The declination of each star changes so slowly that over a period of many years an observer in a given latitude will have the same view of the diurnal circle of that star. The declination of each body of the solar system changes rapidity, and the apparent motions of these bodies changes at a similar rate. The declination of the sun, moon, and navigational planets varies between 25° north and 25: south, and their diurnal circles will lie between these limits.
One of the effects of the earth's rotation on its axis is known as day and night. Since the earth is approximately a sphere, half of it will be in sunlight and half in darkness at any given time. The length of the period of day or night varies with the observer's location on the surface of the earth.
The annual revolution of the earth around the sun shows the 23.5° inclination of the equator to the earth's orbit. About 21 June each year the north pole is at it'maximum inclination towards the sun, and the declination of the latter is 23.5° north. As the earth moves on in its orbit about the sun, the northerly declination off the sun decreases slowly, and reaches 0° on 23 September; it continues to decrease at a constant rate until about 22 December, when it reaches 23.5° south, its maximum southerly declination. Moving on from this point, the declination increases at a constant rate, reaching 0° again about 21 March, and 23.5° north on 21 June.
First point of Aries,the points of maximum declination are called the solstices; the points of 0° declination are called the equinoxes. These words are derived from the Latin, solstice meaning "sun standing still," and equinox meaning "equal night." The point in space at which the March equinox occurs is also called the first point of Aries it is an important reference point in the system of celestial coordinates. It gets its name from when the celestial coordinate system was first established, the sun entered the constellation Aries as it passed from south to north declination.
The annual change in the sun's declination why we have seasons on earth; they are caused by the angle at which the sun's rays strike the earth, and the length of daylight and darkness. The times of 0° declination are termed the vernal (spring) and the autumnal equinoxes; the time of maximum north declination is the summer solstice, and the time of maximum south declination is the winter solstice for the northern hemisphere; summer and winter are reversed for the southern hemisphere.
The revolution of the earth about the sun also affects the apparent positions of the stars, which surround the solar system on all sides. The ones which can be seen from the earth on a given night are those in a direction generally opposite to that of the sun. Because of this the stars appear to make one complete revolution around the earth year independently of their nightly revolution due to the earth's rotation on its axis; each night at the same time at a given place each star will be almost one degree farther west; it requires an average of 365 1/4 days to complete the revolution of 360°.
The combination of the revolutions of the earth and of the planets about the sun results in the rapid change of position the planets. Mars, Jupiter, and Saturn, whose orbits lie outside of earth
are termed superior Planets. The superior planets appear move westward to the sun, meaning that they rise earlier and cross the observer's meridian earlier on each day. They emerge from behind the sun as morning twilight bodies. Motion of planets and continue to rise earlier each day until they again disappear behind the sun, last seen as evening twilight bodies. With respect to the stars, the superior planets appear to move constantly eastward from night to night, except when they are nearest the earth. At this time their motion is retrograde, appearing to move westward among the stars. Mercury and Venus are termed inferior Planets, as their orbits lie inside that of the earth. They appear to oscillate with respect to the sun. Venus always appears comparatively near the sun, it alternates as a morning and evening planet, referred to as the "morning star" or "evening star," because it rises and sets within about three hours of sunrise and sunset.
Mercury is a bright celestial body, but because of its closeness to the sun it can be seen only rarely, and its coordinates are therefore not listed in the Nautical Almanac.
The planets shine by the reflected light of the sun; the inferior planets go through all the same phases as the 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 sun, and are never seen in the "new" phase; they vary only between "full" and "gibbous" when viewed through a telescope.
The most obvious effect of the moon's revolution about the earth is the cycle of Phases through which it passes. Like the planets, the moon shines by the sun's reflected light. Excluding possible eclipses, the side facing the sun is lit, and the opposite side is dark; the moon's appearance from the earth depends on its orientation relative to the earth and sun.
The moon passes through its cycle of phases during a 29.5 day synodic period. The synodic period of a celestial body is its average period of revolution with respect to the sun, as seen from the earth. It differs from the 360 sidereal period because of the motions of the earth and the body in their orbits. Figure 1814 illustrates the positions of the moon relative to the sun and earth during its synodic period, and the resulting phases. When the moon is between the sun and the earth, its sunlit half faces away from the earth, and the body cannot be seen; this is the new moon. As it revolves in its orbit, an observer on earth first sees a part of the sunlit half as a thin crescent, which will then wax or grow slowly through first quarter, when it appears as a semicircle. After passing through the first quarter, it enters the gibbous phase until it becomes full, and the entire sunlit half can be seen. From full it is said to wane, becoming gibbous to the last quarter, and then crescent until the cycle is completed.
The age of the moon at a given time is the number of days which have passed since the preceding new moon, and is an indication of the phase, and the amount of light it sheds. The full moon rises in most latitudes about the same time the sun sets, and sets when the sun rises; the new moon rises and sets with the sun. On the average, the moon rises about 50 minutes later each day, but varies.The limb of the moon is always towards the sun with the points pointed away from the sun.
