~ by Dr. Sarah D. Oktay, Director, University of Massachusetts Nantucket Field Station ~
In just a couple of hours I will go outside along with many people in the United States to watch the total lunar eclipse on a lovely night with some cloud cover. This year is special because the lunar eclipse is occurring during a full moon, and a harvest moon, AND a so called “super moon”: the trifecta of moon gazing nights. All this means is that it is the full moon closest to the fall equinox, when the moon is closer to the earth (“super moon” or perigee moon) and hence looks bigger. The proximity of the moon in perigee on this September 27 event gave this event the grand sounding title, “super moon eclipse.” The last such eclipse happened in 1982, and the next won’t occur until 2033. The lunar eclipse occurs when the earth blocks the sun from reflecting on the moon’s surface will cause the moon to look blood red in color as red light is refracted around our dusty atmosphere and bounces off the moon’s surface. If the Earth had no atmosphere, the Moon would be completely dark during an eclipse. A lunar eclipse can occur only when the sun, Earth and moon are aligned exactly, or very closely so, with the Earth in the middle. Therefore, a lunar eclipse can occur only the night of a full moon. The term for this is being in “syzygy” and yes that was on my spelling word list the past few years. These lunar circumstances line up (if you will pardon a very bad pun) roughly every 20 years and have occurred five times since 1900. The most spectacular part of the eclipse will be the totality phase, when Earth’s shadow completely covers the moon and turns it an eerie red. The moon will dip into the deepest and darkest part of Earth’s shadow, or umbra, during the totality phase, which lasts as long as 72 minutes. Many recent total lunar eclipses have been very short, sometimes only lasting five minutes, so this is an excellent chance to see one thoroughly. The type and length of an eclipse depend upon the Moon’s location relative to its orbital nodes. Unlike a solar eclipse, which can be viewed only from a certain relatively small area of the world, a lunar eclipse may be viewed from anywhere on the night side of the Earth.
This weekend’s blood moon will be the last in a series of four lunar eclipses, dubbed a tetrad, over the last two years. That pattern won’t repeat for another 20 years or so. The Maria Mitchell Observatory scientists will be holding a special viewing session that I hope some of you all attended. I enjoyed watching the moon slowly being covered by the earth’s shadow and then even more slowly remerge.
In the language of science, an equinox is either of two points on the celestial sphere where the ecliptic and the celestial equator intersect. For the rest of us, it’s one of two times a year when the Sun crosses the equator, and the day and night are of approximately equal length. For many people the autumnal equinox is a sign of fall and a harbinger of cool nights and approaching holidays. “Equinox” means literally, “equal night.” As the angle of the earth’s inclination toward the sun changes throughout the year, lengthening or shortening the days according to season and hemisphere, there are two times annually when day and night are of roughly equal duration: the spring and autumnal equinoxes. These celestial “tipping points” have been recognized for thousands of years and have given rise to a considerable body of seasonal folklore. At the autumnal equinox (September 23, at 08:20 UTC), the Sun appears to cross the celestial equator, from north to south; this marks the beginning of autumn in the Northern Hemisphere. The equinoxes are not fixed points on the celestial sphere but move westward along the ecliptic, passing through all the constellations of the zodiac in 25,800 years. This motion is called the precession of the equinoxes. http://yesterdaysisland.com/archives/science/21.php
The moon and lunar cycles are extremely important to biologists and oceanographers. During seasonally high tides more habitat is created for mosquitoes and crabs and fish as greater amounts of land is flooded. After viewing the eclipse, my first thought was that the color and translucence reminded me of an embryo or egg. That image in turn made me think of how dependent horseshoe crab eggs are on the tide which is influenced by the moon. Horseshoe crab spawning season varies according to latitude, but it generally peaks in May and June, with peak spawning occurring on evening high tides during the full and new moons (the higher-than-normal “spring” tides).
If you paid attention last week you might have noticed the higher tides which became greater as the full moon approached. Tides are caused by the gravitational pull of the sun and the moon on the earth. The sun contributes about one third of the pull, and the moon two-thirds. The earth and the moon are attracted to each other in a gravitational sense and the moon pulls at the oceans.
Each day, there are two high tides and two low tides. When the moon is full or new, the gravitational pull of the moon and sun are combined. At these times, the high tides are higher and the low tides are lower than other times of the month. The combined effects of this tide along with the higher tides of a equinox may produce minor flooding on the coast.
Sea turtles are also famously associated with the moon. For many years people thought that sea turtles needed the moonlight of a full moon to find their way to the ocean. This is in fact a myth. Sea turtles do go for the brightest light as they hatch from their nests and prefer to be born at night so they can more easily find the horizon and the starlight or moonlight reflecting off the sea. They will avoid the dark silhouettes of dunes or vegetation and flap cutely on their way to the sea. The colder temperatures of the sand at night is their cue that it is time to be born; under cover of darkness they have a better chance of surviving the short but dangerous journey to the surf. Their sea-finding behavior can take place during any phase and position of the moon, which indicates that hatchlings do not depend on lunar light to lead them seaward. One thing that scientists have observed is that the presence of lights from cars or oceanfront development can confuse the hatchlings and cause them to rush in the opposite direction. http://myfwc.com/research/wildlife/sea-turtles/threats/artificial-lighting/
And you may recall I have written about moths a couple of times in the past, one of my students this weekend repeated the adage that moths are attracted to light because they think it is the moon and once again a likely sounding story turn out to not be true. Moths do use the light of the moon to navigate but they do not head toward it. Dr. Mike Saunders, an entomologist at Penn State explains: “Moths often use the moon to orient themselves during night flight. In visual terms, the moon appears at “optical infinity,” i.e., far enough away that the rays of light it reflects toward Earth are parallel as they enter a moth’s (or a human’s) eye. This constant makes an excellent navigational tool. Using the moon as a reference, moths can sustain linear flight in a given direction.” http://news.psu.edu/story/141283/2008/10/20/research/probing-question-why-are-moths-attracted-light. I have a moth pollinated plant at the foot of my stairs, the evening primrose. The night blooming plant entertains my husband Len and the junior rangers at the Nantucket Field Station. The reason that evening primrose flowers open at night is to lure their primary pollinators, moths. The plants’ flowers are noctodorous; they release a subtle fragrance at night to lure the lepidopterans.
Before this story is due I have had time to see most of the lunar eclipse which lived up to the hype. Relatively clear skies with just a few suspense-buildings clouds allowed us to observe the cloaking and uncloaking of a maroonish-orange orb of light 221,753 miles (356,877 km) away. More about lunar eclipses can be found at https://en.wikipedia.org/wiki/Lunar_eclipse.
Last but not least, the public is welcome and encouraged to attend the Nantucket Biodiversity Conference here on Nantucket on November 13-15th. Scientists working for the island conservation, education, and science originations will present recent research. More information can be found at www.nantucketbiodiversityinitiative.org