The Earth is a spinning top

The Earth is not motionless in space, but is subjected to different movements. The most well-known are the rotational movement around its own axis, which determines the alternation of day and night and the apparent movement of the sky above our heads, and the revolution around the Sun in a slightly elliptical orbit. The two main units of time, days and years, derive respectively from the rotation and revolution movements.
The length of a day can be measured as the time interval between two consecutive transits of the Sun or of a given star on the same meridian. The former is called solar day and lasts 24 hours; the latter is known as sidereal day and lasts about 4 minutes less. The difference between the two periods derives from the fact that the Earth rotates around its own axis as it is moving along its orbit. In doing so, it varies the alignment with the Sun and this entails an additional time to reach the same Earth-Sun orientation once again; this does not occur for the other stars that are so distant that they can be considered fixed.
When measuring a year, things get more complicated. In fact, the sidereal year measures the time interval between two successive alignments of a given star with the Earth, and it corresponds to a complete revolution of our planet around the Sun respect to the “fixed” stars. There is also the solar year which represents the time interval between two successive passages of the Sun through the vernal equinox. The latter is one of the two points of interception of the ecliptic and the celestial equator, an extension of the plane of the terrestrial equator into space. If the Earth’s orbit were immovable in space, these two definitions would coincide. However, in actual fact, bodies have a reciprocal influence on each other within the Solar System; hence, the gravitational attraction of the Sun and the planets modifies the terrestrial movements in relation to their mass and the distance away from the Earth. As a consequence, the Earth’s rotational axis describes a slow movement in the opposite sense respect to the orbital one, tracing a cone-shape in the lapse of time of 26,000 years. This cone-shape is also influenced by nutation, an oscillation with a period of 18 years generated by the Moon’s gravity. The sum of movements causes the slow migration of the North Celestial Pole, currently pointing at the Pole Star in the Little Bear constellation, towards different stars; in 15,000 years, for example, the Vega Star in the Lyre constellation will indicate the North. As the axis wobbles, so does the equatorial plane that is perpendicular to it, modifying some of its orbital parameters; as a result, every year the Earth reaches the equinoxes, i.e. the intersections of the equatorial and ecliptic planes, earlier. Hence the solar year differs from the sidereal year by about 6 hours and for this reason it has been necessary to introduce one day every four years to make up for the difference. This explains leap years and February 29. Time can also be measured in months, which are tied to the cycle of phases of our satellite. In fact, the Moon’s orbit around the Earth is inclined by 5° respect to the plane of the ecliptic. This implies that we can observe the different phases in which our satellite is illuminated by the Sun’s rays; as it changes from new moon to full moon, the lunar phase cycle takes a lunar month (28 days). In the points in which the lunar orbit intersects the Earth’s, the Sun, Earth and Moon are aligned and eclipses occur. There is a solar eclipse when the Moon passes between the Sun and the Earth: it is a coincidence that the small lunar disc is at the right distance in perspective to cover the gigantic Sun. If the Earth is in between the other two bodies instead, a lunar eclipse occurs.

Special reports

From the Multimedia section

Facts

  • 17 May 2011

    Neutrinos

    Neutrinos are extremely light neutral particles...

    The comet’s tail

    Comets are “dirty snowballs” that spend most of their life at the edges of the Solar System...

  • The definition of a dwarf planet

    Astronomy, like all scientific disciplines, is continuously evolving...

    The PP cycle

    Nuclear fusion within the stars begins with the different passages...

  • Children of the stars

    We are made of star dust. The iron in our blood, the oxygen we breathe...

    The Olbers paradox

    Why is the sky dark at night? What seems like an obvious question...