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Given the different Sun incidence in different positions in the orbit, it is necessary to define a standard point of the orbit of the planet, to define the planet position in the orbit at each moment of the year w.r.t such point; this point is called with several names: vernal equinox, spring equinox, March equinox, all equivalent, and named considering northern hemisphere seasons.
The Moon's maximum and minimum declination vary because the plane of the Moon's orbit around Earth is inclined by about 5.14° with respect to the ecliptic plane, and the spatial direction of the Moon's orbital inclination gradually changes over an 18.6-year cycle, alternately adding to or subtracting from the 23.5° tilt of Earth's axis.
Lunisolar precession is caused by the gravitational forces of the Moon and Sun on Earth's equatorial bulge, causing Earth's axis to move with respect to inertial space. Planetary precession (an advance) is due to the small angle between the gravitational force of the other planets on Earth and its orbital plane (the ecliptic), causing the plane ...
The seasons are quadrants of the Earth's orbit, marked by the two solstices and the two equinoxes. Kepler's second law states that a body in orbit traces equal areas over equal times; its orbital velocity is highest around perihelion and lowest around aphelion. [13] The Earth spends less time near perihelion and more time near aphelion.
It is about 18.6 years and the direction of motion is westward, i.e., in the direction opposite to the Earth's orbit around the Sun. This is the reason that a draconic month or nodal period (the period the Moon takes to return to the same node in its orbit) is shorter than the sidereal month. After one nodal precession period, the number of ...
An example of precession and nutation is the variation over time of the orientation of the axis of rotation of the Earth. This is important because the most commonly used frame of reference for measurement of the positions of astronomical objects is the Earth's equator — the so-called equatorial coordinate system. The effect of precession and ...
On Earth, seasons are the result of the axial parallelism of Earth's tilted orbit around the Sun. [2] [3] [4] In temperate and polar regions, the seasons are marked by changes in the intensity of sunlight that reaches the Earth's surface, variations of which may cause animals to undergo hibernation or to migrate, and plants to be dormant ...
As the Earth revolves around the Sun, approximate axial parallelism of the Moon's orbital plane (tilted five degrees to the Earth's orbital plane) results in the revolution of the lunar nodes relative to the Earth. This causes an eclipse season approximately every six months (173 days), in which a solar eclipse can occur at the new moon phase ...