In other words, a light-second is the distance light travels in one second, or 7.5 times the distance around Earth’s equator. If you could travel at the speed of light, you would be able to circle the Earth’s equator about 7.5 times in just one second! The fastest-moving stuff in the universeĪs mentioned above, light travels at an incredible 186,000 miles per second (300,000 km/sec). Click into Atlas of the Universe to find an interactive page that’ll let you keep zooming farther and farther out. There are 33 stars within 12.5 light years of our sun. Okay, the numbers are still pretty big! But hopefully they can help you see that our universe is very vast. Hercules globular star cluster (aka M13): 22,200 miles (35,700 km)Ĭenter of our Milky Way galaxy: 26,100 miles (42,000 km) Pleiades open star cluster: 444 miles (715 km) Scaling the astronomical unit at one inch (2.5 cm), here are distances to various bright stars, star clusters and galaxies: Image via Max Planck Institute for Extraterrestrial Physics. A beam of light from this star takes about 4 years to travel to Earth. See? The red star in the center of this picture is Proxima Centauri, our sun’s nearest neighbor among the stars. Scaling the Earth-sun distance at one inch places this star at 4.4 miles (7 km) distant. The closest star to Earth, other than the sun, is Alpha Centauri at some 4.4 light-years away. If we scale the astronomical unit – the Earth-sun distance – at one inch, then the light-year on this scale represents one mile (1.6 km). This wonderful coincidence enables us to bring the light-year down to Earth. Robert Burnham noticed that, quite by coincidence, the number of astronomical units in one light-year and the number of inches in one mile are virtually the same.įor general reference, there are 63,000 astronomical units in one light-year, and 63,360 inches (160,000 cm) in one mile (1.6 km). Image via Brews Ohare/ Wikimedia Commons. A light beam takes 8 minutes to travel the 93 million miles (150 million km) from the sun to the Earth. One astronomical unit, or AU, equals about 93 million miles (150 million km).Īnother way of looking at it: the astronomical unit is a bit more than 8 light-minutes in distance. Keep reading, for a way to comprehend the vastness of the universe, using units of distance we know and use every day.īurnham started by relating the light-year to the astronomical unit – the Earth-sun distance. – author of Burnham’s Celestial Handbook – devised an ingenious way to portray the distance of light-years in terms of miles and kilometers.

In the late 20th century astronomer Robert Burnham, Jr. Yet miles and kilometers are what most of us use to comprehend the distance from one place on Earth to another. And, if we try to express a star’s distance in miles or kilometers, we soon end up with impossibly huge numbers. And thus a light-year is 5.88 trillion miles (9.46 trillion kilometers).īut stars and nebulae – not to mention distant galaxies – are vastly farther than one light-year away. Light is the fastest-moving stuff in our universe. So we speak of space objects in terms of light-years, the distance light travels in a year. They’re so far away that kilometers or miles aren’t a useful measure of their distance. Objects in our universe are extremely far away. Read more about this image at Wikimedia Commons. The large yellow shell depicts a light-year the smaller yellow shell depicts a light-month.

Unit larger than light years