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The Scale of the Universe

How big is the Earth compared to our Solar system?

On a scale of 1 to 10 billion, the Sun is about the size of a grapefruit. Planets are much smaller, with Earth the size of a ball point and Jupiter the size of a marble on this scale. The distances between planets are huge compared to their sizes, with Earth orbiting 15 meters from the Sun on this scale. 

Seeing our solar system to scale also helps put space exploration into perspective. The Moon, the only other world on which humans have ever stepped, lies only about 4 centimeters (1 inches) from Earth in the Voyage model. 

The trip to Mars is some 200 times as far as the trip to the Moon, even when Mars is on the same side of its orbit as Earth. 

How far away are the stars?

On the 1-to-10-billion scale, it is possible to walk from the Sun to Pluto in just a few minutes. On the same scale, the nearest stars besides the Sun are thousands of kilometers away.

The nearest star system to our own, a three-star system called Alpha Centauri, is about 4.4 light-years away.

How big is the Milky Way?

Using a scale on which the Milky Way galaxy is the size of a football field, the distance to the nearest star would be only about 4 millimeters. There are so many stars in our galaxy that it would take thousands of years just to count them. 

The observable universe contains roughly 100 billion galaxies, and the total number of stars is comparable to the number of grains of dry sand on all the beaches on Earth. 

How do our lifetimes compare to the age of the universe?

On a cosmic calendar that compresses the history of the universe into 1 year, human civilization is just a few seconds old, and a human lifetime lasts only a fraction of a second. 

The cosmic calendar compresses the 14 billion-year history of the universe into 1 year, so that each month represents a little more than 1 billion years. This cosmic calendar is adapted from a version created by Carl Sagan.

BOOK: The Essential Cosmic Perspective with MasteringAstronomy (Sixth Edition) 

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