nutation
Noun
Meaning
The act of nodding.
A very small libratory motion of the earth's axis, by which its inclination to the plane of the ecliptic is constantly varying by a small amount.
The motion of a flower in following the apparent movement of the sun, from the east in the morning to the west in the evening.
Circumnutation.
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A pure nutation is a movement of a rotational axis such that the first Euler angle is constant.
Al-Batani's value, 54.5 arc-seconds per year, compares well to the current value of 49.8 arc-seconds per year (26,000 years for Earth's axis to round the circle of nutation ).
The component perpendicular to the ecliptic is known as the nutation in obliquity.
In simpler terms, nutation (and precession) values are important in observation from Earth for calculating the apparent positions of astronomical objects.
The component that works along the ecliptic is known as the nutation in longitude.
The true or instantaneous obliquity includes the nutation.