Although Guide 8.0 doesn't come with some of the bells and whistles and beautiful pictures of more expensive software packages, it is the best and least expensive software available for use with this book. It provides the ability to construct star charts with labeled magnitudes to the limit of any telescope and for any field of view. With it you can also label and identify variable stars and asteroids and obtain all the parameters necessary for observation of the Sun, Moon and Jovian satellites.
Microsoft Picture It! 7.0 or later editions is a low cost photo processor that has everything needed for the digital camera photographs associated with the activities in this book. With it a grid or scale can be superimposed over an image, faded to transparency and stretched and rotated through a calibrated angle. Any other software you might choose must have the same capabilities.
In order to make useful observations, it is necessary to establish a reference frame for making measurements. For all observations of the position of a star or planet to be related in time and space, certain directions and orientations must be established as standards. To establish such a standard coordinate system, the stars can be considered as fixed to a transparent celestial sphere that rotates from east to west about Earth's axis once every 24 hours. Figure 1.1 is an illustration of this imaginary sphere on which standard coordinates are indicated. The following definitions are referenced to this illustration.
The celestial poles are projections of Earth's poles onto the celestial sphere. The celestial equator is the projection of Earth's equator (e in Figure 1.1) onto the celestial sphere.
An observer at the point p on Earth sees a horizon indicated by the plane NWSE indicating the directions north, west, south and east. The observer's zenith, the point directly overhead, is p'. The local meridian is an imaginary line from the northern horizon, through the north celestial pole, through zenith, to the southern horizon.
As the Earth moves around the Sun at a rate of about one degree per day (360°/365.25 days), the Sun appears to move around the celestial sphere through the fixed stars at the same rate. This path of the sun is called the ecliptic. It is the projection of the plane of the Earth's orbit onto the celestial sphere.
The point at which the ecliptic crosses the equator, south to north going eastward, is the vernal equinox. The opposite point of intercept, 180° away, is the autumnal equinox. The most southern point on the ecliptic is the winter solstice;
the most northern, the summer solstice. Notice that the equinoxes and solstices are points on the celestial sphere not times of the year. Spring begins when the Sun crosses the vernal equinox not when the vernal equinox occurs. In Figure 1.1, the apparent Sun is indicated by a small circle at the winter solstice. The vernal equinox is on the eastern horizon. An arrow indicates direction to the actual Sun.
Due to the gravitational interaction between the Earth and the Moon, the Earth's axis precesses around a 23%° cone with a period of 25,800 years relative to the stars. As a result, the sidereal positions of the equinoxes change with the same period. For the purpose of establishing a coordinate system, however, we can consider them fixed.
Since the vernal equinox represents a fixed point on the celestial sphere, we use this point as the origin of a coordinate system to which we can refer the positions of stars. Suppose an observer at point p observes a star at point s on the celestial sphere. We can imagine a great circle running from north through the star to the celestial equator. We define the angle between this circle and the circle going through the vernal equinox as the right ascension (RA) of the star. For reasons explained in Chapter 2, right ascension is measured in hours, minutes and seconds (0 to 23 hours) eastward from the vernal equinox along the celestial equator.
We define the angular distance of the point s from the celestial equator as the declination of a star at that point. Coordinates of declination are parallel to the celestial equator. They are measured in degrees north or south. North declinations are +0° to 90° and south -0° to -90°. The coordinates of right ascension and declination, called equatorial coordinates, are equivalent to longitude and latitude on the Earth.
The Measurement of Time
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Although we usually tend to think of the digital camera as the best thing since sliced bread, there are both pros and cons with its use. Nothing is available on the market that does not have both a good and a bad side, but the key is to weigh the good against the bad in order to come up with the best of both worlds.