Short essay on Astronomical or Orbital Theories

As we know, the sun is the primary source of heat for the earth. There are three periodic deviations in the relative position of the earth and sun which affect the seasonal distribution of solar radiation on the earth.

These include the eccentricity of the earth’s orbit about the sun, changes in obliquity of the earth’s axis, and the precession, that is, the wobbling of the earth’s axis.

Although the effect of solar distance is of little significance in current seasonal temperature fluctuations, it plays an important role in producing large scale climatic changes (secular climatic variations).

In perihelion the sun is nearest to the earth, while in aphelion it is farthest from the earth. The difference in distance is not more than 3 per cent. This difference means that there is a variation of 6% of the solar radiation received by the earth. But this is not always so.

The earth’s orbit undergoes a change of shape during a cycle of 90,000 and 100,000 years. Sometimes the orbit forms a longer ellipse, and then it returns to a more circular shape.

At the time of the greater eccentricity of the earth’s orbit, the amount of solar radiation received at the earth’s surface at perihelion may be 20 to 30 per cent greater than at aphelion. Variations of this magnitude in the solar radiation would mean different climate from what it is at present.

As a result of the variation in the angle between the earth’s axis and the ecliptic, the Tropics and Polar Circles are displaced. At present, the earth’s axis is inclined by 23.5 degrees.

But according to scientists, this angle undergoes a change. The inclination of the axis varies from 22.1 degrees to 24.5 degrees during a cycle of about 41,000 years. Smaller changes in the inclination of this angle bring about smaller temperature differences between winter and summer.

If the temperature difference between the two seasons is reduced, the ice sheets get an opportunity to grow in thickness and extent. Warmer winters because more snow to fall, since higher temperatures mean increased capacity of air to hold moisture.

On the other hand, cooler summer temperatures do not allow much snow to melt. This would lead to the growth of ice sheets.

Like a spinning top, the earth rotating on its axis wobbles. At present, the axis of the earth points towards the North Star. But by the year 14,000 A.D. the axis will be pointing towards the star Vega, which will then be called the North Star.

Since the period of precession is roughly I about 26,000 years, Polaris will once again be the North Star by the year 27,000. Due to this cyclic wobbling of the axis, climatic change of great magnitude takes place.

As a result of this tilt of the axis towards Vega in about 12,000 years, the northern hemisphere will experience winter when the sun is farthest from the earth (aphelion) and summer will occur when the earth is nearest to the sun (perihelion).

In this way, summers will be warmer and winters colder than at present. It is to be remembered that because of this astronomical factor the variation in the amount of insolation is insignificant.

It is in fact the seasonal contrast in temperature brought about in this way that is important. It is interesting to note that the astronomical theory of Milankovitch once gained wide support and subsequently lost it. But in the light of recent researches it has regained its popularity.

Recently the studies of deep-sea sediments have lent wide support to the astronomical theory. By applying the technique of oxygen isotope analysis, a chronology of temperature variations for the last 450,000 years was established.

The astronomical calculations of the three orbital variables – eccentricity, obliquity and precession were compared with the sequence of climatic changes to determine the correlation.

It was found that the correlation did exist and the climatic variations were closely linked with changes in obliquity, precession and eccentricity of the earth’s orbit.

It was established that changes in the earth’s orbital geometry are the main cause of the succession of quaternary ice ages. The plate tectonic theory has lent further support to the astronomical theory of climatic changes.