As astrotourism continues to take off, more travelers are planning trips on Earth based on night sky events—from solar eclipses and rare supermoons to the annual Perseid meteor shower. And while many of us mourn the early sunsets that fall brings, the extra hours of darkness offer an opportunity to tune into the celestial movements above.
This fall, the skies will hold a special treat for attentive space aficionados: Over the next two months, Earth will have a “second moon.” Beginning on September 29, just after 4 p.m. ET, an asteroid will enter Earth’s orbit and travel in a horseshoe-shaped path around the planet until it departs on November 25, according to the Spanish scientists who first reported the phenomenon earlier this month.
What is the second moon and what is it called?
Earth’s second moon is actually an asteroid named 2024 PT5. It’s considered a temporary “mini moon” because moons are scientifically defined as “naturally-formed bodies that orbit planets,” per NASA Science. But to us Earthlings, it won’t look like the Moon we know and love—because it won’t be visible to the naked eye.
In fact, the second moon is so mini (approximately the size of a school bus) and so distant (some 1.5 million kilometers away) that even a normal telescope won’t be able to capture it. To see it, you’ll need a telescope at least 30 inches in diameter paired with a CCD or CMOS digital detector, Carlos de la Fuente Marcos, astronomy professor at Universidad Complutense de Madrid and co-discoverer of 2024 PT5, tells Condé Nast Traveler.
“The object is too small and dim for typical amateur telescopes and binoculars,” he says. “However, the object is well within the brightness range of typical telescopes used by professional astronomers.”
The brightness of an object in space is what determines if we can see it from Earth—the brightest star visible to the naked eye has a magnitude of 6. “When it was discovered, 2024 PT5 had a magnitude of 17,” Marcos explains. “Now it has a magnitude close to 23. The Sun has a magnitude of -26.78 and the full Moon, -12.7. The magnitude scale is inverted, the dimmer the object, the more positive the magnitude is.”
How can you see the second moon?
For those of us who lack fancy telescopes, the best way to see the second moon before it departs Earth’s orbit is by heading over to a professional space observatory with public access. Here’s a handy list of US observatories categorized by state, located in universities, nature centers, museums, dark sky preserves, and more.
