Saturday, March 14, 2009

Astronomy and Celestial Navigation

This is about the the celestial sphere and apparent motion as it applies to celestial navigation.
The earth has several separate motions in space including three major motions and some minor motions. The terms "major" and "minor" refer to their effect in celestial navigation and the calculations used to solve navigation related problems.

The major motions are rotation, revolution, and precession. Rotation refers to the earth's rotation on its axis once each day relative to the sun and other celestial bodies. The rising and setting of the sun and the passage of the stars, planets, and moon across the sky during the hours of darkness are results from rotation.

The second major motion, revolution, refers to the motion of the earth around the sun once each year. The earth's path or orbit along with the orbits of the other planets in the solar system is an ellipse. The sun is not at the center of the ellipse. If the earth's orbit is used to define a plane, the orbits of all the other planets, except Pluto would lie in the same plane. This plane in which all the planets revolve is called the Ecliptic. The earth's axis of rotation is inclined at an angle of 23.5° to the ecliptic, which also is the plane of the equator being inclined 23.5° to the ecliptic. This inclination causes the changes in the seasons on earth. As the earth revolves the light rays of the sun changes, being greatest in the northern hemisphere in June, when the northern hemisphere is titled toward the sun, and in the southern hemisphere in December when the southern hemisphere is titled toward the sun.

The points in the orbit where the angle of is greatest are called the summer and winter solstices. The points in the orbit when the sun's light shines equally over both hemispheres occurs in March and September of each year. These points are called the vernal equinox in March, and the autumnal equinox in September. With the sun off center, the earth reaches its closest point to the sun in January, after the winter solstice. This point is called Perihelion. The furthest point in the orbit from the sun is reached in July, after the summer solstice, and this is called Aphelion.

The third major motion, precession of the equinoxes, is caused by the gravitational force of the sun, moon and the other heavenly bodies acting together to deflect the axis of the rotating earth. This force causes the earth to act like a spinning top. The axis slowly traces a circle while remaining inclined at the same angle to the plane of the ecliptic. The earth's axis will travel through a circle over a period of 25,800 years. In time, precession will cause the axis to move away from the star Polaris, which we call the pole star.

The earth is located in a system which circles a mid sized star, our sun. The sun is located between the center and the edge of cluster of millions of stars circling the center of what is called a galaxy. This formation is one of other galaxies in the universe. The nearest other star in our galaxy is 4.3 light years away. Considering that light travels 186,000 miles in one second, the distance it would travel in 4.3 years is a long way. The light from the sun reaches earth in about 8.33 minutes. The distances are so great to the stars that the light rays which radiate from them are parallel lines when they reach our solar system. Over these distances the stars appear in the same place relative to the other stars from any point on the earth's orbit around the sun.

The stars are categorized by their brightness, or magnitude, with the brightest called first magnitude and the dimmest which can be seen with the unaided eye categorized as sixth magnitude. A first magnitude star is 2.5 times brighter than a second magnitude, and 6.3 (2.5 x 2.5) times brighter than a third magnitude. Each star is given a magnitude number by astronomers, with the first magnitude stars having numbers of 1.5 or less. Some of the brightest stars are so bright they have been given minus numbers. Examples are stars like Sirius (-1.6) and Canopus (- 0.9). The planets are also given numbers which vary with their positions in their orbits. Sometimes the planet Venus is so bright that it can be seen by the unaided eye in the daytime. As an interesting note, the sun's magnitude number is - 26.7.

Stars appear in different colors in the sky, and this color difference is caused by differences in their internal temperature. Astronomers group stars as red, white, or yellow giants, dwarfs, or mid-size stars depending on their stage of evolution. Only a small number of stars are used for navigational purposes. These are stars with magnitudes of 3 or brighter. There are 57 navigation stars listed in the daily pages of the Nautical Almanac.

Solar System
The sun is the central body of a system of nine principal planets including the earth. Satellites (moons) of the planets, and thousands of minor planets, comets and meteors are included in this system. All of these bodies, encircling the sun, are in motion relative to the stars. This motion is called space motion. The other planets like the earth, revolve around the sun in their orbits. Planets whose orbit lies outside the earth's orbit are called superior planets. The two planets with orbits smaller than the earth, Venus and Mercury, are called inferior planets. Because they are closest to earth, the planets used for navigation are Venus, Mars, Jupiter, and Saturn.
The orbits of all the planets except Pluto, lie in the same plane of the ecliptic as the earth's orbit. This makes it appear that the planets are either preceding or following the sun in its path through the sky as seen from earth. Whether a planet is preceding or following the sun depends on where the planet and earth are in their own orbits. Where a planet is located relative to the sun, when seen from earth at a given time, is known as planetary configuration.

When the sun lies directly between the earth and an inferior planet, the planet is at superior conjunction. When the inferior planet is directly between the earth and sun it is at inferior conjunction. When an inferior planet, while moving in its orbit from superior toward inferior conjunction reaches the point where the angle between the planet and sun as seen from the earth reaches its greatest value, it is said to be at greatest elongation east. After passing inferior conjunction, the inferior planet, continuing in its orbit, will appear west of the sun. When at its greatest angle west of the sun it is at greatest elongation west.

Superior planets are at conjunction when they are at the point in their orbits where the earth, sun and the planet are directly in line with the sun between the earth and planet. When a superior planet is aligned with the earth and sun with the earth in the middle, the planet is at opposition. The points in a superior planet's orbit where the difference in bearing between the planet and the sun, as seen from the earth, is 90 degrees is called quadrature. It is at east quadrature when the planet is east of or following the sun in the sky, and west quadrature when west of or preceding the sun.