A much-anticipated total solar eclipse takes place on Monday, August 21. Fortunately, the ground track for this astronomical event will slice across the middle section of the continental United States, so that some portion of the eclipse can be viewed by millions of Americans. This viewing geometry is quite rare, and the conditions needed for it occur only sporadically. The Moon varies in distance from Earth during the course of its orbit, so for a total eclipse to develop, the Moon must be close enough to Earth to block out the entire apparent disk of the Sun (about half of a degree in angular width—roughly the size of a small pea held at arm’s length). Even when they do occur, because the Moon casts a very narrow shadow, a total solar eclipse is visible from only a small fraction of the Earth’s surface. Moreover, about 70 percent of the Earth’s surface is ocean, so many expeditions use ships for eclipse viewing; a mobile platform also has the advantage of being able to move to areas of likely clear sky for viewing.
If the Earth and the Moon were perfectly smooth bodies, the timing and duration of a solar eclipse would be perfectly regular and predictable. But topography creates issues. We’re familiar with the mountains and valleys of the Earth, but until recently, had imperfect knowledge of the figure (i.e., global shape) and topography (of surface features) of the Moon. Thanks to the Lunar Reconnaissance Orbiter (LRO) spacecraft circling the Moon since mid-2009, we now have a high-precision global map of the surface topography and shape of our nearest planetary neighbor. The mountains and craters of the lunar surface create departures from an ideal circular shape and thus, departures from the starting times and durations of eclipse totality. Such minor variations didn’t used to matter, but today, with atomic clocks, cell phones and hand-held GPS, people want to know when things are going to happen down to the exact second.
NASA has used the new topographic data from LRO to make an eclipse prediction map that allows us to prepare for the exact moment of totality (or partial totality, depending on where one is). The totality zone is a swath across the mid-section of the country about 70 miles across, while a partial eclipse will be seen over most of the country. On Monday, the Moon’s shadow will cross the Oregon coast at about 10:12 am PDT and whisk across the country from west to east at a breathtaking 1,650 mph, taking about 90 minutes before it exits over the coast of South Carolina at 2:45 pm EDT. The exact time of the arrival of totality in your locale can be seen on a map produced by NASA for the event. The duration of totality as it passes over likewise varies, from about 2 minutes on the Pacific coast, increasing to 2 minutes 40 seconds in the eastern third of the country, and leaving the Atlantic coast at 2 minutes 30 seconds duration. So don’t be late or you just might miss the whole show! Not to worry though—this event will be widely covered and a quick online search will give you a choice of places to watch it from inside. You can also make a pinhole projector (camera obscura) to view the eclipse without looking at the sky, not to mention live television on the Internet of the spectacle from around the nation.
As we consider the upcoming eclipse, it is only natural to reflect on the Moon’s closeness. The Moon is the only natural celestial body close enough to cast a shadow on the Earth. Both Mercury and Venus occasionally pass between the Earth and the Sun (called transits), but because they’re so far away, they are mere specks in the glare of the solar disk. In contrast, the Moon covers the Sun during a total eclipse, allowing us to behold the breathtaking beauty of the solar corona—the streams of energetic particles expelled by the Sun that create the “solar wind.” Although always present, the solar corona is much dimmer than the surface of the Sun, and it can only be seen from the Earth during an eclipse.
This stream of solar particles continues beyond the corona, spreading out into space to encounter and interact with all of the planets. On Earth, the particles (solar wind) collide with our global magnetic field and create trapped belts of excited particles that give off a ghostly multi-colored light—the colorful aurora visible in both north and south polar latitudes. On objects where there is no global magnetic field (like the Moon), the solar wind directly impacts the surface. Some of these particles stick to the dust grains, forming a layer of atoms and molecules. Since the most abundant species in the Sun is hydrogen, solar wind protons (hydrogen nuclei) are the most abundant element of these adsorbed gases. Because the lunar dust retains these particles, we can use the soil of the Moon as a source of hydrogen—a rare element on the Moon. Hydrogen is an essential component of water, and many of these solar wind ions may be the ultimate source for at least some of the water ice found in the polar regions of the Moon. So the Sun is not only a life-giving source of energy for Earth, its presence in the lunar surface by implantation of solar wind may eventually provide water—life support—and propellant for future inhabitants of the Moon.
Thus, the forthcoming eclipse is a reminder of the proximity, promise and utility of the Moon. The closeness of the Moon permits us to accomplish much of the early work of lunar return with robotic spacecraft, under the control of operators on Earth—small missions of low mass, permitting the use of existing launch vehicles, off-the-shelf equipment and state of the art technology (no magic beans required). Finding significant deposits of ice will require surface access. The round trip, radio travel time of three seconds between the Earth and the Moon means that remotely controlled equipment can be operated from Earth in near-real time to explore and map the prospects of water ice at the poles to better understand the surface environment (no mission from any country has ever soft-landed at the poles). Once the most desirable deposits are found, we must experiment with ice extraction and processing to understand the issues involved in harvesting the water of the Moon (and by extension, water harvesting on other bodies in space).
The wind from the Sun constantly strikes the day-lit lunar surface, providing an available resource for our use and extraction. Both solar wind hydrogen (which is found everywhere on the Moon) and the polar ice (which may ultimately have its origin as solar wind particles) enable long-term human presence on the Moon. In the most dramatic fashion imaginable, the upcoming eclipse showcases the next logical destination in space.
I’ll close with some cautionary notes. Never look directly at the Sun, either before or after the eclipse. Protective solar glasses, available from reputable vendors, allow you to look at the Sun; welder’s glasses are a good substitute if you cannot find eclipse glasses in your area. Sunglasses will not protect your eyes from severe damage. During totality (and only during totality), you may look at the sky without protective solar glasses. The moment totality is over (evidenced by the first emergence of bright sunlight) have your protective glasses back on. With luck, you’ll have enough clear sky to enjoy one of nature’s most magnificent spectacles—and when you do, be sure to ponder humanity’s future on the Moon, and what it would be like to see a solar eclipse from the lunar surface with the Earth blocking the sun, revealing billions of unseen stars in an otherwise black lunar sky.