Comparing Sizes

You may sometimes find it helpful to compare the size of one astronomical object with another. Such comparions can help you better visualize objects and understand their properties.

In comparing sizes, the usual method is to figure out how many times larger or smaller one object is than another.

For example, if we wanted to find out the relative size of the Earth and Moon, we could compare how many times bigger the Earth's radius is than the Moon's radius.

The Earth's radius is about 6,400 (=6.4x103) km. The Moon's radius is about 1,700 (=1.7x103) km. To find their relative size we simply divide the bigger value by the smaller one. Thus, the Earth is 6.4x103 km/1.7x103 = 3.8 (approximately) times larger in radius than the Moon.

Note: In making such comparisons it doesn't matter if we use the radius or the diameter. Because the diameter is just twice the radius, the factor of two will cancel out when you divide one by the other.

1. Approximately how much bigger across is the Earth than Pluto? The Earth's radius is about 6,400 (=6.4x103) km. Pluto's radius is about 1,200 (=1.2x103) km.
A).. 2
C).. 50

2. Jupiter's radius is about 71,500 km and the Earth's is about 6,400 km. About how much bigger in diameter is Jupiter than the Earth?
B)... 200

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