miles (149,597,870 kilometers); it is often seen rounded off to 93,000,000 miles
(149,598,770 kilometers) and used in reference to great astronomical distances. For
example, the Earth is 1 AU from the Sun; the planet Venus is 0.7 AU; Mars 1.5 AU; Sat-
urn 9.5 AU; and the farthest planet, Pluto, is 39.5 AU from the Sun.A light year is an even larger unit. As the name implies, it is the distance light
travels in one year, or about 5.88 trillion miles (9.46 trillion kilometers). In most
cases, light year measurement is reserved for deep space objects. (For more about
measurement, see “Mathematics throughout History.”)What is the Hubble constant?
Astronomers have always been interested in the age of our universe and the speed of
various objects in space. The Hubble constant was devised by American astronomer
Edwin Hubble (1889–1953). It is the ratio of the recessional speed of a galaxy—
because the universe is expanding—to its distance from the observer. In other words,
the velocity at which a typical galaxy is receding from Earth, divided by its distance
from Earth.The reciprocal of the Hubble constant is then thought to be the age of the universe,
usually written in terms of kilometers per second per million light years. If the number
is high, the universe would be very young; if the number is low, the universe would be
much more ancient. Although there have been numerous theories, the true age of the
universe is usually considered to be somewhere between 12 and 20 billion years old.The most recent agreed-upon rate at which the universe is expanding is approxi-
mately 20 kilometers per second per 106 light years of distance. That makes the uni-
290 verse about 15 billion years old.
Halley’s Comet, named after Edmond Halley, last appeared near Earth in 1986 and will be seen again in the night
sky in 2062. Stone/Getty Images.