Welcome to the second in my series of guides to observing the night sky. This article is written under the Constellations Project, and it builds on How to begin Observing the Night Sky by describing the celestial coordinate system and other details that appear on the sky maps presented in each month’s edition of Scope Out Next Month. As an aid to understanding this material, I have referenced and inserted thumbnail images of graphics that can be clicked for a larger view.
There are two related spherical representations to consider in order to learn how to read a a star chart – a globe and the celestial sphere. Think of the Earth as the terrestrial sphere situated at the center of the celestial sphere. An observer looking outward from the Earth is gazing upon the celestial sphere. A globe is a spherical representation of the Earth’s surface that shares several common attributes with the celestial sphere. For this reason, a brief review of a globe is a good starting point for understanding a star chart. Refresh your understanding of a conceptual celestial sphere, if needed, by reading How to begin Observing the Night Sky.
We can see on a globe that the land areas are apportioned among countries with borders (lines) drawn to demarcate each country’s political, or man-made boundaries. Within the countries are cities. The cities are generally regarded as fixed points that may from time to find themselves in one country or another as political boundaries are redrawn. Highways connecting the cities can be found on maps representing smaller portions of the Earth’s surface.
Constellations are the man-made corollaries to the countries, cities and highways that we find on globes and maps representing the Earth surface. Referring to the accompanying star chart, which is made specifically for Ashton, MD at 8pm EDT on March 15, 2015, find Orion located below and to the right of the zenith (the red circled “z“). There are two ways to think of constellations. First, and foremost, a constellation is a grouping of stars (cities). In this chart Orion appears as a stick figure torso with a belt of three stars and what is usually depicted as a shield and raised club in the chart. The interconnecting blue lines (highways) 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 (country), which is delineated by the purple border. 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.
Now to take on the two coordinate systems. Both spheres have horizontal and vertical circles, sometimes called lines, that represent north-south and east-west spatial relationships. The intersection of two lines is a coordinate that represents a point on the surface of the sphere. For instance, if I know that Ashton, Maryland is located at N39.14° and W77.01°, I can locate Ashton on the globe by finding the intersection of the N39.14° and W77.01° lines on a globe or on a map. Similarly, the intersection on the celestial sphere of the lines representing declination +16.5°, and 4 hours, 36 minutes right ascension is where the star Aldebaran is found in the constellation of Taurus. Aldebaran, a bright star, is not named on the map above, but it can be found at Dec +16.5 degrees, Ra 4:36.
The horizontal lines, or circles, on the globe 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. One could travel around the world while remaining on the equator and never be at a latitude of more than 0° north or south. More circles called parallels of latitude can be found in 10 to 15 degree increments as one moves north or south away from the equator and toward the two attachment points. Note that these circles do not cross one another (they’re parallel), and each circle is smaller than the one before as one moves north or south away from the equator. Latitude is a measure of the angular separation in degrees away from the equator. Imagine yourself sitting at the center of the Earth, with the top of your head pointing toward the north pole while your eyes gaze outward at the equator, the zero-degree latitude. As you being to tilt your head back to look up toward the north pole, the angle between the equator and the point at which you are looking increases, and is the measure of the point’s latitude. If you look upward far enough to see Ashton, Maryland, then the angle, and the latitude, is N39.14°.
The celestial sphere’s corollary to latitude is called declination, and declination is very closely related to latitude. In fact, both are measured in degrees. If one is standing on the equator, then the celestial equator, or 0 degrees declination, is directly overhead at the zenith. Points north or south of the celestial equator are indicated in positive numbers north of the celestial equator, and negative numbers south of the celestial equator. When standing in Ashton, Maryland for instance, the declination at the zenith is +39.14°. This can be observed on the sky chart prepared for Ashton Maryland that was previously examined. Note that the zenith falls just south of the declination line that represents +40°.
The poles on the both the Earth’s sphere and the celestial sphere are two special latitude and declination points. They are defined as the point where the extension of the Earth’s axis beyond its surface intersects the celestial sphere. I referred to the poles as points (as opposed to lines) because the Earth’s celestial poles are at N90° and S90°. There is no east/west component, because the only direction away from, say the north pole, is south. When standing on one of the Earth’s two poles, the corresponding north or south celestial pole will be at the zenith. These points have a single coordinate of +90° or -90° for the north and south celestial poles, respectively.
Longitude, the east-west measure of location in degrees, is represented by the vertical lines on the globe. Note that these lines converge on, and meet at both the north and south poles. The prime meridian, which is the longitude that runs north-south through Greenwich, England, is designated as the zero degree of longitude. Sitting back at the center of the Earth again, turning your head away from the prime meridian increases the longitudinal angle increases. Looking west from the prime meridian to Ashton, Maryland’s location, the angle will be W77.01° from the prime meridian. Longitude can be written as either E or W of the prime meridian, usually no more than 180°. The longitude on the opposite side of the Earth from the prime meridian is correctly written as either W180°, or E180°.
There is a corollary to longitude on the celestial sphere, but it is not as closely related as latitude and declination are. The vertical, or north-south, lines on the celestial sphere are known as right ascension, and are measured not in degrees, minutes and seconds, but in hours, minutes and seconds. This difference in the units of measure 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.
There is a special right ascension line at a given location on the Earth’s called the meridian. The meridian is defined as an arc that starts at the north point on the horizon, extends up toward through the zenith, and back down to the the south point on the horizon. The right ascension of the celestial sphere that is on the meridian is known as local sidereal time. If the 2nd hour of right ascension on the celestial sphere happens to be on the meridian at a given time, and say an hour passes, then the 3rd hour of right ascension will be on the meridian. Let another hour and a half pass, and the right ascension at the meridian will be 4hours 30 minutes, or 4h30m.
The material covered in this post is really all that is needed to understand the coordinate systems and constellations on a star chart. I have alluded to the dynamic nature of a celestial sphere that appears to turn over our heads, and the next in this series of posts will expand upon how this sphere turns with respect to the Earth’s diurnal (day and night) rotation, and with respect to the Earth’s annual orbit about the Sun.
Questions and discussion are always welcome.
© James R. Johnson, 2015.
– Celestial Sphere, ASTR 1230 (Majweski) Lecutre Notes. http://www.astro.virginia.edu/class/majewski/astr1230/. Accessed by Jim Johnson on January 19, 2015.
– Declination on the Celestial Sphere. http://www.opencourse.info/. Accessed by Jim Johnson on January 27, 2015.
– The Globe. www.irishrimes.com. Accessed by Jim Johnson on January 27, 2015.
– Latitude and Longitude. www.mrdowling.com. Accessed by Jim Johnson on January 27, 2015.
– Celestial Sphere with Declination and Right Ascension, Steven Schimmerich, http://hudsonvalleygeologist.blogspot.com/. Accessed by Jim Johnson on January 27, 2015.