VOYAGE OF THE EARTH AROUND THE
SUN
A discussion of the evolution of our universe after the Big Bang is not the scope of this short note. Also a discussion of the present state of the universe, or our Galaxy (Milky Way) or even our solar system could not be satisfactorily completed by a note like this. The main purpose of this article is to discuss briefly the most important facts of the movement of our planet, the Earth, around the Sun because authors of some articles and media presentations provide wrong interpretations and inaccurate statements about the atmospheric characteristics without proper understanding of our voyage around the Sun.
Path, or passage, of our planet, the Earth, is not a circle, but it is an ellipse. While we are moving around the Sun along this elliptical path, the Sun relatively stays at one of the two focuses (foci) of the ellipse. In reality, even the sun does not stay in a fixed position but it moves in the galaxy. If the whole universe is considered, all bodies of the universe are moving continuously as all of them are parts of the still expanding universe.
Let us limit our discussion towards the relative movement of the Earth. As we learn about the ellipse in mathematics, it has two axes, major and minor, which cross at the center of the ellipse. Two focuses with specific characteristics are two points on the main axis. One can follow the instruction of the Activity No. 2.1.1 in the grade 9 Science book provided by the Educational Publications Department to understand what an ellipse is. It instructs ’Fix a white sheet of paper on a wooden board by means of drawing pins. Place two pins apart from each other on the paper. Take a piece of a thread, tie two ends to form a loop, and pass it around the two pins. Now use the pencil and draw an ellipse with the thread tightly stretched. The points where the pins were fixed will be the two centres of the ellipse ’.
Even though the two positions of pins are named as ‘centres’, the real terminology is ‘focuses’. An ellipse drawn according to the instruction will look like one in Figure 1. In the figure 1, AB and CD are main axis and minor axis respectively. Two focuses are marked as F1 and F2.
Even though the relative motion of the Earth and the Sun are three dimensional, they are represented by two dimensional figures and diagrams in most of documents. Therefore, the real facts of our voyage around the Sun are not reflected clearly. In many diagrams the Sun is marked at the center of the ellipse but not at a focus and this also gives an inaccurate picture of the motion of the earth.
As the Sun is located at one focus of the elliptical path of the Earth, the distance between the Earth and the Sun changes continuously. The earth travels more than one hundred thousand kilometers in a hour around the Sun, the shortest distance being during 3-5 January and the longest distance on around 5th July (Figure 1).
The movements of any universal body are not as simple as we observe them and the movement, vibration and wobbling of such objects and particles are very complicated. For example, our planet, the Earth, rotates one round around its own axis in nearly 24 hours while travelling around the Sun. Also its axis has a continuous wobble within an angle about 3 degrees. Earth axis make an angle of 23.5 degrees with the perpendicular direction of the plane of its motion making the voyage more complicated. All these parameters will decide the environmental conditions around us, not only the distance from the Sun which radiates heat as a hot body with temperature about 6000 degrees Kelvin.
Sun does not rise overhead to all the spots on the earth every day and amount of incoming radiation at any spot is not a constant throughout the year. It rises directly overhead to parts of landmass of Sri Lanka in April and again in August – September. But the Sun is not at close distances in these periods. So the warmness of our atmosphere in April is not a matter affected by the distance but a number of other parameters. Similarly, being at very close to the sun in December-January, we feel cold and comfortable climate due to a number of other parameters.
One has to consider a number of facts, not only the distance between Sun and the Earth before making statements about environmental conditions around us.
K. R. Abhayasingha
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