Caption: The diagram illustrates the ancient astronomy lore of planetary configurations.

Features:

1. Definition: A planetary configuration is as an especially significant apparent position of a planet (i.e., its angular position position as seen from the Earth) relative to the Sun projected onto the ecliptic plane.

   A second definition---which we do NOT use here---is just the apparent position of a planet at any time relative to the Sun (see Merriam-Webster definition: planetary configuration).

2. The planets are inferior planets---lowly, degraded planets---if they are inward of the Earth's orbit and superior planets---lordly, conceited planets---if they are outward of the Earth's orbit.

3. The planetary configurations shown are conjunction (inferior conjunction, superior conjunction), opposition, greatest elongation (greatest eastern elongation, greatest western elongation), and quadrature (eastern quadrature and western quadrature).

4. Conjunction is when a planet is aligned with the Sun on the sky: i.e., on the Earth-Sun line. An inferior conjunction is when it is nearer than the Sun and superior conjunction when it is farther.

   Superior planets can only have superior conjunction which are then usually just called conjunctions.

5. Opposition is when a superior planet is 180° away from the Sun on the sky which means that it is opposite the Sun on the sky. A planet in opposition rises when the Sun sets and sets when the Sun rises.

   Inferior planets can never be in opposition.

6. Greatest elongation is when an inferior planet is as far from the Sun in angle on the sky as it can be. Superior planets do NOT have greatest elongations. They have oppositions.

7. Quadrature is when a superior planet is 90° away from the Sun.

   Inferior planets do NOT have quadratures.

8. The diagram omits syzygy, good old syzygy, old syzygy---an alignment of 3 astro-bodies in a gravitationally bound astronomical system.

9. The diagram also omits apparent retrograde motion which can be considered a planetary configuration if one stretches the terminology a bit.

10. The importance of planetary configurations:

    1. Historically, they were important in establishing the orbital elements (AKA parameters) of the planets.

       The orbital elements describe the planet orbits and allow calculations of the positions and motions of the planets: i.e., planetary ephemerides. Nicolaus Copernicus (1473--1543) knew this very well.

    2. In particular, historically, Nicolaus Copernicus (1473--1543) used planetary configurations to determine the orbital radii of the planets which was possible using the heliocentric Solar-System model, but NOT using the geocentric Solar-System model. These determinations were a triumph of heliocentric Solar-System model.

    3. The planetary configurations still have importance since at least some of them allow important observations to be made. For example, superior planets (planets outward from the Earth) are usually closest to the Earth when in opposition. So the superior planets can be especially well observed during oppositions.

    4. They are part of the ancient lore of astronomy.

11. See also Planetary configurations keywords below (local link / general link: planetary_configurations_keywords.html):

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