In my blog, this is the first article on the Basics for Astronomy Olympiads and Quizzes. Hope you will like it! Before starting, I tell you; if you have any doubts on this, feel free to post them in the comment section at the end of the article.
Why do we need Coordinate Systems in Astronomy?
As you know, we use a Coordinate System to find the location of a specific place. For example, if we have the corresponding Geographic Coordinates (Longitude and Latitude), we can tell where that place is situated in the world. (ex:- Coordinates of Paris: 48.8566° N, 2.3522° E) It is simple as that. We use Coordinate Systems in Astronomy for a similar objective; to find a location of a Celestial object in the sky.
Celestial objects like stars do not show observable movements in the universe. It doesn’t mean that they do not move. Some stars revolve around other stars, distances between everything in the universe increases gradually and stars move away from each other,..etc. but it’s just that those movements are not visible to our naked eye as they are thousands of light years away from us. But stars do change their positions in our sky with time due to slow changes that occur in earth like precession. Therefore, they nearly have fixed positions in the sky. Calculations that are done using Celestial Coordinate Systems can determine approximate distances between celestial bodies and distances from them to earth. And also, sailors use Astro-Navigation to find their way at Night.
We get the whole sky with all the stars the people all over the world can see, into a single plane and use it as the common coordinate plane for the main Celestial Coordinate Systems. Celestial Sphere is the imaginary sphere made by Navigators by taking the whole observable universe into a single two dimensional plane which is wrapped around the inner surface of a huge sphere covering the earth.
There are mainly two Coordinate Systems related to the Celestial Sphere.
- Alti-Azimuth Coordinate System
- Equatorial Coordinate System
Alti-Azimuth Coordinate System is specified to a single location. Alti-Azimuth Coordinates of a celestial object differs from place to place. The Celestial Sphere used in this system is centered to the observer. We’ll take it simple. If you stand on the center of a flat ground with no trees or buildings on it and look at the sky around you, you’ll see it as a half-sphere. The circular base of that half-sphere is the circle with you as the center and your horizon as its circumference. The radius of that sphere seems to be the distance between you and the edge of your horizon! Of course we cannot measure it. It is arbitrarily large. In calculations, we take it as one unit. A half of the sphere is above the horizon, and we imagine the other part of the sphere to be below the horizon; the part of the sky with stars that is yet to be risen.
At night, when you observe the sky for several minutes, you will observe the stars move. They move in paths which are kind of parallel to each other. If you are near the equator, you will observe the stars rise from east and set in west. If you are away from the equator, you will see the stars move around a single point. People live in the equator see the rising and setting, because that point around which the stars move is located at their horizon. This motion is due to the rotation of the Earth. The sphere used in the alti-azimuth system is also a celestial sphere, but the term ‘Celestial Sphere’ is mainly used for the sphere in the Equatorial coordinate System.
In the Equatorial coordinate system, the celestial sphere is concentric to Earth, where the center of that sphere is the center of the Earth. The equator of the earth is projected towards the sky and the Celestial Equator is made. The latitudes of earth are projected towards the sky and the ‘Declination’ s are made. The X coordinate of a star is known as its Right Ascension. I will talk about the Equatorial Coordinate system more in the next article.
The Y coordinates (Declination) are calculated by taking the coordinate of the celestial equator as 0 degrees, while in the alti-azimuth coordinate system, Y coordinates are calculated by taking the coordinate of the horizon as 0 degrees.
What we see as rising and setting is the rotation of the Celestial Sphere (which is concentric with Earth) around its rotational axis with celestial poles at its end from East to West. In other words, it is the rotation of the Earth around its rotational axis from West to East.
I hope you have understood what a celestial sphere is.
Alti-Azimuth Coordinate System
As discussed above, horizon is taken as the basis of Y coordinates in this system. If a star is located above the horizon, its Y coordinate takes a (+) value, and if a star is located below the horizon, it takes a (-) value. X coordinates are measured clockwise from North.
- Here, the names given to x and y coordinates are Azimuth and Altitude respectively.
- See figure 2 below.
- Altitude – Angle between the horizon and the specific point in the sky.
- Azimuth – Angle between
- the line joining North and the observer and
- the line joining
- the intersection point of the line drawn from the zenith through the specific point to the nadir and the horizon
- and the observer.
- Most of the time, Azimuth is measured clockwise from North, but some people measure it from South. (If you measure it from south you have to state it beside the measurement)
- Azimuth can take values above 180 degrees too.
- Zenith is the point right above the head of the observer and Nadir is the opposite point of Zenith, in the head of the other part of the sphere below the horizon.
Alti-Azimuth Coordinates differ with time, and the location of the observer on earth.
I promised my Astrophysics teacher to share the knowledge I gained from him with others as well. I taught some of the concepts to some of my friends at school, but due to this Corona pandemic, it no longer works. I will continue this series, so that you, who are at the beginning of your journey in Astronomy, can learn something from this!
That’s it for today! Stay tuned for the next article.Thank you for reading!