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In astronomy, a planet's elongation is the angle between the Sun and the planet, with Earth as the reference point. The greatest elongation of a given planet occurs when this inner planet’s position, in its orbital path to the Sun, is at tangent to the observer on Earth. Given the planets able to elongate are well within the area of Earth's orbit of the Sun, observation of such a phenomenon should not pose that much a challenge, compared to deep sky objects, for example. When a planet is at its greatest elongation, it is farthest from the Sun as viewed from Earth, so its view is also best at that point. When an inferior planet is visible after sunset, it is near its greatest eastern elongation. When an inferior planet is visible before sunrise, it is near its greatest western elongation. The value of the greatest elongation (west or east), for Mercury, is between 18° and 28°; and for Venus between 45° and 47°. This value varies because the orbits of the planets are elliptical, rather than perfect circles. Another minor contributor to this inconsistency is orbital inclination: each planet's orbit is in a slightly different plane. Refer to astronomical tables and websites such as (heavens-above ) to see when the planets reach their next maximum elongations. == Elongation period == Greatest elongations of a planet happen periodically, with a greatest eastern elongation followed by a greatest western elongation, and ''vice versa''. The period depends on the relative angular velocity of Earth and the planet, as seen from the Sun. The time it takes to complete this period is the synodic period of the planet. Let ''T'' be the period (for example the time between two greatest eastern elongations), ''ω'' be the relative angular velocity, ''ω''e Earth's angular velocity and ''ω''p the planet's angular velocity. Then : where ''T''e and ''T''p are Earth's and the planet's years (i.e. periods of revolution around the Sun, called sidereal periods). For example, Venus's year (sidereal period) is 225 days, and Earth's is 365 days. Thus Venus' synodic period, which gives the time between two subsequent eastern (or western) greatest elongations, is 584 days. These values are approximate, because (as mentioned above) the planets do not have perfectly circular, coplanar orbits. When a planet is closer to the Sun it moves faster than when it is further away, so exact determination of the date and time of greatest elongation requires a much more complicated analysis of orbital mechanics. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Elongation (astronomy)」の詳細全文を読む スポンサード リンク
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