Astronomical Significance

The first discovered asteroid (1801, by Giuseppe Piazzi, searching for planets which were assumed to exist between Mars and Jupiter based on Bode's Law) and the largest found to date. Ceres is large enough to affect the orbits of Mars and Earth. Ceres is just slightly smaller than Pluto. Ceres is a spherical C-type asteroid with a diameter of 578 miles (930 km) and a stable orbit. Ceres takes just over four and a half years to orbit the Sun, and completes one full rotation on its own axis in just over nine hours.

Astrological Significance

The general overall interpretation of the position of the asteroid Ceres in the horoscope is that it will help to describe how a person takes care of themselves and others. Because of the Goddess Ceres' mythological ties to the role of the single mother and to seasonal changes, the astrological interpretations of Ceres can also involve the menstrual cycle, single-motherhood, and pregnancy. Expansions on this theme include the relationship to food and eating (Ceres is the goddess of agriculture), exercise, and pampering as they relate to personal stress reduction, well-being, and self-esteem.

For example, in my natal horoscope, Ceres is placed within Virgo (keywords: health-conscious, precise, critical, introverted). This indicates that there may be more of an outward focus on the well-being of others than on the self. Inwardly, the Virgoan tendency toward health, perfection and introversion could could indicate the use of food in an obsessive manner to nurture the self.

Aspects to other planets will help fine-tune these interpretations. For example, in my natal horoscope, Ceres is square to Neptune, which can indicate the use of drugs or self-deception to reduce stress. The combination of Ceres in Virgo with this square aspect to Neptune could indicate a detailed and perfectionistic (but disillusioned) focus on health and body to nurture the soul. Food and the relationship to it could be disturbed and health affected (notably, the menstrual cycle, which is commonly affected by eating disorders).
references: astronomical data is from NASA
Addendum, August 26, 2001

Ceres is no longer the largest asteroid found to date. a large body, designated 2001 KX76, was found within the Kuiper Belt by astronomers from the Lowell Observatory, the Massachusetts Institute of Technology, and the Large Binocular Telescope Observatory. This body is potentially larger than Plutos' moon, Charon. Discovered on July 2, 2001, 2001 KX75 is 6.5 billion kilometers (4 billion miles) from Earth. The actual size of the object is still in debate, however the figures in debate are 1,200 kilometers or 1,400 kilometers, each bigger than Ceres.
"This object is intrinsically the brightest Kuiper Belt Object found so far," says Lowell Observatory Director Robert Millis, leader of the survey team. "The exact diameter of 2001 KX76 depends on assumptions that astronomers make about how its brightness relates to its size. Traditional assumptions make it the biggest by a significant amount, while others make it larger by at least 5 percent."
Ceres is now the second largest asteroid known to date. It is, however, still the first asteroid discovered.