Try this exercise to see how far the moon is from Earth

Saturday, Jul 13, 2019 4:03 AM

Greetings, Stargazers.

This month marks the 50th anniversary of the Apollo 11 moon landing. Some of us were lucky enough to be around when that happened.

I was a space enthusiast since before I can remember. For the landing on the moon, my dad took me and a friend “camping” in one of the screened-in shelters that had electricity at Huntsville State Park near Houston. We watched the event on a small black-and-white television, and it was certainly memorable.

I remember not being able to discern most of what was shown with the stark black-and-white images, and I certainly didn’t catch the now-famous “one small step” quote as Neil Armstrong stepped onto the lunar surface. In the weeks that followed, I was one of many my age who were certain that there would be lunar colonies before the end of the 20th century. Maybe we’ll get them in the 21st century.

The moon really is a long way from the Earth. To get an idea of scale, you can use a standard 12-inch globe for the Earth and a baseball to represent the moon. A baseball is slightly small for this scale, but is a much better approximation than a softball. The International Space Station would orbit this globe every 92 minutes at a distance of less than half an inch from the surface. Geostationary weather and communication satellites with orbital periods of 24 hours would be almost 3 feet away.

To place the baseball moon at the correct distance from the globe, it would need to be almost 30 feet away. At this distance, you could hold your little finger at arm’s length and easily cover the baseball. In the same way, you can always cover the real moon with your little finger. Covering the moon with your finger is also a good way to overcome the optical illusion of the moon appearing much larger when it is near the horizon.

The moon’s orbit is somewhat eccentric or, in other words, elliptical rather than circular. The farthest point in the orbit is almost 12% farther than the closest point. The moon’s orbit is also tilted by 5.2 degrees when compared with the Earth’s orbit around the sun. This is why we don’t see eclipses every month – the shadows simply miss.

Besides causing tides in the oceans, the moon’s gravity also causes the interior of the Earth to deform a bit. This tidal friction is actually slowing down the rotation of the Earth. Today, a day is about a millisecond longer than it was a century ago. As a result, there are occasionally leap-second days, the last one being in 2016. Unfortunately, the details of these frictional forces are hard to predict, so leap seconds are only added when needed to keep the local noon at the same time of day.

This month

The full moon is July 16, but this would not be the best day to look at it through binoculars or a telescope. Shadows are what make the surface details stand out. The line between the light part and the dark part is called the terminator, and it provides the most details. Because the location of the terminator changes every day with different phases of the moon, you can see different surface details throughout the month.

Saturn reached opposition July 9, so this is the month it is closest to Earth. Saturn and Jupiter are both far enough away that the monthly distance changes make very little difference in how they look through a telescope. The most important thing is to wait until they are well above the horizon so our atmosphere doesn’t interfere with your views.

The next Chimney Rock dark sky event is scheduled for July 26.

Charles Hakes teaches in the physics and engineering department at Fort Lewis College and is the director of the Fort Lewis Observatory. Email him at hakes_c@fortlewis.edu.