Earth in Motion
Just like the other planets in the solar system, the Earth moves both around the Sun, and spins about its axis. In the pages to come, we will look at these 2 basic motions of the Earth.
Rotation of the Earth
Man has observed the rising and the setting of the Sun with much curiosity since the dawn of time. Stories have been weaved about why the Sun perpetually rises in the east and sets in the west, stories that have now become myths and legends.
Past theories proposed that the Earth was at the centre of the solar system, with the Sun and Moon and other celestial bodies moving around it. To them, the Sun and Moon all rise in the east and set in the west, while the Earth appeared motionless. But there were those who chose to differ, suggesting that these celestial movements were due to the movements of the Earth.
One such non-conformist was Copernicus, who proposed that it was the Sun, and not the Earth, which was at the centre of the solar system, around which the other planets orbited. He wrote a book that contained his views and evidences on how the movements of the planets could be accounted for in a Sun-centred system.
Since Copernicus's time, accurate measurements of the Earth's rotational rate have been made. The Earth is a spheroid with a diameter of 12,756 km and thus a circumference of 40,074 km. The Earth makes a complete rotation once every 24 hours, with the equator moving at about 1,600 km/h as a result.
The sky is full of tell-tale signs that indicate that the Earth is actually rotating. By taking a long-exposure photograph of the night sky with a fixed camera, "star trails" can be captured on film. These "star trails" are actually the paths of the stars across the sky as the Earth rotates. The path of the "star trails" can be observed to be roughly circular.
By observing the weather patterns, we can also conclude that the Earth spins on its axis. The spin on the Earth makes the clouds in the weather system look like giant spirals
Certain instruments can also give indication of the Earth's spin. An example would be the Foucault pendulum. With the help of a Foucault pendulum, the rotation of the Earth can actually be witnessed. The Foucault pendulum is specially designed so that it is neither subject to any external forces like air currents, nor affected by any built-in faults, such as a string that untwists. Using a Foucault pendulum, it can be observed that its path changes gradually over a few hours. The reason is because the pendulum, once it has been set swinging, remains moving in a straight line while the Earth continues to rotate beneath it.
The Earth in Orbit
The Sun rises in the east and sets in the west. Even a five-year old could tell you this. But what many people believe, is actually wrong! The sun only rises due north and sets due east on just 2 days in the year – during the 2 equinoxes. The rest of the time, it rises and sets either to the north or to the south of due east or west.
The reason for this is due to the Earth's orbit around the Sun.
The Earth's axis of rotation points in the same direction throughout the year, but at an angle of 23 1/3 o from the perpendicular to its orbit. This gives rise to the seasons – in June, the Sun is highest in the northern sky, while in December, it is highest in the Southern Hemisphere. On one day in March and September all parts of the world have days and nights of exactly 12 hours – during the 2 equinoxes. At all other times, the day and night lengths are different depending on where you are; in temperate regions, the winter nights are long and the days are short, and vice versa in the summer.
In winter, when observed from mid latitudes, the Sun rises later than usual and never rises very high in the sky, and sets early. During summer, the opposite occurs. And while the Northern Hemisphere experiences winter, it is summer time in the Southern Hemisphere, and vice versa. Evidently, it is this axial tilt of 23 1/3 o that gives us the 4 seasons.
Al these can be witnessed by using a sundial and observing the shadow cast by the gnome (a high Sun will cast a short shadow). The length of the sun fluctuates as the height of the sun changes throughout the year. However, even the most accurately constructed sundial will be off by about 16 minutes at different times of the year. This is due to the fact that the Earth's orbit is slightly elliptical, not a perfect circle.