Celestial Phenomena

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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.

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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!

Lunar Eclipses

The motion of planets around the sun, and of their Moons around themselves, frequently causes phenomena known as Eclipses. Let us see how this happens with earth and its moon and the sun.

Earth has only one moon. The moon cannot glow by itself. It only reflects the sun’s rays. If the earth obscures the sun’s rays from falling on the moon, the moon appears to disappear, either completely or partially. When does this happen? When the earth aligns itself between the sun and the moon, the shadow of the earth falls on the moon making the moon ‘disappear’ for a while. This is called a Lunar Eclipse. If the earth’s shadow falls completely on the moon, it’s a Total Lunar Eclipse. A partial shadow of the earth on the moon creates a Partial Lunar Eclipse.

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”.

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Lunar Phases

The changing shape of the moon, known as the Lunar Phases, is also explained by the amount of sunlight reflected by the moon towards the earth, depending on the alignment of the moon with respect to the sun and the earth. The surface of the moon is lit by the sun’s rays at different angles depending on their relative positions. On a dark night the moon has completely blocked out the sun’s rays, therefore, although the moon is in the sky you don’t see it. This is known as the New Moon. Over the next few days, the moon moves to one side and a part of the moon starts reflecting the sun’s rays and becomes visible to us. At first, it’s just a small bit, known as a Crescent Moon, then it grows larger and becomes a Half Moon and finally a Full Moon. Then the cycle repeats month on month.

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.

Every so often, due to contending gravitational forces of the planets and nearby asteroids, an asteroid may spin out of its orbit and hurtle away from our solar system or towards the sun. They may even pass dangerously close to earth and some are known to have crashed into earth, pulled by earth’s gravity. They mostly burn up in the atmosphere as spectacular fireballs, and rarely cause damage on earth. Their sudden and unpredictable movements keep astronomers wary of them at all times.

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 _______.

Answer Key

  1. b) Rotation
  2. b) Revolution
  3. A lunar eclipse
  4. Lunar phases
  5. Elliptical

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