Planets, according to current astrologers, can symbolize underlying unconscious impulses or desires, or energy flow regulators indicating facets of experience. They manifest themselves in various ways in the twelve signs of the zodiac and the twelve houses. The planet itself may have an effect on the situation it is connected with; for example, if Jupiter is located in the house it rules, then expansion and abundance are likely properties of this relationship. The connection between planets and signs was first noted by Philo of Alexandria, who lived in Egypt during the first century AD. He believed that each planet had an influence on certain signs of the zodiac. Today, this connection is widely accepted among astronomers. Philo's ideas were later developed by Placidus, a Roman scholar born about 250 AD. Placidus proposed a system where every zodiac sign was associated with a particular planet. This system remained popular until about 1400 when it was replaced by another system called the "modern" system. The modern system was developed by Nicolas Copernicus who was born in 1473. He proposed that the Earth was not the center of the universe but instead revolved around the Sun. His idea was rejected for several centuries after his death but is now accepted as true. Under the modern system, which is still used today, the sign that Pluto occupies determines what type of behavior is shown by the Planet it represents.