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oblateness: how much a planet has been flattened at its poles by its rotational speed. If the Earth’s size and rigidity were proportionately reduced, it would be similar to a basketball of butter. Now for all spinning bodies, the part furthest from the spin axis spins the fastest, like the person at the end in crack the whip. And since the parts of a planet at its equator are furthest from its pole (which is its spin axis), it follows that a planet’s equatorial part has the greatest tendency to fly away. Thus all spinning heavenly bodies are wider through their equators (their "equatorial diameter") than through their poles (their "polar diameter"). This effect increases as a planet spins faster, and decreases the denser the planet is. The Earth, with a density of 5.5 gm/cm3, has an equatorial diameter 27 miles greater than its polar diameter, or an oblateness of 0.3%. Saturn, a gas giant that would float in water with a density of only 0.687 gm/cm3 (the density of water is 1 gm/cm3 ), is 8078 miles thicker through its equator than through its poles. Its oblateness is 10%. If a planet’s equatorial diameter is De and its polar diameter is Dp, then its oblateness is defined as (De - Dp)/De .

obliquity: the angle between a planet’s axis and the plane of its orbit (see pages 42 and 43). Unless acted upon by an outside force, a spinning body’s spin axis never changes the direction it points in space as it orbits its parent body. This is the principle by which a gyroscope (a spinning body) directs and stabilizes a vehicle in flight.

occultation: when a body passes between another body and an observer, hiding the second body from view. Most often used when the Moon passes in front of a star, since the Moon has no atmosphere, the occultation is clear and can be timed.

on the (house) cusp: when a house cusp falls within a sign, that sign is said to be on (or to "fall" on) the cusp of that house. That sign and its ruling planet are then said to rule the affairs of that house.

Oort cloud: A Dutch astronomer, Jan Oort, observed in 1950 that no comet had ever been seen with an orbit indicating that it came from interstellar space, nor did comets seem to have a preferred direction of origin. He therefore hypothesized that the Sun is surrounded by an immense cloud of comets, the Oort cloud, extending from about 1200 times Pluto’s distance from the Sun out to about three light years. (The nearest star, Proxima Centauri, is 4.25 light years away.) So distant that its members are only weakly bound to the Sun, the Oort cloud is occasionally disturbed by passing stars that fling bits of it either into the inner solar system as comets or out into interstellar space. The Oort cloud is thought to contain as many as a trillion comets, but since they are so small and at such a great distance, there is no direct evidence that it actually exists at all.

opposition: the 180° aspect (orb 10-12°) denoting change, polarity, awareness, objectivity, and often a particular way of relating. The second strongest aspect, it symbolizes a conflict between an internal (initially not seen as such) and apparently external energy—that one fails to see in oneself, and, like the seventh house, projects onto the outer world. The opposition signifies others we bring into our life to externalize unconscious or rejected aspects of our own being. Accepting these is the growth in consciousness the conflict provides. When an opposition is resolved the energy of the opposing planets is then released and available.

orb: the greatest deviation an aspect can have from being exact and still be in effect. Orbs are highest for the Sun and Moon, smaller for the inner planets, and smallest for the outer planets (see the table on page three). Orbs decrease as the aspect’s harmonic number increases (e.g., the orbs for the semisextile and conjunction of the Sun and Moon are respectively 2° and 12°).

orbital eccentricity: the deviation of an orbit from a perfect circle: an eccentricity of 0 is a perfect circle, while an eccentricity of 1 is a straight line. Only Mercury (0.20) and Pluto (0.25) have orbital eccentricities > 0.1, and of the rest, only Mars’ orbital eccentricity > 0.05 (see the tables on pages 102 for the planets, page 147 for the asteroids, and page 206 for TNOs). Eris, Makemake, Haumea and Sedna, on the other hand, have respective orbital eccentricities of 0.44, 0.16, 0.19 and 0.855. This 0.855 means Sedna’s orbit is long and narrow: its maximum distance from the Sun (aphelion) is 91 billion miles, but its closest approach (perihelion) is only 7 billion miles.

orbital inclination: the angle between the plane of a body’s orbit and the plane of the ecliptic (the Earth’s orbit). All the planets except for Mercury (7°) and Pluto (17°) have orbital inclinations < 3½°. Eris, Makemake, Haumea and Sedna, on the other hand, have respective orbital inclinations of 44°, 29°, 28° and 12° (see the tables on pages 102 for the planets, page 147 for the asteroids, and page 206 for TNOs).

orbital resonance: The time it takes an orbiting body to revolve once around its parent body is called its period. For example, the Earth’s (sidereal) period around the Sun is 364¼ days; the Moon’s (synodic) period about the Earth is 29½ days. When the periods of two bodies orbiting about a common parent are in a small integer ratio to each other (like Pluto completing 2 orbits in the time it takes Neptune to complete 3, a ratio of 2:3), an orbital resonance occurs between the two bodies. They then exert a regular, periodic, and greatly enhanced mutual gravitational influence on each other. This usually results in an unstable interaction in which the two bodies exchange momentum and change orbits until a resonance no longer exists. Unstable resonances between small ring particles and Saturn’s inner moons are resolved in favor of the larger body, thus causing the gaps in Saturn’s rings. When celestial bodies have the same or similar orbital radii, this special case of a 1:1 resonance causes large Solar System bodies to "clear the neighborhood" by ejecting nearly everything else around their obits; this effect was used in the IAU’s 2006 definition of a planet. Sometimes resonant systems can be stable and self correcting so that the bodies remain in orbital resonance. The 2:3 resonance between Pluto and Neptune and the 1:2:4 resonance of Jupiter’s moons Ganymede, Europa, and Io are examples of stable orbital resonances. Please refer to pages 144-145 for the asteroids’ orbital resonances with Jupiter, and to the diagram on the bottom of page 207 for TNO orbital resonances with Neptune.

Orrery: a mechanical device showing the motions and positions of the planets and moons. Usually driven by a clockwork mechanism, the Sun was at the center and planets moved at the end of arms extending from the center. The earliest known Orrery is the Antikythera Mechanism (c. 100 BC) found in 1901 off the Greek isle of Antikythera. The first in the modern era was made in 1704 and presented to the Earl of Orrery, hence its name.

outer planets: Uranus, Neptune, and Pluto. Since they remain 7, 14 and 12-33 years respectively in a sign, their effect by sign is generational, collective and social rather than individual. Their effect by house, however, is very individual, particularly in the angular houses. (Pluto stays 12-33 years in a sign because its orbit is so highly elliptical.)

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