Sommers-Bausch Observatory - University of Colorado


The Harvest Moon - Not!


Original article by Keith Gleason appeared in The Old Lyons Recorder, Lyons, Colorado, on September 19th,1996.


The most famous of the full moons is the Harvest Moon, the one occurring nearest the time of September's autumnal equinox. Contrary to popular belief, the Harvest Moon isn't necessarily bigger or brighter than other full moons - but it comes along at the time when farmers are out in their fields harvesting crops, and can make productive use of the extended evening twilight that the full moon provides.

Not so this year! On the evening of Thursday, September 26th, the Harvest Moon will provide little assistance to farmers - but instead, will put on a celestial show for the remainder of us: our last total lunar eclipse of the century.

That night the Moon will rise above the eastern horizon at 6:39 p.m. with the beginnings of the eclipse already in progress. Looking carefully, you'll be able to note that the lower-left portion of the Moon will be somewhat darker than the upper-right, because some sunlight that would normally reach the lunar surface is being obscured by the Earth. Over the next half-hour, the phenomenon becomes increasingly more obvious as the Moon's orbit carries it eastward and deeper into the Earth's shadow - while simultaneously, the Earth's rotation makes both the Moon and the shadow appear to rise higher into the sky towards the west.

At 7:15 p.m., the lower-left edge of the Moon will enter the Earth's "umbra" - the region opposite the Sun where all direct sunlight is blocked by the Earth. For the first time, the curved edge of the Earth's shadow will become visible, falling across the face of the Moon. This curved shadow was used by Aristotle as the first observational proof that the Earth was "round" (actually, a sphere) - 1800 years before Columbus! Shortly thereafter, Aristarchus correctly deduced that the Earth was about four times larger in diameter than the Moon, simply by comparing the gentle arc of its shadow to the size of the lunar disk.

For the next 70 minutes, as more and more of the lunar surface slips into the Earth's umbral shadow, you'll be able to see stars nearby that would normally be drowned out by the glare of the Moon - and you'll be able to watch the Moon move slowly eastward relative to the stars and to Saturn, the bright "star" to the lower-right of the Moon about 4 lunar diameters away.

At 8:25 p.m., the total eclipse officially begins, when all portions of the Moon have been plunged into darkness, completely hidden from direct sunlight by the Earth. But not complete darkness! Even though the light will be faint (typically only 1/10,000 the brightness of the uneclipsed moon), the lunar surface will likely remain bright enough to be seen, glowing a dull coppery red. The source of the illumination comes from sunlight passing through the thin layer of atmosphere around the rim of the Earth's disk. The light is both filtered and bent, as if passed through a lens, so that it shines onto the darkened Moon. Because blue light from the Sun has been scattered in other directions by the atmosphere (that's why our daytime sky appears blue!), only the reddest colors of sunlight makes it past the Earth (that's why sunsets are red!) to reach the lunar surface and give it its peculiar coloration.

Total eclipse lasts until 9:29, when the southeast edge of the Moon peeps out of the shadow and receives direct sunlight again - then the entire scenario plays out in reverse, with all portions of the Moon receiving at least some sunlight at 10:36. By 11:35 p.m., the Moon will have completely cleared the shadow, and the eclipse will be over.

The eclipse can be easily viewed with the naked eye and with binoculars from your own backyard. However, for those wishing to view the event through telescopes, Sommers-Bausch Observatory and Fiske Planetarium on the University of Colorado campus in Boulder will be holding a special public Open House the evening of September 26th, from 7 p.m. until 10:30, weather permitting. Call Fiske at 492-5002 or the Observatory at 492-6732 if you need further information or directions.

More information about the eclipse and other celestial phenomena can be found at the Observatory's homepage at the network address "http://lyra.colorado.edu/sbo/".