From the Orion Arm
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From the Orion Arm

The Celestial Sphere

Creating a map of the Heavens above!

Representational Image: An Armillary Sphere — The different rings show the positions and markings of various night sky objects. These rings include the Ecliptic (that includes the markings of the Zodiac) and the Celestial Equator among others. (“Armillery Sphere 1”, by Stew Dean, Licensed under CC BY-NC 2.0)

Centralizing Coordinates

Figure 1: The North Celestial Pole depicted directly above the Geographical North Pole — The bright spot towards the slight left of the image is the Sun, the concentric circles depict the Declination Circles, while the apparent straight lines appearing to radiate out from the center depict the Right Ascensions Circles. Note that in this image, the RA circles are appearing as straight lines due to the large field of view of 176°. Similarly, the South Celestial Pole is such that it appears at the top at the South Celestial Pole. Shown in red is the Ecliptic.
Figure 2: Ecliptic and the Celestial Equator. ♈︎(𝛾) represents the first point of Aries. The position of the Sun is below the Celestial Equator implying that it is higher in the sky for Southern latitudes than for Northern latitudes (Joshua Cesa, CC BY 3.0, via Wikimedia Commons, converted to JPG).
Figure 3: Measuring the coordinates of any point located on the Celestial Sphere using Right Ascension and Declination (from Wikimedia Commons).
Figure 4: The Equatorial (RA-Dec) Coordinate System at Port-de-Bouc, France. Note that this time, the North Celestial Pole is not right at the top and is inclined at some non-right angle to the Horizon. This is because the Equatorial system is not observer-based and is fixed to the Universe. So how it appears in the Night Sky keeps on changing from place to place. The degree measure of the NCP (North Celestial Pole) or the SCP (South Celestial Pole) gives us the Latitude of the place. As previously discussed, the RA circles too move from East to West during the Night Sky. This non-observer-oriented thingy prompts us to introduce another coordinate system fixed to the Observer — the Alt-Az Coordinate System.
Figure 5: Various astronomical coordinate grids — Equatorial Grid (depicted in Blue), Ecliptic Grid (in Pink), Galactic Grid (Green), and Supergalactic Grid (in Yellow).

Decentralizing Coordinates

Figure 6: The Alt-Az Coordinate System at Port-de-Bouc, France. Notice that in this image, the green circles (representing the Altitude and Azimuthal lines) are in contrast to Figure 4 showing the RA-Dec system at this place. The Altitude lines here, meet at the exact top and the concentric circles depicting growing Altitude too tend to coalesce there. Hence, this coordinate system is Observer-based, making it more convenient for amateur night sky observers.
Figure 7: The Observer-based Alt-Az Coordinate System (CheCheDaWaff, CC BY-SA 4.0, via Wikimedia Commons)
Figure 8: Polar points of different coordinate systems — Zenith (in Blue), North Ecliptic Pole (NEP, in Brown), North Celestial Pole (NCP, in Red), North Galactic Pole (NGP, in Green), North Supergalactic Pole (NSGP, in Yellow).

Some Other Elements

Figure 9: ¹This 4-minute difference between sidereal and solar day is responsible for the apparent movement of the stars along the Ecliptic which is what causes the change in Zodiacs. (Tauʻolunga, CC BY-SA 3.0, via Wikimedia Commons)
  1. You might have observed a ¹ appearing earlier on in the text when I mentioned that the position of the stars remains fixed each night “for a couple of days”. Well, then what happens over a couple of weeks? The stars change their positions. And why is that? It's because of the 4-minute difference between the duration of a Sidereal Day and a Solar Day that causes this slow relative rotation of the Equatorial Coordinate System with respect to the day and night period on Earth. This gap between the ‘two types of days’ is almost compensated over a year after which, the same stars appear to be in the same spot as they were the last year…
  2. …almost. Even the annual compensation fails to account for the Precessional changes of the Earth that change the orientation of the Equatorial grid over time, albeit very slowly. Yet this change is enough to cause deflections of about 1.38° per century. Hence, it becomes crucial for astronomers to ‘reposition’ the EQ system for consistency from time to time (about every 50 years or so). This repositioned system is then used as a reference for the next few years and is known as ‘Epoch’. The current epoch J2000 is fixed at the coordinates of the night sky objects as of 1 January 2000 at 12 Noon UTC of the Julian Calendar (that gives its ‘J’ and ‘2000’).
  3. For the ease of nomenclature and classification of stars, the whole celestial sphere is divided into 88 fixed regions known as the Constellations. These Constellations, as an official astronomy term, are not groups of stars, but rather regions on the Celestial Sphere, named after their constituent groups of stars (the erstwhile ‘constellations’).
  4. An important point to note is that the positions of all the heavenly bodies are not fixed on the Celestial Sphere. These include the Moon, the planets, and the Sun. These ‘wanderers’ (Planeta) are wayyyy closer than any other celestial body, so, are in constant motion with respect to the background stars. Hence, they cannot be designated with any particular coordinates on the Celestial Sphere.
  5. Talking about the constellations, due to the effects of sidereal-vs-solar-day, the constellations along the Ecliptic appear to move eastwards with time and the Sun appears to ‘enter into a new constellation every month. This constellation is ‘obscured’ by the Sun and hence, the Sun is said to be in that particular ‘Zodiac’ during that time interval!
  6. But even the Zodiacs have not been untouched by the phenomenon of Precession. Due to precession, our Zodiacs have changed quite a lot since they were first used. Vernal Equinox (though called the First Point of Aries) doesn’t happen in Aries anymore but in Pisces. Neither does Autumnal in Libra (it happens in Virgo). In fact, the dates of the zodiacs (the period when the Sun lies in the region spanned by that Constellation) themselves have shifted to about a month after where they were when they were set up, along with the descent of Scorpio (that now spans only about a week) and the ascent of a new constellation on the Ecliptic, Ophiuchus.
Figure 10: Constellation Names and boundaries visible from Port-de-Bouc, France.



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Agrim Arsh

Physics Fanatic. Night Sky Enthusiast. Amateur Android Developer. High-Schooler