Here are some new mnemonic methods by Peter Hobbs for recalling the order of size of the various planetary objects in our Solar System.

Firstly, for the five largest bodies (including planetary moons) within the solar system:

" SUN - J - SUN! "

(Sun - then Jupiter, Saturn, Uranus, Neptune)
Of diameters (miles x 1000): 865, 89, 75, 32, 30.

This can be recalled phonetically as:
"The biggest? - Son Jason!"

It is no coincidence that all five bodies are gaseous objects composed of between 75% and 90% hydrogen. The four planets are gas giants rather than terrestrial (ie. rocky) objects. Each has a large mass and consequent strong gravity. (Jupiter for example has 318 times the mass of Earth, yet all of the planets together have only 0.001 the mass of the Sun). The strength of each giant's huge gravity prevents fast-moving atoms of hydrogen from escaping, while the relatively small gravities of the other planets (Earth, Venus etc.) allowed them to lose their hydrogen billions of years ago to leave each with a solid rocky surface.

The order of size of the four gas giants also happens to match the order of distance of each planet, ie. Sun-x-x-x-x-J-S-U-N-x

After the sun and gas giants the Earth is the next (6th) largest body, followed by Venus, Mars, Ganymede (Jmoon), Titan (Smoon), Mercury, Callisto (Jmoon), Io (Jmoon), The Moon (Emoon), Europa (Jmoon), Triton (Nmoon), then (17th) Pluto.

These lesser bodies have diameters of (miles x 1000):

7.9 (E), 7.5 (V), 4.2 (M), 3.3 (gJ), 3.2 (tS), 3.0 (M), 3.0 (cJ), 2.3 (iJ), 2.2 (moon-E), 1.9 (eJ), 1.7 (tN), 1.4 (P)

Four of the seven largest moons circle Jupiter, the largest planet. They are Io, Europa, Ganymede and Callisto, known collectively as the Gallilean moons because they were discovered by Gallileo Gallilei in 1610. Jupiter has 16 moons in total, in an outward order of 4 small, then 4 Gallilean, then 8 more small moons. Astronomers tend to quote the Gallileans in order of distance outward (ie. I-E-G-C), for which the innermost (Io) is easy to recall as "I/O" is a computer term for "In/Out"!

For a new mnemonic to recall the entire order of planet-sizes excluding the various moons and Pluto, think of the "family" as:

" Son, JaSUN, EVen MaM! "


Lastly, for a mnemonic to help distinguish between the relative sizes of Ganymede (J), Titan (S) and Triton (N) think of them alphabetically:

" Titan? Triton? Ganymede takes the lead! "


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