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Lunar Directions A Digital Supplement to Astronomy Insights Astronomy Magazine Shoot the Moon How to Capture Stunning Lunar Detail with Modest Equipment April 2016 • Astronomy.com Moon-watching You should observe the Full Moon! Mountains and lava plains, rays, and a hint of color invite exploring a bright Moon. by Michael E. Bakich Anthony Ayiomamitis Anthony Observer 1: “It’s clear. Let’s set up the scope.” Observer 2: “Nah. It’s Full Moon.” Observer 1: “So, we’ll look at the Moon.” Perhaps the oddest ray system on the Moon Observer 2: “Are you nuts?” originates at the craters Messier and Messier A. I’m betting that if you haven’t had this rise as much as you can so you’re looking can reach 22 percent. Likewise, basalt has a conversation, you’ve gone through the gist through less of Earth’s image-distorting maximum of 2 percent of the mineral tita- of it in your mind. Observe at Full Moon? atmosphere. Also note that if the view is nium dioxide (TiO2) on Earth, whereas Observe what, exactly? The Moon’s intense uncomfortably bright, many manufacturers lunar basalt can contain up to 13 percent light scatters through the sky, essentially sell neutral density filters, which reduce the TiO2. Iron and titanium are dark materials eliminating every deep-sky object that’s not amount of light reaching your eyes but not and contribute to the overall murky appear- a double star. And you certainly don’t change it in any other way. ance of the maria. observe the Moon when it’s Full because Furthermore, because different lava that’s when the Sun lies highest in its sky on Explore the seas flows have different concentrations of these the part facing us, killing all details. Early lunar observers dubbed the large, flat substances, the floors of the maria reflect Not so fast. Believe it or not, several of expanses visible to the eye maria (Latin for different percentages of light. You can our natural satellite’s features are at their “seas”) because they look like dark water- prove this to yourself by looking at the best when its shape is roundest. That’s filled basins. Today we know that, rather Moon the next time it’s Full. Even your because in addition to formations that cast than water, these large depressions contain unaided eyes will show that some maria are shadows (like mountains and crater walls), vast solidified flows of lava made of basalt. slightly darker than others. For example, the Moon also contains albedo features, The lava on the floors of the maria take a look at the three connected maria on which depend on differences in reflectivity appears much darker than the surrounding the Moon’s eastern side: Mare Serenitatis, and color. (Oh, yes. Luna’s surface displays highlands, which are mostly anorthosite. Mare Tranquillitatis, and Mare Fecundita- a range of subtle colors.) Albedo features That rock contains between 90 and 100 tis. Each of them appears darker than either don’t cast shadows under any lighting. percent of the reflective mineral plagioclase Mare Imbrium or Oceanus Procellarum on Three types of features are especially feldspar. The contrast between the maria the western half. great to observe when Earth’s nearest and the highlands is highest at Full Moon. To see bright and dark areas within a neighbor hangs like a ripe honeydew melon While Earth basalt tops out at about 14 single mare, point your telescope at Mare in the night sky. Remember to let the Moon percent iron, on the Moon that percentage Serenitatis. Its center looks lighter than its edges. The differences in composition are because the two regions have different ages. The outer areas date from between 3.8 and 3.85 billion years ago while the center region may have formed as recently as 3.2 billion years ago. The Moon also features a few craters with ultra-dark lava-covered floors, and these stand out best at Full Moon. The prime example is Plato Crater, which Michael E. Bakich is an Astronomy senior editor and author of 1,001 Celestial Wonders to Plato Crater’s floor is so dark that early observers called it the “Greater Black Lake.” Dave Tyler See Before You Die (Springer, 2010). 2 ASTRONOMY INSIGHTS • APRIL 2016 Plato Endymion Lunar directions Mare Imbrium Mare Serenitatis It’s easy to determine the Moon’s north- ern and southern halves because they Le Monnier correspond to those directions in our Aristarchus sky. Figuring out east and west, how- Oceanus ever, is not so intuitive. Procellarum In 1961, the International Astronomi- Copernicus Kepler Mare Tranquillitatis cal Union adopted the same system of A directions for the Moon as that used on Mare Insularum Messier Earth. This means an observer on either Mare world sees the Sun rise in the east and Fecunditatis set in the west. So, when we look at the Moon from the Northern Hemisphere, the eastern half (the portion lit at First Quarter) is to the right and the western half (Last Quarter) lies to the left. astronomers as early as the 17th century Tycho referred to as the “Greater Black Lake.” Like the maria, Plato filled with dark lava approximately 3.84 billion years ago. Other craters with dark floors are Le Monnier and Endymion. Numerous lunar features are visible when the Moon is Full. This image corresponds to a “naked- During Full Moon, compare Plato with eye” view with north at the top and contains all boldface features in this story. Lick Observatory Aristarchus Crater, our satellite’s brightest such feature. It appears so dazzling because it’s young — only 450 million years old. The crater with the next most promi- Look for the colors When you’re viewing lunar features with nent rays is Copernicus. Because it formed While it’s true that the Moon doesn’t dis- similar compositions, older ones will 810 million years ago, researchers struggled play a vivid rainbow of hues, you can use appear darker because the process of space for decades to explain why its rays — which color filters to enhance what differences weathering (by the solar wind, cosmic rays, spread out for 500 miles (800km) — remain exist. The basalt in the maria, for example, and micrometeorites) has affected them bright. Current theory suggests that the appears a bit bluer than the anorthosite in longer. In Aristarchus’ case, the material crater’s impactor excavated bright highland the highlands. So, to darken the maria and thrown out by the impact that formed it material and sprayed it across the dark sur- enhance the contrast between them and the hasn’t darkened much. face of Mare Insularum. highlands, try a red filter. Copernicus, along with Kepler Crater to Note: Several manufacturers make filters Trace lunar rays its west and Aristarchus to Kepler’s north- in different shades, for example, light-red, Sometimes a dark surface covers lighter west, form a right triangle of rayed craters medium-red, and red. Because you’re material. If, as has happened millions of on Oceanus Procellarum that merits your observing the Full Moon, use the darkest times, a meteor hits the Moon, the impact attention. A low-power eyepiece in your filter. It will have the added benefit of cut- can splash some of what’s underground scope reveals rays from each overlapping ting down our satellite’s light. across the landscape. And the bigger the those from the other two and bright pairs To get slightly better views of the rays, meteor, the longer and brighter the splash. of rays connecting all three. try light- or medium-green or yellow filters. Astronomers call such ejecta “rays.” Most You’ll find a unique ray system in Mare Depending on the color sensitivity of your resemble the spokes of a wheel and extend Fecunditatis associated with two small cra- eyes, one of these filters may enhance the for many times their crater’s diameter. And ters — Messier and Messier A — that you contrast between the material splashed out Full Moon is the best time to view them. simply must observe. Two straight, slightly when the meteor created the crater and the Tycho Crater has the most extensive ray diverging rays more than 60 miles (100km) darker surface rock. system. Some of its spokes reach as far as long extend westward from Messier A, So don’t be too hasty to cross off the 930 miles (1,500 kilometers) from the leading most observers to describe the dates, as the song goes, “when the Moon impact point. Tycho and its ray system are scene as “comet-like.” hits your eye like a big pizza pie.” To many so bright that you even can see them when Here a meteor first formed Messier and amateur astronomers, that’s observing! only earthshine (sunlight reflected from then bounced to form Messier A. The Earth to the Moon’s night portion) illumi- oblong shapes of the craters and the direc- Find more lunar features to target at www.Astronomy.com/toc. nates the thin crescent Moon. tion of the rays indicate a low impact angle. WWW.ASTRONOMY.COM 3 ASTROIMAGING he Moon has a special place in my heart. I took my first lunar photo- graph in 1958, but it was more than four decades later before I achieved results that matched my love for the Shoot the TMoon. A look at the history of lunar pho- tography reveals I was not alone; for a cen- tury, professional astronomers also faced a resolution barrier. The invention of the dry photographic plate process in 1871 by Richard L.
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