THE MOTION OF THE EARTH
The Motions of the Earth have been a topic of confusion for most of the time in the history of astronomy. For the longest time in the history of THE ASTRONOMY the Earth was assumed to be stationary and everything else in the universe was assumed to be revolving around it.
This idea is known as the geocentric model proposed by Claudius Ptolemy around 100AD. The stars were supposed to be fixed on the celestial sphere whose center was Earth. however, some of them were changing their position daily, those were the planets.
The name planet itself came from the GREEK word planets means wanderers. However, after the introduction of the Heliocentric model given by Nicolaus Copernicus (1473–1543) the perspective of seasonal changes and other phenomena was changed completely.
There are several types of orbital motion of the Earth and as well as other planets. So, let’s discuss it one by one and find the phenomena causing them.
ALL of us might be aware of the 24-hour cycle, of SOLAR DAY, is our basic social unit of time. This apparent progression of the Sun and other stars across the sky, known as DIURNAL MOTION, is a consequence of Earth’s rotation.
But the stars do not repeat their exact position from night tonight. There is always a small shift. Because of this shift, a day measured with respect to a star(other than the Sun) is called a SIDEREAL DAY which differs in length from a SOLAR DAY(w.r.t. the Sun).
The reason behind this difference is shown in the picture below. The Earth moves simultaneously in two ways, it rotates on its axis while revolving around the Sun.
Therefore after rotating a full cycle on its axis, it moves a small angle on orbit around the Sun, due to which it completes its sidereal day ahead of its solar day.
We can calculate this difference easily: Earth takes 365 days to orbit the Sun which is a 360° angle around the Sun. So, for one day it takes 360°/365 =0.986° extra angle to complete, which takes 3.9 minutes extra for Earth to get the Sun on its exact previous position. Therefore the solar day is 3.9 minutes longer than the sidereal day.
The previous motion we discussed is unnoticeable for a human in practical life.
But the seasonal changes are very easy to notice. If you are a keen observer of the sky, then you might have noticed that some constellations in the night sky are only visible for a specific interval in a solar year. Some examples are Scorpio, Virgo, Gemini, etc.
They, collectively, are called the zodiac. The reason behind this phenomenon is just a coincidence that these constellations lie in the plane of the orbit of the Earth around the Sun, and around different months different constellations are on the opposite to the Sun w.r.t. the Earth.
There is 12 zodiac and each of them shows for approximately one month. So, the ancient Astrologers (not Astronomers) connected this idea to our birth month and made it the symbol of fate (which we know is totally fake).
CHANGES IN SEASONS
The axis of the Earth is tilted by the angle of 23.5° w.r.t. plane of its orbit around the Sun. therefore, for six months (21 sept-21 march) the southern hemisphere is closest to the Sun and the rays of the Sun have to cross the thinnest layer of the atmosphere (perpendicular to the surface) so, most of its energy reaches the surface.
While the northern hemisphere makes some angle to the Sun rays, therefore, energy has to cross the thickest layer of the atmosphere (almost parallel to it). For the other 6 months (21 march – 21 Sept) the northern hemisphere is closest to the Sun.
The time at which the north pole is closest to the Sun is called summer solstice and when the southern pole is closest to the Sun it’s called the winter solstice. There becomes situations when both the north and south poles are equidistant from the Sun, it’s called equinox.
It happens twice a year autumnal equinox(21 Sept) and the vernal equinox(21 march). At the equinox the lengths of day and night are equal. The time interval between two vernal equinoxes is called TROPICAL YEAR.
There is a misconception that season changes because of the variation of distance between the Earth and the Sun. however, this variation has nothing to do with it because this variation is about 3% which is insignificant to change the temperature of the Earth.
LONG TERM CHANGES
The time required by the Earth to complete its orbit around the Sun w.r.t. any other star is called sidereal year it’s 365.256 days and about 20 minutes longer than the tropical year. The reason behind it is the phenomenon called precession.
You might have seen this phenomenon, in a spinning top when it’s slowing down its axis starts to rotate around the vertical, making a conical shape. Similarly, It happens to the Earth. However, this rotation is very slow and takes around 2600 years to complete one cycle. Because of this The Polaris the pole star won’t remain absolutely north.
After a century or so it will go further away from Earth’s north pole.