Scale of the solar system

Most books and other information sources attempt to portray the size, scale and emptiness of our solar system, but most fail to do so successfully. Why?

We invariably are given ONE scale for the planets sizes, and ANOTHER scale for their locations -- two DIFFERENT scales !

For example, the nice graphic on The Nine Planets home page shows their SIZES to scale:

and the Starry night movies of the inner and outer solar system shows the orbits NEARLY to scale:

Movie of inner solar system

Movie of outer solar system

but boy oh boy, are they different scales!! While the use of two scales makes for pretty pictures and interesting T-shirt designs, it gives a VERY UNREALISTIC view of what the solar system is really like (that is, just how EMPTY it is).

Here is a very simple yet useful little exercise to show the real scales. My wife used it during 2000, teaching year 9 science at Kelmscott Senior High School in Perth; it is adapted from the excellent book The Evolution of a Planetary System (from the SETI Institute).

Stage 1: The planets' sizes

Materials

The 2.8 cm sphere or disk represents the sun. At the same scale, the planet diameters are:

Planet	Actual diameter (km)	Scale diameter (mm)	Suggested item

Mercury	4,900	0.10	A typical grain of salt is 0.5 mm in diameter. Pick the smallest grain you can find, and advise the students that it is still 5 times bigger than it should be to make this model.
Venus	12,100	0.24	Grain of sand or salt.
Earth	12,800	0.25	Grain of sand or salt.
Mars	6,800	0.14	A tiny grain half the size of the earth grain.
Asteroid Belt			Chalk dust.
Jupiter	143,000	2.9	Cake decoration or similar, of correct size.
Saturn	120,000	2.4	Cake decoration or similar, of correct size.
Uranus	51,800	1.0	Cake decoration or similar, of correct size.
Neptune	49,500	0.99	Cake decoration or similar, of correct size.
Pluto	2,300	0.046	A typical grain of salt is 0.5 mm in diameter. Pick the smallest grain you can find, and advise the students that it is still 10 times bigger than it should be to make this model.

Why such tiny sizes for the planets? Because we'll use the same scale for their distances...

Find an outdoor area where the model can be built. If you want to include Pluto, it needs to be 118 metres long ! [Without Pluto, 90 metres will do.]

Place the model of the sun (the 2.8 cm sphere or disk), taped to an index card, on a post (or similar) at one end of the area.

Ask the students where, at this scale, they think Mercury should go. After they've made their estimates, measure off 1.16 metres from the sun, and place the Mercury card (index card with grain of salt attached). Repeat for each planet (probably easier to measure from the last planet than from the sun). Here are the scale distances:

Planet	Actual distance from sun	Scale distance from sun	Scale distance from previous planet

Mercury	58 million km	1.16 metres
Venus	108 million km	2.16 metres	1.0 metre
Earth	150 million km	3.0 metres	0.84 metre
Mars	228 millin km	4.56 metres	1.56 metre
Asteroid Belt	approx. 400 million km	approx 8 metres
Jupiter	778 million km	15.56 metres	11 metres
Saturn	1,430 million km	28.6 metres	13.04 metres
Uranus	2,870 million km	57.4 metres	28.8 metres
Neptune	4,500 million km	90 metres	32.6 metres
Pluto	5,900 million km	118 metres	28 metres