Imagine if you could see only the front of your home and that no matter what you did, the back side remained hidden. Weird and frustrating? That's how astronomers feel a lot of the time because humans can observe only one side of some of the most intriguing places in the universe.
The most obvious example is the moon, which always keeps one hemisphere facing Earth. That is why every month we see the same man-in-the-moon pattern. This friendly looking formation—created by a group of ancient lava flows that are slightly darker than the surrounding surface—is most noticeable when the moon is roughly full, August 7 to 12 this month.
Astronomers had long assumed that the farside was just like the near side. Wrong. Pictures of the hidden hemisphere, first taken in 1959 by the Soviet spacecraft Luna 3, show a vastly different landscape. Now we know that the moon is lopsided because Earth's gravity pulled the moon's mass slightly off-center toward us. As a result, volcanoes never really got going on the farside, and the surface there has few dark blotches but lots of huge craters that were never swallowed by lava (above, right side). As no astronauts have ever landed on the farside, we know little about its composition and history.
Every major moon in the solar system similarly shows only one face to its parent planet. An astronaut sitting on the farside of Europa would never get a glimpse of Jupiter, sitting just 420,000 miles away.
Sometimes our view of the farside is blocked because the object spins so slowly. Galaxies typically take 100 million to 300 million years to rotate; through all of modern history we have barely seen them budge. In some cases, as in the turbulent galaxy M82, dramatic, violent processes remain unknown to us because we cannot take a peek around the corner.
Our galaxy, the Milky Way, has interesting things hidden behind its farside too. One of the nearest galactic neighbors—the Sagittarius dwarf spheroidal—lurks invisibly behind the Milky Way's center. (It was discovered only by tracking the occasional stars that peek through.) When our solar system swings around in 100 million years, the curtain will rise and this billion-solar-mass galaxy will appear as the largest entity in the night sky, 40 times wider than the moon. By then, the beautiful Andromeda galaxy will be obscured and go unseen for tens of millions of years.
Fortunately, some of the most intriguing cosmic objects are so small and swift that we see both sides nearly at once. Overhead this month in the constellation Cygnus is a distant star called HDE226868. It orbits an enigmatic thing that emits a stream of X-rays. Those rays flicker incredibly quickly, within a fraction of a second—indicating that the near and farsides of the source are located at virtually the same place. Simple physics calculations show that the strange brawny object is so tiny, it can be only one thing: a black hole.
How's that for a paradox? We can watch the back side of a black hole, but we can't see around our very own moon.