If you are up at 6 or 7 a.m. on the 24th, a glance out an east-facing window will give you a nice view of the waning crescent Moon about 5 degrees above Jupiter, or on the 25th, 5 degrees below. It’s a nice juxtaposition of two objects close in the sky, but in reality nowhere near each other in space. The Moon orbits at a distance of around 400,000 kilometers (250,000 miles); Jupiter’s currently 2300 times further away, at a little less than a billion kilometers. That’s pretty far. Kinda makes you wonder exactly how far you can see using only your eyes.
Well, Uranus is visible without optical aid from a non light polluted location by someone with 20-20 vision. It’s 3 billion kilometers away. That’s far too.
Once you’re past our solar system’s planets, meteors and comets, virtually everything else you can see with your naked eye is a star in our galaxy. Our galaxy is some 100,000 light years (10 trillion kilometers) across with 100 to 400 billion stars. Oddly, the furthest star you can see with your naked eye is also one of the brightest. At number twenty in brightness, but number one in naked eye distance, is Deneb, the bright super giant found at the head of Cygnus, aka The Northern Cross. So far away that the distance measurements vary widely, Deneb is anywhere from 1425 to 7000 light years distant, depending who you ask. That’s really far away.
Step outside around 8pm on a clear night and find a darkish area that gives you a good view of the eastern sky. About two-thirds of the way up you will find The Great Square of Pegasus, not surprisingly a great square made up of four prominent stars, with very little in the middle. The square is about 15 degrees on a side (the distance between your pointer and pinkie fingers extended at arms’ length). Draw an imaginary line from the bottom left to the upper right stars, and extend it another 30 degrees further. That bright star is Deneb. Deneb is the furthest thing you can see from the Northern Hemisphere with eye alone... with one exception.
Go back to the Great Square. Draw another diagonal line, this time from the bottom right star to the one at the top left. Extend that line another 30 degrees, and you will find the familiar ‘W’ which makes up the vowel-rich constellation Cassiopoeia. Half way along that line you will find a Moon-sized smudge on the sky. Wipe off your glasses and look again. If it’s still there, you’ve found the Andromeda Galaxy.
Difficult but possible from the city, and a pushover from a dark country sky, at the really, really far distance of 2.5 million light years, our sister galaxy in space is the most distant object the unaided eye can see, and it’s very possibly available from your own back yard
