Tonight is a "special" one, astronomically speaking, because for the first time in nearly 20 years the Full Moon is on the same night as the Autumnal Equinox. In fact, these two moments occur a mere six hours apart from each other. NASA's Dr. Tony Phillips writes, "There hasn't been a comparable coincidence since Sept 23, 1991, when the difference was about 10 hours, and it won't happen again until the year 2029." As a result of these two big events occurring on the same night, tonight's full moon has been dubbed the "Super Harvest Moon." The full moon closest to the Autumnal Equinox is always nicknamed the "Harvest Moon" because farmers rely upon clear skies and the light of the full moon to work past sunset into the night to bring in crops (at least before or without artificial lighting). But astronomers have added the "Super" onto it this year because it is the "ideal" Harvest Moon, occurring at the start of Fall! The Full Moon rises tonight in the east around sunset, and will be overhead at the Autumnal Equinox at 11:09 PM. While you are observing that full moon overnight, you will see a bright white light near it, and that's the planet Jupiter! It's very close to Earth this month.
(A Moon-Jupiter conjunction on Aug. 26, 2010. Credit: Tom Cocchiaro.)
The Autumnal Equinox, or the official start of Fall, is tonight at 11:09 EDT. This is the moment when the Earth's axial tilt is aligned with the Sun, and from our perspective on Earth, the Sun's most direct rays are over the Equator. For the Northern Hemisphere our days become shorter than the nights. The reason why we have seasons is because of that axial tilt I just mentioned. Earth's rotation axis is not perpendicular to its orbital plane, as seen in this diagram:
So as the Earth orbits the Sun in a year, different parts of the Earth will be more "exposed" to the Sun than others, leading to seasons in the Northern and Southern Hemispheres. The tilt of the Earth never changes, but from our perspective on Earth as we move around the Sun, the Sun appears to be higher or lower in our sky through the year...seasons!