbral (total) shadow. Anyone standing on the moon would see the Earth completely eclipse the sun.
Although the Earth is blocking sunlight from reaching the moon during a total lunar eclipse, Earth is much smaller than the sun, so indirect sunlight still manages to get around Earth and illuminate the moon. However, this sunlight is filtered through the Earth's atmosphere, eliminating most of the blue lightwaves and leaving deep red or orange light to land on the moon's surface. The next total lunar eclipse will be on April 15, 2014, and will be visible from the Americas.
This month's more subtle penumbral eclipse will be visible throughout most of the world, with the exception of Australia, Alaska and extreme eastern Siberia. Eastern Canada will see the entire event while the rest of Canada and the U.S. will encounter moonrise (around 6 p.m. that day in New London) with the eclipse already in progress.
Next month: Comet ISON
Oct. 8: You might see some stray meteors from the Draconids meteor shower, a minor shower that produces about 10 meteors per hour that peaked on the night of Oct. 7. It is produced by dust grains left behind by comet 21P Giacobini-Zinner, which was first discovered in 1900. Meteors will radiate from the constellation Draco, but can appear anywhere in the sky.
Oct. 9: Mercury at greatest eastern elongation. The planet Mercury will be at its farthest angle from the sun, known as greatest elongation. It will be at its highest point in the night sky after sunset. This is the best time to observe Mercury since it hugs the sun and doesn't usually climb very high above the horizon.
Oct. 12: International Observe the Moon Night, an annual event dedicated to encouraging people to look up and take notice of our nearest celestial neighbor.
Oct. 18: Full moon. This will be the smallest full moon of the year because it will be near apogee, its farthest point from the Earth. Which also brings us to...
Oct. 18: Penumbral lunar eclipse.
Oct. 21, 22: Orionids meteor shower, an average shower producing up to 20 meteors per hour produced by dust grains left behind by none other than Halley's Comet. The shower runs annually from Oct. 2 to Nov. 7, peaking this year on the night of Oct. 21 and the morning of Oct. 22. This year's waning gibbous moon will block some of the meteors, but the Orionids tend to be fairly bright. Best viewing will be from a dark location after midnight. Meteors will radiate from the constellation Orion, but can appear anywhere in the sky.