Saturn is on display in the southern sky every night and is the most distant planet we can easily see with the naked eye.
Saturn is very far away from us. It takes light, travelling at 186,000 miles per second, nearly 90 minutes to travel the one billion miles that separate the Earth and Saturn. As far as that is, it’s nothing compared to the distance to even the closest star.
Proxima Centauri, the smallest member of the Alpha Centauri triple star system, is the nearest star to us. It lies at a distance of about 25 trillion miles from us. It takes light from Proxima Centauri over four years to reach Earth!
Most of what we can see in the night sky are stars, and most of those stars are relatively close — less than 5,000 light years away. There is one object visible to the unaided eye from even a moderately dark sky whose distance dwarfs that of the stars. This object is the Andromeda galaxy, and it is currently well placed for viewing in the night sky.
The Andromeda galaxy has been known since ancient times, but it's true nature was only discovered about 100 years ago. In the early 20th century, a brilliant astronomer and observer at the Harvard College Observatory named Henrietta Swan Leavitt discovered a special relationship between the inherent brightness and period of variability of cepheid variables, a type of star that dims and brightens on a regular schedule.
The discovery of this relationship, now known as Leavitt’s law, gave reliable distances to stars as far away as 20 million light years. Using this relationship along with his own observations, Edwin Hubble determined that Andromeda was not a star cluster or cloud of gas in our own Milky Way galaxy, but instead was an entirely separate galaxy from ours.
Up until this point, it was thought that the Milky Way was the entire universe. We now know that Andromeda is only one among hundreds of billions to trillions of galaxies within the observable universe.
Andromeda is about the same size as the Milky Way but contains around a trillion stars — twice as many as the Milky Way. If you happen to observe Andromeda, consider that the light reaching your eyes took 2.5 million years to travel to Earth from Andromeda, a mind-blowing distance.
The Morning Sky
Mars is returning to visibility, rising in the east about an hour and a half before the sun. Watch as the moon moves from west to east and how its crescent gets thinner from one morning to the next this week. Sirius, the brightest star in the night sky, sets in the west just as the sun rises in the east.
The Evening Sky
Venus, Saturn and Jupiter form a straight line in the southwestern evening sky. Venus is the brightest of the trio, shining six times brighter than Jupiter and nearly 100 times brighter than Saturn. Andromeda is near the highest point in the sky around 9 p.m.
Use an astronomy app or star chart to help you find the nearest large galaxy to us. A pair of binoculars will begin to give form to this hazy mist, shining with the light of a trillion suns.
Dan Price is a NASA/JPL Solar System Ambassador and informal educator. He leads the Night Sky Tours at Josephine Sculpture Park. Have a question about astronomy or space science? Send an email to firstname.lastname@example.org and it might be featured in a future column.