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in astronomy, the characteristic relative positions of the sun, planets, moon, and other bodies of the solar system on the celestial sphere. Of greatest interest are the planetary configurations (see Figure 1).
In the case of Mercury and Venus, which are known as the inferior planets because they revolve within the earth’s orbit, we distinguish between superior and inferior conjunctions with the sun. At these conjunctions, the planet and the sun have the same celestial longitude. Eastern and western elongations correspond to the greatest apparent angular distance of the planet from the sun. The inferior planets at superior conjunction are situated behind the sun, and being hidden by its rays are inaccessible to observation. Directly before inferior conjunction and after it, the inferior planets are visible as a narrow crescent. In some cases, at inferior conjunction, such planets can pass across the sun’s disk (transit of planets across the sun’s disk). At the elongations, the inferior planets have the form of a bright half-disk. The values of the angular distances of the planets at the moment of elongation vary somewhat because of the ellipticity of the planets’ orbits. For Mercury it is about 28°, for Venus about 48°.
For superior planets (planets whose orbits are outside the earth’s orbit), we distinguish between conjunction with the sun, opposition, and eastern and western quadratures. At conjunction with the sun, the planet has the same longitude as the sun. At opposition, the celestial longitudes of the planet and the sun differ by 180°. Finally, at the eastern and western quadratures, the difference between the celestial longitudes of the planet and the sun measures 90°, with the planet in the first case located to the east of the sun, and in the second, to the west of the sun. The superior planets are not visible near the conjunctions. The best conditions for their observation are near the oppositions, when the planets are closest to the earth and have the sunlit hemisphere presented to it.
The configurations of the moon, asteroids, comets, and space probes are determined in the same manner as the planetary configurations. The configurations of the moon are characterized by the lunar phases. For example, a new moon begins at the conjunction of the moon with the sun and a full moon begins at the opposition. The first and last quarters are observed when the moon is at the eastern and western quadratures, respectively. Data on all the planetary configurations, conjunctions of the planets with the moon, and the conjunctions of the planets with each other, as well as the phases of the moon, are published in astronomical yearbooks.