This is a frequently observed phenomenon and is also a mnemonic which enables you to remember the order in which the planets have their orbits outwards from the sun, though it is hard to see why anyone should want to remember this. If you forget which M comes first, remember that you should never put a Mars bar near the sun.
It is depressing to find that if you put planets mnemonic into Google you are offered 372,000 pages. This suggests that all over the world thousands of people have spent hours of their lives thinking up new ones, and no doubt many are busy thinking up more now that the International Astronomical Union this year agreed that there are currently eight planets and three dwarf planets known in the solar system*.
But here is a pretty visual representation of the relative sizes of some of them. On the same site there are similar pictures showing the others and the sun.
*The 2006 definition of "planet" by (the IAU) states that, in the solar system, a planet is a celestial body that:
• is in orbit around the Sun
• has sufficient mass so that it assumes a nearly round shape, and
• has "cleared the neighbourhood" around its orbit.
A non-satellite body fulfilling only the first two of these criteria is classified as a "dwarf planet", whilst a non-satellite body fulfilling only the first criterion is termed a "small solar system body". The definition was a controversial one, and has been both criticised and supported by different astronomers.
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