Space Historian

What is a space historian?  As was briefly mentioned in the post "Orion the Hunter", stars are not very close to the Earth, with the exception of our Sun, which is approximately 93 million miles (1 AU) away.  That might not sound very close, but in astronomical terms, it's right next door.  The distance between the Sun and the Earth is called an astronomical unit (AU).  This is the standard unit of measurement for objects in our solar system, but once outside of the solar system, an AU is not large enough to feasibly measure distance. That's where the light year comes in.  In review, one light year is the distance that light can travel in one year which is roughly 6 trillion miles.

The Sun, which is our closest star is still so far away from us that it takes light about 8 minutes to reach us. The next closest star to us is Proxima Centauri, which is about 4.2 light years away.  It is a member of a star system containing two other stars; Alpha Centauri A and Alpha Centauri B. Unfortunately, we cannot see this star system from the northern hemisphere.  One of my goals is to travel to the southern hemisphere so I can see this system.  Even though it is the closest star to our solar system, it is not the brightest star in the night sky. Sirius is the brightest star in the sky, followed by Canopus which is about half as bright as Sirius. Alpha Centauri A is the third brightest star.  Without a telescope, an observer would not be able to see the complete star system and could only see Alpha Centauri A.

As mentioned, Proxima Centauri is 4.2 light years away, meaning it takes 4.2 years for the light to reach Earth and our eyes.  Most objects in the night sky are hundreds to millions of light years away.  The Ring Nebula (M57) is about 2300 light years from Earth.  The light that we are seeing from this nebula today was produced around the fall of the ancient Egyptians.  Our closest neighboring galaxy to our own Milky Way Galaxy is called the Andromeda Galaxy (M31).  It currently lies some 2.5 million light years away from us. When looking at this galaxy with our unaided eyes, binoculars, or telescope, we are seeing the galaxy as it was 2.5 million years ago!  This is before homo sapiens started to evolve. The Andromeda Galaxy is currently on a collision course with the Milky Way.  But don't worry!  This won't happen for another 4 billion years.

We are even viewing our closest celestial neighbor in the past, even if  by a small amount. The moon is about 239,000 miles away from the Earth.  Light travels at about 186,000 miles a second, so when viewing the moon, we are seeing the moon as it was about 1.3 seconds ago.

Next time you look up at the stars or even the moon or sun, remember that you are looking back in time. Even light, the fastest moving thing known to us, isn't instantaneous.