The Milky Way has a hidden feature you've probably never noticed. Here's how to find it

The Summer Triangle in July, with the Great Rift running through the Milky Way.

(Image credit: Alan Dyer/VW Pics/UIG via Getty Images)

If you've not seen the Milky Way this year from the Northern Hemisphere, it's time to make a plan. August's dark sky window opens around Aug. 6, when the moon begins to rise late at night as a waning crescent, meaning dark moonless skies for about 10 nights. That's when to see the Milky Way (and if you can time it for Aug. 12/13's Perseid meteor shower peak, all the better).Five things are required to successfully see the Milky Way:the correct time of year (The brightest part is visible March-October)the correct moon phase (last-quarter moon through slim crescent moon)a dark sky away from light pollutiondark-adapted eyes (stand in total darkness for 25 minutes)an idea of what you're going to seeThe best way to dark-adapt your eyes is to start stargazing. Look south and begin picking out stars you know, paying special attention to the Summer Triangle. These three stars — Vega, Deneb and Altair — are your major signpost for finding the dense arc of starlight that will gradually emerge. Specifically, focus on the sky from Deneb through Altair — the left-hand side of the Summer Triangle — and you'll slowly see not only the Milky Way, but something else in front of it that few stargazers properly appreciate.Sure, you can see the bright halo of gas and stars that indicate the center of the Milky Way galaxy, which is particularly bright below the Summer Triangle in the Sagittarius constellation. The closer you are to the equator, the higher in the sky, and the brighter, this region — known as the Galactic Center — will be.Now look again. See those dark patches within the arc winding from Deneb to Altair? That's the Great Rift, so named because it appears to tear the Milky Way in half. It's an interstellar cloud of gas and dust (dark nebulas) between the solar system and the bright center of the Milky Way, which blocks and scatters visible light, essentially covering up bright stars and nebulas from our point of view. It looks like someone has spilled black ink across the Milky Way.There are two ways of seeing the Great Rift. The most modern way is to observe it in infrared and radio light, which passes through the dust without being absorbed. This is exactly what some observatories are built to do, from radio dishes to the James Webb Space Telescope, which can probe the inner Milky Way — including the billions of stars in the center of our galaxy — which are on the other side of the Great Rift. Meanwhile, the new Vera C. Rubin Observatory has begun observing the entire visible southern sky every few nights over the course of a decade, mapping the Milky Way (including the Great Rift) to answer questions about its structure and formation.The second way to see the Great Rift is to see it for what it is. Astronomers call the section we're looking at the Cygnus Rift, also called the Northern Coalsack, 1,000-3,000 light-years away in the constellation Cygnus. It's just below Deneb (which is 1,500 light-years distant), slightly outside of the Summer Triangle over an otherwise bright, dense region of stars called the Cygnus Star Cloud (around 2,600 light-years away). About six degrees in diameter, the Northern Coalsack blocks the light of background stars. However, it's just the northern tip of the Great Rift, which stargazers also call The Dark River or Dark Cloud.Beyond the Northern Coalsack, however, the Great Rift is something to travel to see more of. It stretches across about 120 degrees of the night sky from Cygnus and Sagittarius in the Northern Hemisphere to Centaurus in the Southern Hemisphere. The latter is home to the famous Southern Cross and, besides its star Acrux, the original Coalsack Nebula. Another reason to travel to see more of the Milky Way is to appreciate how Southern Hemisphere cultures view the Great Rift. While modern astronomers see it as a dark cloud that needs to be peered through, indigenous cultures see its shapes. The Wiradjuri in Australia see the silhouette of an emu (its head is the Coalsack Nebula) and use its position in the night sky as a seasonal calendar. Meanwhile, the Inca in South America saw a llama (and, beside it, a baby llama).The Great Rift is one of my favorite things to see, because it reveals what we're doing when we see that arc of light stretch across a summer night sky — we're looking at a galaxy from the inside, with all of its depth and complexity.How and when I'm watching it