The earth, together with eight other known planets, their moons, and the sun, make up the solar system. Like the other planets of the solar system, the earth follows an elliptical path of revolution about the sun called an orbit; it takes about 365 days, or one year, for the earth to complete one circuit around the sun. The earth's axis of rotation is inclined at an angle of 23 1/2 degree's to the plane of the earth's orbit, called the plane of the ecliptic. As will be explained more fully later, this inclination is responsible for the different seasons experienced on the earth's surface, as well as the changing length of the solar day.
Because of the tilt of the earth's axis with respect to the ecliptic, it seems to an observer on earth as though the sun is continually shifting its position with respect to the earth's equator as the earth moves in its orbit around the sun. When the sun is at it's northern most point with respect to the earth, it is called the summer solstice which occures about June 21 of each year 12 days before aphelion, the orbital point farthest from the sun. Because the angle of the sun's rays is large at this time in the northern hemisphere and relatively small in the southern hemisphere, the northern hemisphere experiences summer and the southern hemisphere experiences winter at this point in the earth's orbit.
As the earth continues in its orbit the sun seems to move lower, until it appears to cross over the equator from north to south latitudes. This time is referred to as the autumnal equinox, and occurs about September 22. When the sun is at its southern limit 23 1/2 degree's south latitude which occurs about December 21 is called the winter solstice, and precedes perihelion, the earth's closest approach to the sun, by about 12 days. The northern hemisphere is now having winter, and in the southern hemisphere it's summer. When the sun crosses the equator from south to north latitudes, about March 21. This is called the vernal equinox, which is important in celestial navigation.
As the early astronomers observed the sun in the course of the earth's revolution, it seemed to them as though the sun was a ball moving across the heavenly sphere, transiting 12 different constellations in the course of a year. These 12 constellations comprise the Zodiac. The zodiac has no navigational meaning today, but it is important today in the practice of astrology.
When they observed the sun at the time of the vernal equinox, it seemed to the ancients as though the sun were located at a point within the constellation Aries. They chose this point the First Point of Aries, as a reference point by which all bodies on the celestial sphere could be located. The First Point of Aries continues to be used as the fixed reference point for the location of bodies on the celestial sphere. The sun now appears to be in the constellation Pisces at the time of the vernal equinox.
With the exception of the outer most planet Pluto, the other known planets of the solar system also revolve about the sun in about the same plane as the plane of the earth's orbit. They move at different speeds, all the planets revolve about the sun in the same direction. Mercury, the planet nearest the sun, and Venus, the second planet, are referred to in celestial navigation as the inferior planets, as their orbits lie inside that of the earth, while Mars, Jupiter, and Saturn, the fourth, fifth, and sixth planets, are called the superior planets because they are the visible planets whose orbits are outside that of the earth. These three planets,along with Venus, are sometimes referred to as the navigational planets. Uranus, Neptune, and Pluto, the seventh, eighth, and ninth planets, are so far away from the sun they are not used for navigational purposes most of the year.
In addition to the nine known planets of the solar system, several astronomers and mathematicians currently theorize the existence of a tenth as yet undiscovered planet beyond the orbit of Pluto, based on certain aberrations in the orbits of Uranus and Neptune.
Because of the closeness of the other planets of the solar system, when they are viewed from earth they appear to move across the background of the unchanging stars and constellations. Because the planets are illuminated by the reflected light of the sun, the inferior planets go through phases similar to the earth's moon, being "full" when on the opposite side of the sun from the earth, and "new" when on the same side. The superior planets never pass between the earth and the sun, they never appear in the "new" phase.
