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Lunar Craters

Lunar Features: July 2009

You can use this month’s focus on the Moon to learn more about lunar features. The most prominent lunar features, as seen from Earth, include impact craters, basins, and mountains.

Lunar Craters: Lunar craters result from the collisions of meteorites and comets with the Moon’s surface. There are many hundreds-of-thousands of craters on the lunar surface, ranging in sizes from mere millimeters to the largest, the South Pole-Aitken Basin, at 2,500 km. About three-hundred thousand craters larger than one-kilometer adorn the near-side of the Moon. The most prominent of those are Tycho and Copernicus.

Larger craters appear to have streaks, or rays, extending from their outer edges, like spokes surrounding the center of a bicycle wheel. These features are the result of lunar material ejected by an impact. Better-known ray systems include those surrounding Craters Tycho and Copernicus, as well as Aristarchus and Kepler.

Catenas, or crater chains, are another interesting lunar feature sometimes associated with craters. These occur when weaker incoming objects are fragmented by tidal forces before impacting. The comet ShoeMaker-Levy 9, which struck Jupiter in 1994, is a good example of an object being first fragmented by tidal forces and then impacting as several individual objects to form a chain of craters. The Moon’s Davy Crater Chain, is believed to be the result of such an event.

Interestingly, not only does the Moon’s near-lack of atmosphere contribute to its higher impact rates, those impacts ultimately contribute to what little lunar atmosphere there is, through a process known as sputtering. When atoms released by an impact are not fast enough to escape into Space, they become gravitationally bound within the lunar corona. This process is similar to that which contributes to the formation of planetary rings.

Lunar Plains/Planitia (Maria): This graphic names the better-known lunar maria, as seen from Earth.

Maria appear as darker splotches on the lunar surface. These are lower altitude regions, or basins, that are layered in solidified basaltic lava from ancient volcanic eruptions. Although the name maria means seas, these basins were likely never filled with water. Closer inspection will reveal many additional features within and along maria, including dorsa (wrinkle-ridges,) craters, and rilles (long grooves or canyons.)  Measuring seventy-five miles long and up to nine-hundred-fifty feet deep, Hadley Rille is one of the largest on the moon and marks the landing site of Apollo 15.

Lunar Mountains (Mons): While Earth’s mountains are created by plate tectonics, lunar mountains result from impact events. These landforms peak as high as five kilometers, the largest of which typically border maria. The tallest lunar mountain is Mons Huygens, of the Appenninus Mountain Range and near the aforementioned Hadley Rille and the Apollo 15 landing site.

As the moon wanes from Full to New this month, take advantage to explore the lunar surface.  To help you make the most of your observing sessions, I have some free lunar charts noted here (near the bottom of the post) and some information about lunar phases, here!

Our Night Sky: July 2009

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In honor of the 40th anniversary of the first manned lunar landing, I’ll feature Earth’s only natural satellite, the Moon, for the month of July.

(Neil Armstrong audio: “That’s one small step for man, one giant leap for mankind.”)
NASA Astronaut, Neil Armstrong, spoke those famous words on July 20, 1969, as he and fellow astronaut, Edwin (Buzz) Aldrin, became the first humans to set foot on the lunar surface.

While you and I will likely never know the thrill of actually touching the Moon’s surface, we can observe it up-close, using binoculars and telescopes. In fact, with its vast system of craters, plains, and mountain ranges, we can spend countless enjoyable hours exploring the lunar surface.

Each phase of the Moon offers something special. With a fuller moon, greater illumination means more surface area to investigate. As the Moon waxes and wanes, less illumination offers greater contrast to lunar features, especially those along the terminator – that line where light meets dark.

Knowing what you are looking at is always more fun than simply looking; so, exactly where and what should you explore when observing the Moon? For where to look, I recommend starting with one of the free online lunar charts or atlases; and I can give you a start with what to explore …

Craters, recognizable as brighter, circular features, were formed by impacts from asteroids and comets.

Plains, also known as maria, are those dark splotchy regions. Maria means seas, which is exactly what original observers once thought these ancient lava flows were.

Mountain ranges look like elongated ridges. Because of the moon’s low gravity and lack of erosion, lunar mountains can be very tall and especially steep and rugged.

Since this month does mark that 40th anniversary of the first humans on the moon, why not observe those regions where Apollo astronauts have landed?

That very first manned landing, Apollo 11 in 1969, touched down in the Sea of Tranquility. Four months later, Apollo 12 landed a few hundred kilometers west, in the Ocean of Storms. The last manned lunar mission occurred in 1972, with Apollo 17 landing at the Sea of Serenity.

Whenever, wherever, and however you observe the moon, be safe and be comfortable. Try keeping a journal for notes about, and even sketches of, what you see. And don’t forget to check my blog entries for lunar observing tips, information about the six manned lunar missions, and where you can get free lunar maps.

MORE ABOUT:

Lunar Atlases / Maps: You can find user-friendly lunar maps in bookstores, online, and in some libraries.

Books:

Ernest H. Charrington, Jr.‘s extensive book, Exploring the Moon Through Binoculars and Small Telescopes includes labeled images, science data, and detailed background information.

Peter Grego has two excellent guide books – The Moon and How to Observe it is a comprehensive guide that includes images, science data, background information, and in-depth observing tips for telescopes, binoculars, and the unaided eye. His Philip’s Moon Observers Guide is a more practical introduction handbook, complete with a daily observing diary, images, and tips.  You might also enjoy Peter’s archived Astronomy Now lunar observing articles.

Online Maps: There are many lunar charts, atlases, and maps available online, including premium software applications and free charts.

Created by Christian Legrand and Patrick Chevalley, The Virtual Moon Atlas is a free application that you can download directly to your desktop.  While this user-friendly program is reasonably self-explanatory for general use, the Cloudy Nights website does offer a Beginner’s Tutorial for basic configurations.

The Full Moon Atlas is a series of interactive maps that can be accessed either online as a free-preview version or via your desktop as a nominally-priced full version.  This program includes catalogs, detailed images, mouse-over identification, and a lunar phase calculator.

The Observatoria ARVAL Moon Map is an online one-page feature of an annotated Full Moon image and is perfect for quick identification of various lunar features.

The Moon Society offers an excellent learning opportunity on their website, as co-sponsors of the American Lunar Society‘s “Lunar Study and Observing Certificate Program.”

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