The horizontal (east-west) lines, or circles, on a globe or map are called parallels of latitude. The equator, or the largest circle half way between the two points were the globe is attached to its stand, is a special parallel that is designated as the zero degree latitude. As viewed from the center of the Earth, latitude is measured in degrees of separation from the equator. Ashton MD, for instance, is at N39.14° latitude, which means that it is separated from the equator by 39.14°. The Earth’s north pole, which would be directly overhead for an observer at the center of the Earth, is at latitude N90°.
The vertical, or north-south, lines on the celestial sphere are known as right ascension, and are marked not in degrees, minutes and seconds, but in hours, minutes and seconds. Lines of right ascension are somewhat related to longitude, but since the Earth turns about its axis within the celestial sphere, the relationship is not fixed. The difference in the units of measure, degrees of longitude vs hours of right ascension, accounts for the changing relationship between the “fixed” surface of the Earth and the celestial sphere that “rotates” across the sky.
There are twenty-four hours of right ascension on the celestial sphere, which is related to the length of a day on the surface of the Earth. Two different hour lines of right ascension are one hour, or 15° apart. Fifteen degrees is exactly how much the celestial sphere appears to rotate in one hour. Twenty-four right ascension hours spaced 15° apart represent the 360° circle around the celestial sphere.
Declination is the measure in degrees of separation between the celestial equator, and a point on the celestial sphere, and it is the celestial sphere’s corollary to terrestrial latitude. Like lines of latitude, lines of declination usually run horizontally, or east-west on a chart. Declination lines more distant from the celestial equator, or 0° declination, have higher numbers. North declination is identified as a positive number (+40°) while south declination is identified as a negative number(-40°).
To illustrate the relationship between declination and latitude, consider the terrestrial equator, which is 0deg latitude. The sum of all zenith points at the terrestrial equator define the celestial equator. Similarly, zenith for an observer in Ashton, Maryland (N39.15° latitude) is +39.15°. This can be observed on the sky chart below that prepared for Ashton, Maryland that was previously examined. Note that the zenith falls just south of the declination line that represents +40°.
All of the stars and other objects that might be seen in the sky are at varying distances from us. Since even the nearest of these objects are so far away, our mind perceives them as all being at the same indeterminable distance away. For this reason and for the purpose of this amateur stargazers, the sky and all that it holds can be thought of as the celestial sphere – much like the inside of a planetarium dome. Stars are fixed points upon this sphere that all move together across the sky. Imagine the earth at the center of this sphere, and that the sphere surrounds the entire earth.
In reality, the earth spins on its axis beneath the celestial sphere. The point on the earth’s surface from which we happen to be observing circles the earth’s once each day, and we observe the celestial objects passing through our field of view, or the sky, from east to west. Our perception is that the earth is fixed, and the celestial sphere turns above us. For casual observational purposes, and for ease of explanation, the remainder of this discussion will assume the perceived motions.
There are two ways to think of constellations. First, and foremost, a constellation is a grouping of stars that usually represent some mythological being. In the first chart Orion appears as a stick figure torso with a belt of three stars and holding what is depicted as a raised club and perhaps a dead lion. The interconnecting lines aid in identifying the spatial relationships between the constellation’s stars by guiding our eye from one star to the next. Secondarily, a constellation is an area of the sky, which is delineated in the second chart by the white field. If the Sun, Moon, a planet, or some other celestial object happens to appear within a constellation’s borders, it is said to be located in that constellation.
Defining the horizon is rather simple: walk out into the middle of a large, open field, or just imagine yourself standing there. Lift one arm until it is horizontal, or at 0° elevation, and point in any direction at the “line” where the sky meets the earth. This is a single point (see what I just did there?) on the horizon. Keeping your arm extended horizontally, turn about in a complete circle, or 360°, and the horizontal circle traced out by the extended finger has traced out all of the points that make up the horizon, which we will simply refer to as the horizon.
Image Credit: – Zenith. Addison Welsey. cse.ssl.berkeley.edu/ Accessed by Jim Johnson on January 19, 2015.
The southern constellations are those that fall within the Southern Celestial Zone, which is located at the bottom, or southern, portion of the sky map below. This zone is bounded by the celestial equator (0° declination) to the north, and the southern horizon to the south.
Stars in this region tend to hang low in the southern sky, never rising very far above the horizon. From our N39° latitude on the terrestrial sphere, these constellations generally rise in the southeast, make a brief appearance above the horizon, and then set in the southwest. As a the southern declination of a star increases, the amount of time that it will appear above our horizon decreases.
To gain a sense of how a star’s declination affects its time above the horizon, examine the -20° and -40° declination lines in the chart below. The -20° declination lines spans from the 0th hour (0h) of right ascension line to the 10th hour (10h) of right ascension line, which means that a star traversing the sky at -20° will be above the horizon for ten hours. Now consider an object just 20 degrees farther south on the -40° declination line. This line spans from the 2nd to the 8th hour of right ascension, which means that the star will appear above the horizon for only six hours. The effect accelerates as the declination increases even more. At five degrees farther south, or -25° declination, we approach the southern limit of our view from N39° latitude. An object at this declination would only appear above a perfect southern horizon for just a few minutes.
In contrast, objects that have a more northerly declination remain above the horizon increasingly longer. At +39.14° declination and higher on the celestial sphere, which corresponds to my N39.14° latitude on the terrestrial sphere, objects remain above the horizon all night!
Northern Constellations are those that fall within the Northern zone of the celestial sphere, which is zone that spans from east to west across the central area of the sky chart below. This zone is bounded on the north by the circle that defines the Circumpolar region, and on the south by the equator, or 0° declination, that spans from the east to west points on the horizon.
Stars in this zone generally rise in the east, pass directly or nearly directly overhead, and then set in the west. These are the most easily observed constellations when they are on the meridian, because views in this region are the least affected by the atmospheric haze that degrades views near the horizon. This region is also the least likely to be blocked by obstructions such as trees or buildings on the horizon.