Volume 15 Issue 4April 2009
What's Up? -- April 2009
Around the Sun: Conjunction, Quadrature and Opposition
    The angle separating a planet from the Sun, elongation, is based on measurements made from the Earth, and is used to describe four positions along a planets orbit. Inner planets have elongations that range from 0 to 90 degrees, while the outer planets can have elongations that range from 0 to180 degrees.
    Using the term elongation we can see that if a planet is on an imaginary line from the Earth to the Sun then its elongation is 0 and the planet is described as being in conjunction with the Sun. Inferior conjunction is for inner planets and is when the planet is between the Earth and the Sun, while superior conjunction occurs when the planet, either inner or outer, is on the opposite side of the Sun from the Earth.
    Quadrature of a planet occurs when an outer planet has an elongation of 90 degrees. For example this month Mars is at its western quadrature on the 18th and Jupiter reaches its eastern quadrature on the 29th.
    Opposition of a planet occurs when an outer planet has an elongation of 180 degrees. No planets are in opposition this month but when a planet is in opposition it rises at sunset and sets at sunrise, thus providing more viewing time than when the elongation is less.
    Interestingly our moon shares these orbital positions with the planets. At new phase the moon is in conjunction with the Sun; first quarter it has reached quadrature; full phase it is in opposition; and by last or third quarter is at quadrature again.
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