Before we leave for our trip into the realms of the cosmic spheres, it’s a good idea to get our bearings and understand our observational platform, the Earth.
To fully understand the academics of Astronomy and Astrophysics, one has to conceptualize ideas in the 3rd and 4th dimensions. To make this subject matter easier to explain, Astronomers often utilize visual aids to help others understand this science dealing in the abstract. One of those tools is the Celestial Sphere.
I was first introduced to the Celestial Sphere in college, and the utilization of this very simple visual aid helped me understand the night sky as seen from Earth. Although this particular visual aid is conceptualized as the outdated Platonic Geocentric model (Earth Centered), rather than the accepted Copernicus Heliocentric model (Sun Centered), it still has practical applications.
The Celestial Sphere aids in the understanding of the constellations and motions of the planets as we see them fixed in the night sky. While the stars and planets look as though they are rigid, they are very much in motion. The Celestial Sphere helps us to understand these varying motions and what they mean for our observational proposes.
For example, have you ever noticed that the Sun and Planets follow a particular path across the sky? Maybe you have not stopped to think about it, but the Sun and Planets follow a very precise path through the sky called the Ecliptic. The Ecliptic is an imaginary line that is represented on the Celestial Sphere.
In the image above, I have outlined the Ecliptic in red. The Ecliptic traverses at a shallow angle across Earth’s sky, concerning the Celestial Equator.
The Celestial Equator is an extension of Earth’s equator onto the Celestial Spheres. I have outlined the Celestial Equator in blue in the image above. This line is important because it represents the 0h to 24h mark or latitude on the Celestial Sphere. Instead of using latitude in degrees as we do on Earth, astronomers use “hours” or Right Ascension on the Celestial Sphere to represent these coordinates.
Another very important coordinate system on the Celestial Sphere is that of Declination. Declination on the Celestial Sphere runs from North to South, just like longitude on the Earth. I have marked examples of Declination on the Celestial Sphere in green. This line is used to express how many degrees above or below the Celestial Equator an astronomical object is positioned.
Declination is expressed in 0 to 90 degrees and -0 to -90 degrees. The positive degrees being above the Celestial Equator or in the Northern Celestial Hemisphere, negative being below the Celestial Equator or in the Southern Celestial Hemisphere.
Running from the North Celestial Pole (NCP) to the South Celestial Pole (SCP) through the 0h mark on the Celestial Equator is another imaginary line called the Celestial Prime Meridian.
The Celestial Prime Meridian allows an observer to express their Zenith position relative to the NCP and SCP. Allow me to express this complex idea in a picture. See the diagram below.
The Earth is our observational platform. While all observers view the night sky from the same platform, each observer has a different position.
The coordinates on the Celestial Sphere help us to express these different positions in scientific terms. This allows us to facilitate a cohesive understanding of astronomical objects relative to another observer’s position.
Now let’s move on to another important application that comes with understanding the Celestial Sphere. Astronomical explanations behind the Summer/Winter Solstice and the Vernal/Autumnal Equinox. All of these imaginary positions can be found on the Celestial Sphere. Once again, allow me to explain this with a diagram.
The Vernal Equinox (First day of Spring), is defined as the point on the Celestial sphere where the Sun traveling along the Ecliptic intersects the Celestial Prime Meridian at the 0h or 0-degree point on the Celestial Equator.
Likewise, the Autumnal Equinox (First day of Fall), is defined as the point of the Celestial Sphere where the Sun traveling along the Ecliptic intersects the Celestial Prime Meridian at the 12h or 180-degree point on the Celestial Equator.
The Summer Solstice (The first day of Summer) is when the Sun reaches the highest point in the sky along the Ecliptic. The Winter Solstice (First Day of Winter) is the opposite of the summer solstice when the Sun reaches the lowest position in the sky along the Ecliptic. All of these motions along imaginary lines can be easily understood through the aid of the Celestial Sphere. Now let’s see how this model can help us understand the stars.
When we look up at the stars at night, we see points of light. These points of light belong to a larger group of stars we refer to as the Constellations.
Each of the constellations has boundaries. The Celestial Sphere marks these boundary lines helping us to understand where each of the boundary lines is drawn. This is of uttermost importance when locating celestial objects. Knowing where to look, and in what part of the sky, is essential to understand the locations of various points of interest in the night sky.
Let’s take a constellation that everyone knows very well because it lies on the Celestial Equator, Orion. I have outlined Orion’s constellational boundary in yellow and highlighted some of the brightest stars forming his figure. One of the most spectacular stellar nebulas lies within the boundary of Orion, M42 (circled in purple). If an observer wants to know where they can find this celestial gem, they will know that it lies within the boundary of Orion.
Above is an image taken by the Hubble Space Telescope of the Orion Nebula. This region in Orion is a huge stellar nursery where new stars are evolving. I will do a post that focuses on the astrophysics of this incredible stellar nursery at a later date. But for now, Orion is a reference point in understanding constellational boundaries on the Celestial Sphere.
The Celestial Sphere helps us to understand and make sense of the points of light that we see from Earth. The Celestial Sphere surrounds the Earth like a blanket on which all the stars that we see reside. This model however is incorrect in assuming that the stars are at all the same distance from Earth. The stars that make up the constellations are all at varying distances, we just see them as homogeneously correlated due to our observational platform and vantage point. That aside, the Celestial Sphere is correct in showing how the patterns of these points of light are associated with each other, forming the constellations.
Now that we have a better understanding of Earth’s Sky as we see it from our observational platform, we are nearing the assent into the realm of the cosmic spheres. But before we go, I want to make one last stop. To understand the knowledge of the spheres, we need to first understand our sphere, the Earth.
Next time, we will take a closer look at our planet from an astronomical perspective. I will dig deep and uncover the secret and unlikely reason that the Earth sustains life. By understanding our astronomical history we can then understand the history of the other cosmic spheres around us. We are but a tiny part of the cosmos, understanding our place allows us to gain the ability to begin to grasp the Knowledge of the Spheres.
Thank you for coming along with me as we traverse the cosmos and discover the Knowledge of the Spheres.
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