This article introduces three types of spherical coordinate systems - equatorial, elliptical, and horizontal.
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Celestial Sphere
The celestial sphere is an imaginary dome upon which the stars and planets are projected. You can imagine this as the dome of a gigantic planetarium in which you are in the center.
You know that the Earth rotates around its axis. But for the earthy observer, this is the celestial sphere, which makes a revolution in 24 hours while the Earth stays unmoved.
The celestial sphere rotation axis coincides with the one of the Earth and points to the North Star or North Pole (P). Therefore, the celestial hemisphere directed toward the North Pole is called the northern hemisphere.
Spherical coordinates
In the spherical coordinate system, you express the planetary position on a celestial sphere.
- First, you divide the celestial sphere into two equal parts.
- Then, you choose the starting point (a zero-degree) on the dividing circle.
- Finally, you project the position of the star or the planet into the dividing circle
The projection arc is the first coordinate (latitude). The arc from the starting point to the projection point is the second coordinate (longitude).
Equatorial plane
A celestial equator is a plane that divides the heavenly sphere into two equal parts, separating the northern and southern hemispheres.
Right Ascension and Declination
Since the equator constantly rotates with the sphere, each equatorial degree ascends on the East. So they called the latitude degree along the equatorial plane a right ascension, RA. The name "right" is used to separate this coordinate from another, called oblique ascension, which we'll discuss below.
They used to call the latitude (a north/south deviation from the equator) a declination, dec.
When a planet ascends, its RA is usually below or above the horizon.
Astrologically, we are often interested in the equatorial degree that ascends simultaneously with the planet. This degree is called oblique ascension, OA.
The difference in degrees between right and oblique ascension is called the ascension difference, AD.
Ecliptic Plane
During the year, the Earth revolves around the Sun in its orbit. The plane of this orbit is called the ecliptic plane.
For an earthly observer, this is the Sun, who makes a complete revolution around the Earth against the background of fixed stars on the celestial sphere. The line along which the Sun moves throughout the year is called the ecliptic circle.
Celestian Longitude & Latitude
All planets in the solar system revolve around the Sun in roughly the same ecliptic plane. Even visually, you may notice in figures 1, 2, and 3 that the planets follow each other on the same line.
The ecliptic circle is the circle of the Zodiac. They used to call the latitude (a north/south deviation from the ecliptic) a celestial latitude, lat, while the zodiac degree - a celestial longitude, lon.
Inclination of the Ecliptic
Since the axis of Earth's rotation around the Sun does not point to the North Pole, the plane of the celestial equator slightly deviates from the plane of the ecliptic. They often call this deviation the inclination of the ecliptic. It is approximately 23-24° depending on the era.
Astrologers agreed to consider the intersection of the ecliptic and the equator as a starting point for equatorial and ecliptic coordinates.
When the Sun is at this point, the day is equal to the night. Indeed, all the planets and stars during the day rotate with the sphere in parallel to the celestial equator.
When the Sun is at 0° Aries, it is at the same time at the celestial equator. Therefore, on that day, the Sun moves along the equatorial line. It is plain that any equinox degree spends precisely 12 hours above the horizon and 12 hours below it. And the same applies to the Sun at 0° Aries.
Horizontal Plane, Azimuth and Altitude
Astrologers agreed to call longitude, that is, the angle of deviation from the North - azimuth and the deviation from the horizon in up / down direction - altitude.
Azimuth degrees follow clockwise, from North to East, and the body's altitude above the horizon is positive.
Here is a nice picture that represents all three types of coordinates of the same point
Bottom Line
You have learned essential terms:
- Celestial sphere
- Celestial equator
- Right ascension and declination
- Oblique ascent and ascent difference
- Ecliptic plane and ecliptic circle
- Ecliptic tilt
- Azimuth and altitude.
You also got acquainted with the concept of spherical coordinates.
You are now ready to move on with your understanding of primary directions.
