Where and how to see Earth's “second moon” this fall

As astrotourism continues to rise, more and more travelers are planning trips on Earth based on events in the night sky – from solar eclipses and rare supermoons to the annual Perseid meteor shower. And while many of us mourn the early sunsets of fall, the extra hours of darkness provide an opportunity to tune in to the movements of the sky.

This fall, the sky has a special treat in store for attentive space lovers: in the next two months, Earth will have a “second moon.” Starting September 29, just after 4 p.m. ET, an asteroid will enter Earth's orbit and move in a horseshoe-shaped path around the planet until it departs on November 25, according to Spanish scientists who first discovered the phenomenon earlier this month had reported.

What is the second moon and what is it called?

Earth's second moon is actually an asteroid called 2024 PT5. It is considered a temporary “mini-moon” because moons are scientifically defined as “naturally formed bodies that orbit planets,” according to NASA Science. But to us Earthlings, it won't look like the moon we know and love – because it's not visible to the naked eye.

In fact, the second moon is so small (about the size of a school bus) and so far away (about 1.5 million kilometers away) that even a normal telescope cannot capture it. To see it, you need a telescope at least 30 inches in diameter and a digital CCD or CMOS detector, explains Carlos de la Fuente Marcos, astronomy professor at the Universidad Complutense de Madrid and co-discoverer of 2024 PT5 Conde Nast Traveler.

“The object is too small and too dark 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 determines whether 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,” explains Marcos. “Now it is at magnitude near 23. The sun is at magnitude -26.78 and the full moon is at magnitude -12.7. The size scale is inverted, the darker the object, the more positive the size.”