from UniverseToday Website
and its relation to the rotation axis and plane of orbit.
But thanks to the ongoing efforts of
astronomers, we have come to understand that it is in fact a sphere,
and one of many planets in a system that orbits the Sun.
In addition to knowing the precise distance from the Sun, we also know that our planet orbits the Sun with one pole constantly tilted towards it.
Such a tilt results in there being a difference in how much sunlight reaches a given point on the surface during the course of a year.
the northern hemisphere's North Star,
and south to dim Sigma Octantis.
Credit: Bob King
In the case of Earth, the axis is tilted
towards the ecliptic of the Sun at approximately 23.44° (or
23.439281° to be exact).
When the North Pole is pointed towards
the Sun, the northern hemisphere experiences summer and the southern
hemisphere experiences winter. When the South Pole is pointed
towards the Sun, six months later, the situation is reversed.
In winter, the days become shorter and the Sun is lower in the sky. In northern temperate latitudes, the Sun rises north of true east during the summer solstice, and sets north of true west, reversing in the winter.
The Sun rises south of true east in the
summer for the southern temperate zone, and sets south of true west.
In the southern hemisphere, the
situation is reversed, with the South Pole oriented opposite the
direction of the North Pole and experiencing what is known as a
"midnight sun" (a day that lasts 24 hours).
In the northern hemisphere,
In the southern hemisphere, the situation is reversed, with the summer and winter solstices exchanged and the spring and autumnal equinox dates swapped.
However, Earth's axis does undergo a slight irregular motion known as nutation - a rocking, swaying, or nodding motion (like a gyroscope) - that has a period of 18.6 years.
Earth's axis is also subject to a slight
wobble (like a spinning top), which is causing its orientation to
change over time.
When this happens, summer will occur in
the northern hemisphere during December and winter during June.
Precession of the
and the precession of the Equinoxes.
Essentially, this is a phenomena where
the dates of Earth's perihelion and aphelion (which currently take
place on Jan. 3rd and July 4th,
respectively) change over time. Both of these motions are caused by
the varying attraction of the Sun and the Moon on the Earth's
During the Scientific Revolution, it was a huge revelation to learn that the Earth was not a fixed point in the Universe, and that the "celestial spheres" were planets like Earth.
But even then, astronomers like
Copernicus and Galileo still believed that the Earth's
orbit was a perfect circle, and could not imagine that its rotation
was subject to imperfections.
And what we know is that they lead to
some serious variations over time - both in the short run (i.e.
seasonal change), and in the long-run.