–Charlene and Jim Rohr
A light-year is the standard measuring stick that astronomers use to calculate large distances in the Universe. Light travels at approximately 186,300 miles per second. It can circle the Earth 7.5 times in one second. Cruising for one year at this velocity, light covers a little less than 5.8 trillion miles. http://www.learnastronomyhq.com/articles/ how-many-times-can-light-travel-around-the-earth-in-1-second.html
Altair is the brightest star of the constellation Aquila, the Eagle, and one of the vertices of the asterism, the Summer Triangle (the other two are Deneb and Vega). Altair is 16.8 light years from Earth. It is the twelfth brightest and one of the closest visible stars viewed with the unaided eye.
When we see light from Altair, we are not seeing how it looks today, but as it looked 16.8 years ago. So when we see light coming from stars, constellations, and galaxies in the night sky, we are looking back in time. A star could explode as a supernova, and we wouldn’t know until the light arrived on Earth.
The center of the Milky Way Galaxy is 26,000 light-years away. The nearest large galaxy, Andromeda, is 2.5 million light-years away.
On October 11 we discussed the Andromeda Galaxy and you may wonder just how far away is it? If the speed of light is 186,300 miles per second and one light-year equals about 5.8 trillion miles, then the Andromeda Galaxy is 2.5 million X 5.8 trillion miles away. Remember that the Apollo spacecraft took 3 days to get to the Moon and back in 1969? Going at that speed, it would take Apollo over 600 billion years to reach the Andromeda Galaxy. The Hubble telescope has detected galaxies 13 billion light years away. The universe is