Evection

Not to be confused withEviction.

Main article:Lunar theory

Inastronomy,evection(Latin for "carrying away" ) is the largestinequalityproduced by the action of theSunin themonthlyrevolution of theMoonaround theEarth.The evection, formerly called the moon'ssecond anomaly,was approximately known in ancient times, and its discovery is attributed toPtolemy.The current name itself dates much more recently, from the 17th century: it was coined byBullialdusin connection with his own theory of the Moon's motion.

Evection causes the Moon'secliptic longitudeto vary by approximately ± 1.274° (degrees), with a period of about 31.8 days. The evection in longitude is given by the expression$+4586.45''\sin(2D-\ell )$,where$D$is the mean angular distance of the Moon from the Sun (itselongation), and$\ell$is the mean angular distance of the Moon from its perigee (mean anomaly).

It arises from an approximately six-monthly periodic variation of theeccentricityof the Moon's orbit and alibrationof similar period in the position of the Moon'sperigee,caused by the action of the Sun.

The evection opposes the Moon'sequation of the centerat the new and full moons, and augments the equation of the center at the Moon's quarters. This can be seen from the combination of the principal term of the equation of the center with the evection:$+22639.55''\sin(\ell )+4586.45''\sin(2D-\ell ).$

At new and full moons, D=0° or 180°, 2D is effectively zero in either case, and the combined expression reduces to$+(22639.55-4586.45)''\sin(\ell ).$

At the quarters, D=90° or 270°, 2D is effectively 180° in either case, changing the sign of the expression for the evection, so that the combined expression then reduces to$+(22639.55+4586.45)''\sin(\ell )$.

Bibliography

• Brown, E.W.An Introductory Treatise on the Lunar Theory.Cambridge University Press, 1896 (republished by Dover, 1960).
• Brown, E.W.Tables of the Motion of the Moon.Yale University Press, New Haven CT, 1919, at pp. 1–28.
• H Godfray,An Elementary Treatise on the Lunar Theory,(London, 1871, 3rd ed.).
• O Neugebauer,A History of Ancient Mathematical Astronomy(Springer, 1975), vol. 1, at pp. 84–85.
• R Taton & C Wilson (eds.),Planetary astronomy from the Renaissance to the rise of astrophysics, part A: Tycho Brahe to Newton,(Cambridge University Press, 1989), at pp. 194–195.