What are the Ecliptic, Equatorial, and Horizontal Coordinates?

This article introduces three types of spherical coordinate systems - equatorial, elliptical, and horizontal.

[toc]

Celestial Sphere

As we discussed in the previous post, 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).

Ecliptical Coordinates — Celestial Longitude & Latitude

Planets’ orbits are not strictly in the ecliptic plane but are slightly inclined to it. Consequently, planets can be slightly above or below the plane of the zodiac circle.

To describe a planet’s coordinates on the zodiac circle, we can use its zodiacal degree, counted from 0 Aries, and the deviation from the ecliptic plane (in degrees).

The first coordinate is celestial latitude, and the second is celestial longitude. Together, these two coordinates are called ecliptic coordinates.

Equatorial Coordinates — Right Ascension & Declination

A celestial equator is a plane that divides the heavenly sphere into two equal parts, separating the northern and southern hemispheres. The celestial equator is always perpendicular to the axis of rotation of the celestial sphere.

In astrology, it is customary to count degrees of the equator from the point of intersection with 0 Aries. In this case, the direction of the degrees of the equator coincides with the direction of the degrees of the ecliptic.

Then, the planet’s position can be described by its equatorial degree and the magnitude of deviation from the equator. The first coordinate is right ascension, and the second one is declination. Together, they form equatorial coordinates.

Horizontal Coordinates — Azimuth & Altitude

Finally, you can measure the position of a celestial body relative to the horizon. The projection of the planet onto the horizon is called azimuth. Usually, azimuth is a degree of the horizon relative to the true north. The elevation of the planet above the horizon (in degrees) is called altitude. The pair of azimuth and altitude form the horizontal coordinates.

Oblique Ascension & Ascension Difference

At the moment the planet rises over the horizon, we observe which degree of the equator is ascending. We call that degree an ascending coordinate of the planet. Let’s assume we observe the rising of a planet at zero geographic latitude. Here, the Earth’s rotation axis coincides with our local horizon and points directly north. Meanwhile, the plane of the celestial equator is perpendicular to the horizon.

In that case, the rising degree of the equator coincides with the projection of the planet onto the equatorial plane, which is at a right angle to the horizon. Hence the name of this equatorial coordinate - the right ascension.

However, such a scenario occurs only at zero latitude. A different ascending equatorial degree will mark the planet’s rising if we find ourselves further north. And this ascending coordinate of the planet will not coincide with the place of its projection. This ascending coordinate is called the oblique ascension.

Thus, the planet has two equatorial coordinates—its right ascension (a universal coordinate) and its oblique ascension (a coordinate associated with local latitude).

The difference between these two coordinates is called the ascension difference (AD). It equals zero when we are at the equator.

Bottom Line

You have learned essential terms:

• 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.