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For general observing, one should check the On-line Calendars to see if there are any Solar System events of interest. Also check the circumstances of the Moon that might hinder observation of Aurora, Comets, Deep Sky Objects, and Meteor Showers from your place of observation. Planning really enhances your observing session. Don't forget to check the Weather, Clear Clock Sky and Amateur Astronomer's Notebook.
The Earth rotates about its polar axis once a day and produces an apparent motion on the night sky about the celestial poles. In the Northern Hemisphere the north celestial pole is elevated above the horizon. Facing away from the elevated celestial pole, an observer sees the Sun, Moon, planets, and stars rise in the east and set in the west. They reach their highest altitude as they cross the local meridian. When the observer turns to face the elevated celestial pole, stars nearest the pole neither rise nor set. They become circumpolar and cross the meridian each day once above the pole at their highest altitude and once below the pole at their lowest altitude. In the Northern Hemisphere, circumpolar stars appear to rotate about the north celestial pole counterclockwise. In the Southern Hemisphere the effect is reversed and they appear to rotate clockwise.
Superimposed on the diurnal rotation is an annual rotation caused by the Earth's orbiting the Sun. Since the stars are seen by the naked eye after sunset, the constellations appear to move from east to west, and to return to the same position after a year. Relative to the Sun, the stars rise and set roughly four minutes earlier each day. In the course of a month, the night sky appears to move two hours in right ascension to the west. Also because of this orbital motion of the Earth, the circumpolar stars in the Northern Hemisphere appear to rotate once a year in a counterclockwise direction around the north celestial pole and in a clockwise direction about the south celestial pole.
The Moon moves in an orbit inclined to the ecliptic by 5.1 degrees; the Moon makes one revolution about the sky from west to east in about a month. During this period the phases on the Moon complete a cycle from new to full and back to new. The orbit of the Moon is moving around the ecliptic, so that other aspects of the Moon's position in the sky, such as its maximum and minimum declination, change from one month the next. It is important to know when the planets are in the most favorable position for observation. The outer planets, for example, are best seen around opposition. They are in their least favorable position around conjunction.
The inner planets are different--they are in their most favorable position near greatest elongation, even though they are not at full phase. At superior conjunction the phase is around full, but the planets are difficult to see because they are further from Earth and usually too close to the Sun. At inferior conjunction the inner planets are nearest to the Earth, but again they are difficult to see because their phase is small, and they are too close to the Sun.
Often the times of phenomena need not have any great precision; sometimes the nearest hour, day, or even the nearest week are sufficient for observational purposes. The dates and times, however, usually depend on the coordinate system. For historical reasons the conjunctions and oppositions of planets have always been calculated in geocentric ecliptic coordinates. On the other hand, the conjunctions of planets with other planets, bright stars, or the Moon have always been calculated using equatorial coordinates; the phenomena are then observed more easily with an equatorially mounted telescope. In some cases the times of phenomena have been defined as the maxima or minima of the distances from the Sun or the Earth or the elongation from another body. In such cases, the phenomena are independent of the coordinate system.
© Copyright 2007 - Samuel J. Wormley