How fortunate to have been in Flagstaff, AZ last evening. The group of high school students I am leading on an earth science trek through the Grand Canyon state had the opportunity to witness celestial splendor on Mar. 13, 2013. They saw comet PANSTARRS and a glorious sliver of a waxing crescent moon.
The experience would not have been as grand, had the wonderful folks at the Lowell Observatory not gone out of their way to make sure we were part of the crowd (numbering somewhere between 100 and 200 people). Standing in clear, crisp, mountain air at an altitude of 7,200 feet, those atop the observatory’s highest viewing point, eagerly looked toward the west. Approximately 30 minutes after sunset, people started to see the comet, a faint dot in the western sky, just below and to the right of the crescent moon. Some looked through binoculars; others through a comparable resolution telescope. I opted for my Nikon D5100 camera with a 300mm zoom lens. I opened the shutter to its widest setting and applied the sports setting speed (Fig. 1).
The comet appeared exactly as advertised, with its tail pointing upward, away from the Sun. This because the solar wind (moving away from the Sun at speeds of about a million miles an hour), literally pushes charged particles in the comet’s tail away from it. As PANSTARRS continues its trek around the Sun (just extend pathway in Fig. 2) toward the right and then swinging around and behind the Sun’s position on a left-moving track as it leaves our Solar System, the comet’s tail will continue to point away from the Sun and also point away from Earth. According to NASA, the solar wind is, “…the plasma of charged particles (protons, electrons, and heavier ionized atoms) coming out of the Sun in all directions at very high speeds -- an average of about 400 km/sec, almost a million mph! It is responsible for the anti-sunward tails of comets and the shape of the magnetic fields around the planets. Solar wind can also have a measurable effects on the flight paths of spacecraft.”
Due to the comet’s path, only observers at high northern latitudes will be able to see PANSTARRS sometime overnight for the next month or so. All the while, the comet will be getting fainter because it will be moving away from Earth.
While all the fuss over PANSTARRS continued at Lowell Observatory, I turned my camera upward to capture the crescent moon. In addition to the brightly lit bottom edge (the part of the Moon that we can view from Earth that is facing the Sun), it is easy to see the darker “seas” (also known as “mares”) and the lighter mountainous areas (Fig. 3). This brightening of the normally “invisible” dark part of the moon during non-full moon events is due to sunlight being reflected from the Earth and illuminating the rest of the Moon’s surface (earthshine as opposed to “moonshine”).
The “seas” are newer formations (lava flows) and, thus, have fewer pock marks (a.k.a., meteor impact craters). The “land” areas are more mountainous and are loaded with meteor impact craters. Scientists believe that most of the cratering and volcanic activity took place more than 3 million years ago.
For more information about the moon, check out these links.
Earth's moon: formation, composition and orbit (space.com)
10-surprising-moon-facts (space.com)
Geology of the Moon - University of Virginia Department of Astronomy
© 2013 H. Michael Mogil
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