When Galileo first viewed the moon through a telescope in the early 1600s, he saw a surface that looked surprising to many. It was not smooth or pristine — as celestial objects were assumed to be — but rather rocky and rugged.

Prior to this, most people assumed objects in the heavens were fundamentally different than objects on Earth. Earth was the center of the cosmos and made of changing, heavy, imperfect matter. Things in the sky — such as the stars, planets and the moon — were bright, spherical, perfect and eternal.

But Galileo’s view challenged this assumption. You can see for yourself what he saw by viewing the moon’s surface through a good telescope — especially when the moon is only partially full and the division between the light and dark portions (known as the terminator) makes every mountain and crater wall stand out in stark detail. Galileo saw the moon as a giant rock in the sky.

But if that was true, what kept it from crashing down to Earth? It was questions like this that opened the door to a new understanding of gravity and motion.

Since Galileo’s surprising insight, we have come a long way in our understanding of our nearest celestial neighbor. And that understanding took a “giant leap” forward 50 years ago this month, as humanity set foot on that giant rock in the sky for the first time.

We mark a half-century since the Apollo 11 mission put humans on the moon. Since then, the moon remains the only planetary object — besides our own planet — visited by humans, and the things discovered in that epoch of exploration have led to a host of insights about how planets form and the nature of our own world.

As just one example, consider rocks. Reflected light allows us to learn some things about the composition of solid objects in space, but until Buzz Aldrin and Neil Armstrong brought back the first samples of rocks from the moon, we had never been able to study firsthand rocks as they existed naturally on the surface of another world.

Of course, we had studied meteorites, which occasionally bring bits of rock to Earth, but there’s a huge difference between studying rocks in their natural setting to studying fragments that have survived a fall through the Earth’s atmosphere.

Besides the immense technological achievement that the first lunar landing represented, it was our first chance to touch another planet. The lunar samples returned still are yielding scientific discoveries today.

I was recently asked about what specific day celebrates the anniversary of Apollo 11, but the mission itself was an endeavor years in the making that only culminated in July 1969. And even that culmination was an extended, eight-day mission.

That means, there actually are several days to note, and recognizing them helps get a sense of the enormity of the whole journey to the moon’s surface and back.

Apollo 11 mission

The spacecraft composing the Apollo 11 mission consisted of the Command Module, called Columbia, and the Lunar Module, called Eagle, and were launched together on a huge Saturn V rocket at about 8:30 a.m. (EDT) on July 16, 1969.

After a single orbit around the Earth, the linked spacecraft commenced the 240,000-mile trip to the moon with three astronauts aboard: Armstrong, Aldrin and Michael Collins. Reaching a maximum speed of about 24,000 mph, the trip to the moon still took more than three days.

For a sense of scale on that distance, when I teach about the size of the solar system in class, we create an outdoor scale model that stretches the width of campus. With the sun roughly the size of an orange and sitting just in front of College Church of the Nazarene, the orbit of Pluto is about the distance of the Burke Administration building on the corner of Convent Street and state Route 102.

On that scale, both the moon and the Earth are invisible specks, and the distance between them is the width of your thumbnail.

Armstrong, Aldrin and Collins reached lunar orbit just after noon on July 19. After a day in orbit, the two spacecrafts separated and the Eagle began its descent to the lunar surface at 1 p.m. July 20.

The slow descent took about two hours, with the Eagle landing on the surface of the Moon at 3:17 p.m. This craft carried only two of the astronauts —Collins remained aboard the Columbia still orbiting the moon. Once the Eagle had landed, six hours passed before Aldrin and Armstrong emerged on the surface.

The landing site for the Apollo 11 mission was the Sea of Tranquility, one of the large maria or dark spots on the moon’s surface that indicate lunar lowlands. These regions are younger and less rugged than the lunar highlands.

Though comparatively flat, the surface beneath the Eagle — as it came in for a landing — was filled with craters and boulders, and Armstrong had to pilot it in for a precision landing. You can’t see the site through a telescope, but the Lunar Reconnaissance Orbiter that mapped the moon in high resolution in 2009 captured images that show the descent portion of the Lunar Module left behind.

When Armstrong stepped onto the moon’s surface at 9:56 p.m. July 20, his first words actually were “That’s one small step for a man, one giant leap for mankind,” but for reasons of both pronunciation and radio static, the “a” wasn’t audible in the transmission.

Armstrong and Aldrin together spent just two hours on the surface, collecting samples and images and taking a phone call from President Richard Nixon.

The next day, just after noon, the Eagle launched from the surface to rendezvous with the Command Module and began the return trip to Earth. The craft splashed down July 24 in the North Pacific Ocean, and the astronauts were recovered by the USS Hornet.

Their ordeal wasn’t over though, as they were required to spend 21 days in quarantine to ensure they hadn’t brought back any harmful pathogens from the moon.

There were more missions to the moon. In all, 12 men walked on the moon in missions that also included Apollo 12, 14, 15 and, in December 1972, the last lunar mission, Apollo 16. Since then, NASA and other space agencies have focused on robotic exploration of the solar system and studied the long-term effect of human habitation of space in places such as the International Space Station in low Earth orbit.

The moon, however, remains the obvious next step for manned exploration of the solar system, and NASA has plans to return astronauts to its surface by 2024. Besides the scientific and technological benefits of such a return, the moon remains — 50 years out from the day we first set foot upon that giant rock in the sky — a symbol for much of humanity’s hopes for the future and a reminder of what unified endeavor has accomplished in the past.

Steve Case is director of Olivet Nazarene University’s Strickler Planetarium and an assistant professor in the department of chemistry and geosciences.

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