Today at 15:44 UT (GMT) the earth achieve that moment of balance that occurs each spring and fall.
When and why the fall equinox happens:
The seasons of the year are caused by the 23.5° tilt of the earth's axis. Because the earth is rotating like a top or gyroscope, it points in a fixed direction continuously -- towards a point in space near the North Star. But the earth is also revolving around the sun. During half of the year, the southern hemisphere is more exposed to the sun than is the northern hemisphere. During the rest of the year, the reverse is true. At noontime in the Northern Hemisphere the sun appears high in the sky during summertime and low in the sky during winter. It is highest at the summer solstice (about June-21) and lowest at the winter solstice (about December-21). The half-way points in the year are called the equinoxes. It is time of the year when the sun rises exactly in the east, travels through the sky for 12 hours, and sets exactly in the west. 1,2 Everywhere on earth experiences close to 12 hours of daylight, and 12 hours of nighttime.
Crystalinks gives a good round-up of Autumnal Equinox observances from around the world, and offers this more general advice.
There are those who believe the equinox solar affect produces a reduction in the magnetic field of the Earth, providing easier access to other dimensions beginning around 24 hours before, and ending around 24 hours after the exact Equinox point.
Doorways or thresholds into the mysteries are more easily accessed during equinoxes and when we consciously engage this timing we are taking advantage of the opportunity to further activate our own experience of these sacred timings and what they have to offer us. This is a great time to be on the land, in a power spot that calls to you, whether that is in a forest, near a body of water, on a mountain, in a sacred site or in your back yard. What is important is to create the time and space that supports a direct experience of the mysteries that are ready to reveal themselves to you.
Also see Edna Spennato's diary for an explanation of a global event planned for today, as well as an awesome image of Kukulkan's (Queztalcoatl's) Pyramid at Chichen Itza.
In Heaven's Mirror, Graham Hancock describes what occurs at this magnificent temple.
On 21 September 1996, the autumn equinox, we were at Chichen Itza in the Yucatan, a Mayan site thought to date to the sixth century AD and to have been continuously developed thereafter until at least the thirteenth century. The central pyramid is dedicated to Kukulkan (Quetzalcoatl). Consisting of nine superimposed platforms, it towers 30 metres high and measures 55.3 metres along each side. Like the Pyramid of the Sun at Teotihuacan its principle axis is deliberately tilted east of north and west of south. Here, however, the angle selected is not designed to signal the days of the sun's zenith passage but instead the spring and autumn equinoxes, when the sun rises precisely due east and the hours of light and darkness are equal.
By about 5.15 in the evening it was clear what was happening. So skillfully was this magnificent pyramid aligned to the trajectory of the setting equinoctial sun that it had been possible for the ancient builders to contrive a pattern of light and shadow on the western side of the northern stairway. Very gradually, as the minutes ticked by and the sun fell lower in the sky, this pattern, which was projected by the north-western corner of the pyramid, gained in shape and substance. By around 5.30 p.m., it had manifested itself fully as a gigantic undulating serpent with seven coils of shadow defined by seven triangles of light. The tail of the serpent reached the top platform of the pyramid, with its body extending down the balustrade all the way to the ground where a huge sculpted serpent's head with gaping jaws completed the illusion of the stairway.
This almost magical epiphany of an ancient deity as a signal of the equinoxes indicates that an advanced geodetic and astronomical science must at one time have been practised at Chichen Itza -- for only a civilization with highly skilled surveyors, astronomers and setting-out engineers could have achieved the razor-sharp alignments necessary to materialize such an image in such a way at precisely the desired moment. It is impossible to say for sure when this science might have first begun to express itself in the architecture of Chichen Itza, because the Temple of Kukulkan -- like the pyramids of Teotihuacan -- surmounts an earlier structure that stood on the same site and had the same orientations.