Secular (astronomy)

In astronomy, very small but long-term changes in the movement (orbital elements, rotation) of celestial bodies are referred to as secular (from the Latin saeculum for century ) . The most important of these secular trends are:

the slowing down of the earth's rotation and thus the increase in the length of the day (currently just under 0.002 s per century),

slow, mostly monotonous changes in the orbital elements of the earth, planets and their moons, especially with regard to the orbital planes (precession) and the apsidal lines ,
- similar also with artificial earth satellites (here, however, clearly noticeable in days to weeks),
the increase in solar luminosity (by a few percent in 500 million years) and the (weaker) decrease in its mass due to its nuclear fusion,
the expansion of the universe by about 0,000,000 01 percent per year.

The greatest effect - and already discovered by ancient astronomers - is precession , a slow displacement of the earth's axis. The other phenomena were suspected as early as the 18th or 19th century, but only proven in the last 120 years.

causes

The most important causes of such orbital disturbances and changes are deviations from the spherically symmetrical gravitational field of the sun, on which the Kepler laws are based. As a result, the orbital elements of the earth and all other planetary orbits are subject to slow changes (see three-body problem ).

The symmetry in the solar gravitational field in which the earth moves is "disturbed" mainly by the existence of the moon and the other planets. The earth's moon has the greatest effect , but also the (relatively close) Venus and the giant planet Jupiter . In addition, all rotating celestial bodies deviate from the ideal spherical shape ( flattening of the earth 0.3%, Jupiter 15%), whereby they deform their own gravitational field.

Smaller influences come from the different axes of rotation of the planets, from somewhat irregular mass distribution in their interior (as the central body of their moons), from mutual planet-moon interactions, from slight braking by interplanetary matter , from the light pressure of the sun, the gradual decrease of the solar mass and from by the general theory of relativity explainable influences.

In contrast, the Earth's rotation slows down due to the tides caused by the moon . However, they only have a braking effect under shallow seas , especially the Bering Sea .

Secular trends in the earth-moon system

On earth , secular trends of the types (1) and (2) are particularly important for astronomers and geoscientists - and through precise observations they become noticeable relatively quickly - because

the earth's rotation is the natural timer for all natural sciences and also has a decisive influence on the shape of the earth ,

the position of the earth's equator together with the plane of the earth's orbit (ecliptic) defines the most important astronomical coordinate system,

the ecliptic itself represents the reference system for all planetary orbits .

In addition, the movement of the earth also influences the orbit of the moon and vice versa.

Precession of the earth's axis

The most important secular phenomenon on earth is its precession , a regular change in direction of the earth's axis, which in around 26,000 years describes a cone shell with an opening angle of currently 23.5 °. The precession becomes noticeable as a continuous, monotonous change in the nodal line (line of intersection of the planes of the earth's equator and earth's orbit ), which serves as the coordinate axis of the celestial coordinates. This shifts z. B. all star locations parallel to the ecliptic by about 50 ″ (0.014 °) per year (see epoch (astronomy) ).

Change of the ecliptic inclination and the apsidal line

The opening angle of the above Kegels slowly changes, which manifests itself as a change in ecliptic skew. Although it appears secular over a period of millennia, it is largely a long-period change of around 41,000 years.

The position of the orbit ellipse in the plane of the earth's orbit also changes secularly (or very long-periodically like the ecliptic inclination). This can be seen in the rotation of the vestibule line (straight line between the point of the earth's orbit closest to and furthest from the sun).

Both effects together were believed to have triggered the ice ages over the past million years.

Lunar orbit

Due to the flattening of the earth (21 km to 6370 km), the position of the lunar orbit and its nodal line in space changes by around 20 ° or 40 ° per year. What appears to be monotonous in the short term is periodic when viewed from decades to centuries . The best known of these periods are the Saros cycle (18.03 years) and the almost 19-year nutation period . The orbital inclination of the moon is also somewhat variable.

In addition, there are slow drifts of some parameters, which can already be proven by measurement for the orbital axes and the period of rotation (length of the month). The slowing down of the earth's rotation has a small effect on the radius of the lunar orbit, which increases annually by about 4 cm.

In the case of artificial earth satellites , these effects of the flattening of the earth are much stronger and can be up to 20 ° per day in low orbits .

Planetary influences

The above variations are mainly caused by asymmetries in the Earth-Moon system. However, there are also influences from the other planets that can be detected today despite their small size:

Eccentricity of the earth's orbit

The orbital ellipse of the earth varies over the course of many millennia from almost circular ( eccentricity ) to slightly elliptical ( ). Some periods of about 100,000 years (see Milanković cycles ), for which the giant planets Jupiter and Saturn are mainly responsible, are superimposed on a very long-term change . ${\ displaystyle e = 0 {,} 005}$ ${\ displaystyle e = 0 {,} 058}$ ${\ displaystyle 0 {,} 028 \ pm 0 {,} 012}$

Currently, there is a seasonal variation in solar radiation from . In the history of the earth these influences could increase up to 20%, which also contributed to the rocking of some ice ages due to the unequal distribution of radiation between the climatic zones and the duration of the seasons. ${\ displaystyle e = 0 {,} 017}$ ${\ displaystyle \ pm 3 {,} 4 \, \%}$

Planetary precession

Most of the precession of the earth axis effective in the earth-moon system is caused by the gravitational effect of the moon (2 thirds) and the sun (1 third) on the non-spherical earth figure and is therefore called lunisolar precession . A small proportion (some 0.1 ″ of around 50 ″), however, comes from the other planets, with numerous periods overlapping to form a mean value.

literature

Manfred Schneider : Celestial Mechanics. Volume 4 Theory of satellite motion, orbit determination. Spektrum-Verlag Heidelberg 1999, ISBN 3-8274-0484-3 .
Karl Stumpff , H.-H. Vogt: The Fischer Lexicon Astronomy. Revised 8th edition, Fischer Taschenbuch Verlag, Frankfurt / Main 1972.