By Chris Fellows, Serenity Mobile Observatory
Understanding and observing our nearest celestial neighbor
As I looked through my previous articles and started planning for this week’s installment I realized that I have neglected one of the biggest and brightest objects in the sky, our moon. Well, let’s remedy that this week.
I need to be honest here. Although I find the moon beautiful and endlessly observable, it normally is more of an aggravation than a joy to astrophotographers such as myself. Due to the brightness of the full moon (it is magnitude -12.7), it tends to blot out the thousands of other targets I would typically be shooting. Astrophotography targets are normally in the magnitude 8 to 12 range or even dimmer. Remember, the human eye can only see down to about magnitude 6, so the targets I am hunting are completely invisible to the unaided human eye. Anyway, enough whining. Let’s take a look at the moon.
Like I mentioned in the subtitle, the moon is our closest, non-manmade, celestial neighbor. The average distance to the moon is 382,500 kilometers (237,674.4 miles). The distance varies because the moon travels around Earth in an elliptical not a circular orbit. At perigee, the point at which the moon is closest to Earth, the distance is approximately 360,000 kilometers (223,693.6 miles). At apogee, the point at which the moon is farthest from Earth, the distance is approximately 405,000 kilometers (251,655.3 miles). On astronomical scales this is very close. If the Earth were the size of a standard basketball (about 24 cm or 9.4 inches), the Moon would be about the size of a standard tennis ball (6.7 cm or 2.6 inches) and on average would be about 7.37 meters or 24 feet away from the basketball. Now to put that into perspective, on this scale the sun would be about 2.8 km or 1.75 miles away and be 26 meters or 85 feet across, the size of a very large house. Space is BIG.
Another very interesting feature of the moon is its rotation. The moon does rotate but it is what is called tidally locked to the Earth. What this means is that the moon’s rotation is on a 1:1 ratio with its orbit. With each orbit of the earth the moon rotates once, which keeps a single side of the moon always facing the Earth. You often hear people speaking of the “dark side of the moon.” This is completely misleading – the moon has days and nights just like the Earth. The only difference is that a day on the moon lasts 29.5 Earth days, the same amount of time it takes it to orbit the Earth. That is why the moon always looks the same when you observe it – you never get to see half of its surface from the Earth.
Have you ever noticed that the moon sometimes looks huge and sometimes small in the sky? People have many “armchair theories” about why this is. “The moon is at perigee when it looks big.” Or, slightly more sophisticated: “The Earth’s atmosphere magnifies the apparent size of the moon when it is close to the horizon.” Both of these are wrong and the true answer is both surprising and you can test it yourself! The reason the moon looks bigger when it is low in the sky is your brain is lying to you: It is a fantasy, a mirage that you can easily test. Next time you see the moon coming up over the horizon and it looks HUGE, perform this little experiment. Take a dime out of your pocket and while the moon still looks big, low in the sky, hold the dime at arm’s length and cover the moon, it should easily cover the moon. Now wait a few hours until the moon is high in the sky, looks different, eh? Now perform the experiment again. What did you discover? Let me know in the comments section – I would love to hear your feedback on this.
According to the most recent theories about the formation of the moon, it is truly a child of the Earth. Called the giant-impact hypothesis, our current ideas about the formation of the moon is that about 4 billion years ago, while our solar system was still forming and very chaotic, a Mars-sized planetoid collided at a glancing angle with the still-forming Earth. This fantastic impact stripped the lighter materials, mostly rock, from the surface of both worlds and tossed them into orbit around the primeval Earth, while the denser material, mostly iron and nickel, stayed with the larger body. The orbiting rock coalesced into the moon as we see it today and the nickel-iron sank to the center of the earth and remains there today, generating our very helpful magnetosphere and protecting us from deadly cosmic rays and the solar winds high energy particles. If it weren’t for this happy accident, life as we know it wouldn’t be possible on Earth, and Earth would look more like Mars.
OK, enough of this boring theory and history lesson. Let’s actually look at the moon. First of all, you don’t need anything but your eyes to observe the moon. With the naked eye you can pick out and identify many features of the moon. Space.com has consolidated and published very detailed maps that were put together but NASA and USGS that you can use to identify everything from the large Maria (Latin for seas) that you can see with the naked eye, down to the smallest craters you can see in the largest backyard telescopes. The first map on this webpage also shows the lunar landing sites of the Apollo and Surveyor missions.
Stepping up from naked eye to a decent set of binoculars will show a lot more detail on the moon’s surface. With a pair of 10X60 inexpensive binoculars, like these from Amazon, will let you see mountain ranges and large crater details. If you take the next step and purchase a small backyard telescope like this 127 mm Celestron PowerSeeker, you will be able to see some of the finest details on the moon.
However you observe, make sure to get out on several nights during the moon’s different phases. The moon truly changes from night to night depending on its level of illumination. The best observing is along the terminator, the line between night and day or the shadow line of the moon. When the moon is full it looks flat and the features are very difficult to pick out. You need to observe the moon when it is waxing and waning as the light at the terminator is at a very steep angle and shadows of mountains and craters are visible. It really make a huge difference from night to night.
The moon is in the sky every day and for more than half the month it is big and bright and up at night. Take a little time and get to know our closest neighbor: It will provide hours of stark beauty and exploration right in your backyard.
Till next time …
Chris Fellows, Serenity Mobile Observatory
Find Chris on Facebook (or, if you’re lucky, at your campground). (Editor: Check out his amazing photos on his Facebook page!)