Planets are the nearest and brightest celestial objects in the night sky. They are a good start for novice astronomers as they are easily visible and their movements are really interesting. As was mentioned in the earlier posts all the visible planets of the solar system (all the planets except Neptune) appear along a line known as ecliptic. Now here is a simple explanation why it is so:
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| The Solar system (not to scale; Pluto is no longer a planet) |
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The solar system is nearly planar i.e. all the planets lie in a same plane. Now if you are standing on earth the planets will all be in the same line which is known as the ecliptic. To understand this look at the diagram below:
Now if we remove the orbits, the sun and the two planets will appear in one line (as seen from the earth {blue}):
Similarly the outer planets will also appear along this line:
Thus from earth the planets and sun will always appear along the line called the ecliptic line. This line can be easily traced along the sky as it is the line along which the sun and moon moves. Once you establish the location of this line the other objects can be easily seen in relation to it. The planets appear along this line as non twinkling objects which differentiates them from nearby stars. The planets move relative to their background, you can easily plot this movement and it can be very exciting and interesting.
Now that we have determined where the planets will appear now let us see when they will appear. The orbital lengths of all the planets are different as is the time taken by them to complete their orbits. They can be ahead of earth in their orbit or behind ( all planets revolve in the same direction) at any time. If you see the above diagrams you will notice that the planets appear either to be ahead of the sun or behind it. The planets ahead of the sun appear in the sky just before the sun i.e. in the morning whereas the planets behind the sun appear at dusk. If a planet is crossing in front of the sun or behind it relative to the earth it will not be visible until it is at least 10 degrees ahead or behind the sun. Thus the planets can either be dawn planets of dusk planets.
Thus up-till now we have established:
1. All the planets appear in the ecliptic line.
2. They may appear early in the night the so called dusk planets or late in the night just before the sunrise the so called dawn planets.
I will finish this post with a brief overview of the individual planets
Mercury:
It is the closest planet to the sun with the smallest orbit. It is most of the time in front of or behind the sun relative to earth. It does not get 10 degrees from the sun( the minimal angle for a planet to be visible) often and is an elusive planet.
Venus:
The brightest object in the sky following the sun and the moon and one of three objects capable of forming a shadow. It appears as a dawn planet for half the year and a dusk planet for the other half. It was thought to be two different planets because of this.
Mars:
The red planet, getting its distinctive color from iron oxide alternates with venus in being closest to us. The most visited of our neighbors it appears in the night sky as a red object. It just passed through the Leo constellation.
Jupiter:
The largest planet of the solar system by far, this planet is the first of the gas giants (the first four planets are called rock planets). The most bright object after Venus it is visible as a morning planet nowadays, it was one of the three planets that came together about two months ago. On its surface there is a storm having a diameter of two and half earths and it can be seen easily with a normal telescope. It is called the great red spot.
Saturn:
The second gas giant, and the second largest planet, Saturn is renowned for its rings. Easily visible with a telescope they have fascinated astronomers for centuries. They are in fact belts of dust and small particles in orbit around Saturn.
Uranus:
The 7th planet of the solar system it is the last planet visible with the naked eye. Similar in size to and consistency to Neptune, scientists sometimes call them ice giants. It is dim planet and moves slowly through the night sky.
Neptune:
The second of the ice giants and fourth of the gas giants, it is the only planet not visible to the naked eye. It is the farthest planet of the solar system. The objects beyond it are called Transneptunian objects. It is the coldest planet of the solar system.
