Celestial Phenomena

Planets and their motion

Every planet in our Solar System rotates around an imaginary line connecting its two poles, called its Axis. The spinning motion makes the sun appear to move from east to west. Our days and nights are therefore determined by this spinning motion, known as the Rotation of the planet.

The Rotation Period– the time it takes to make one spin – varies from planet to planet. On earth we include one day and one night together to make one Solar Day, which is 24 hours. We call this one Earth-day. Every planet has a different solar day. Venus spins the slowest – it takes the equivalent of 243 earth-days for Venus to spin once. Jupiter spins the fastest – in only 10 hours. So, a solar day on Venus is 243 times that of Earth’s solar day. And a solar day on Jupiter is only 0.42 times that of earth’s solar day.

Every planet also moves around the sun in an elliptical orbit and in the same direction. As a result, every planet is closer to the sun at some points in time and far away at some other points. The closest and farthest points of a planet from the sun are called the Perihelion and Aphelion. This movement around the sun, called a Revolution, determines the Solar Year for the planet. On earth, this takes 365 earth days. Mercury, being the closest planet to the sun, has a solar year of just 88 earth days – it takes only that long to complete one revolution around the sun. A solar year on Neptune, the farthest planet, would last the equivalent of 60,188 earth days!

Solar Eclipses

Eclipses can also occur of the sun. This happens when the disc of the moon passes right across the sun’s surface. Although the moon is much smaller in size (diameter) than the sun, it is much closer to the earth than the sun. Therefore, it can physically block out the sun for a while.

From where you are standing on earth, either the entire surface of the sun could be blocked –called a “Total Solar Eclipse” – or only some part of it is blocked – called a “Partial Solar Eclipse”.

Other Orbiting Bodies

Imagine a humongous snowball hurtling through space leaving a long tail of gas and dust in its wake. That’s a Comet. There are millions, possibly billions, of such comets in the universe but science has so far counted only about 3,653 of them. They most likely originate in distant reaches of our Solar System, areas known as the Oort Cloud and the Kuiper Belt. Some may even come from outside our solar system – interstellar visitors!

Comets too circle our sun in extremely elliptical orbits, which means that they have an extremely large aphelion as compared to their perihelion. They may be outside our solar system for hundreds of thousands of years before they return close to our sun.

Hundreds of thousands of irregularly shaped rocks also revolve around the sun in roughly elliptical orbits. These are Asteroids. They are mostly in an area known as the Asteroid Belt, somewhere between the orbits of Mars and Jupiter. Due to their irregular shapes they often have erratic orbits.

Check Point

1. The movement of a planet around its axis is called –
   1. Revolution
   2. Rotation
   3. Orbit
   4. Solar year
2. The movement of a planet around the sun is called its –
   1. Solar day
   2. Revolution
   3. Rotation
   4. Spin
3. When the earth aligns itself between the sun and the moon, it causes ______.
4. The changing shape of the moon is known as its ______.
5. The orbit of comets is extremely _______.

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