Tomorrow evening, gather your family and friends, pack snacks, and settle into the backyard or wherever the due-east horizon is visible to watch an eclipsed harvest moon rising above our wind-swept landscape.
Several reasons make this eclipse special.
First, Sunday’s moon is a supermoon — a full moon that is larger and brighter than usual.
Second, we’ll have to wait two years to watch the next total eclipse of the moon and 18 more years to glimpse the next supermoon eclipse.
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Third, this eclipse is the fourth and last in what astronomers call a lunar tetrad. That’s a series of four consecutive total eclipses, each separated by six months.
And last, the total phase of Sunday’s eclipse conveniently takes place between 7:11 p.m. and 8:23 p.m. No setting of alarm clocks to awaken us for this eclipse.
A total lunar eclipse occurs when the full moon passes through the dark shadow (umbra) cast behind the Earth. This requires a nearly perfect alignment of the moon, Earth and sun.
Tomorrow evening, the moon begins to enter the shadow at 6:07 p.m. However, because the moon does not rise in the Pacific Northwest until shortly after 6:40 p.m., most of the early partial phase of this eclipse remains hidden from view. When the moon does lift above the horizon as the sun is setting, it will already be two-thirds covered by the Earth’s shadow.
Initially, the moon may appear dim if haze or clouds mask the horizon. If so, the eclipsed moon will be easier to spot as it ascends and darkness rolls across the country side.
With each passing minute, the moon buries itself deeper inside the umbra until totally eclipsed at 7:11 p.m. This marks the beginning of totality.
The shadow covering the lunar surface may glow ruby red to a singed pineapple yellow. These colors are tinted by sunlight passing through a continuous rim of sunsets and sunrises encircling the dark side of the Earth. Like children, each eclipse is different.
During totality, the moon’s southern edge will shine slightly brighter than the rest of the moon. This happens because the moon is skimming just above the bottom inside edge of the shadow.
On the other hand, the moon’s north pole will appear darker because it crosses deeper inside the shadow.
Totality ends at 8:23 p.m. as the moon’s left side brightens and nudges from the umbra.
During the next hour, the moon continues to slip outside the shadow. By 9:27 p.m., the eclipse has finished and the moon’s familiar charcoal gray colors will have returned.
Lunar eclipses are easily seen with the unaided eye. Binoculars will enhance viewing enjoyment.
Notice the fuzzy, curved shape of the shadow’s edge as it slides across the lunar landscape. That fuzziness originates from scattered sunlight passing through the Earth’s atmosphere before shading the moon.
And if there is any doubt, the shadow’s curve shape again proves that the Earth is round.
Sunday’s moon is traditionally called the Harvest Moon because it’s the first full moon closest to the start of autumn.
Supermoon is seemingly a fast-food word coined over 30 years ago by an astrologer (not an astronomer) and then popularized by the news media and internet in recent years.
A supermoon sounds gargantuan, but it’s only slightly wider (by 14 percent) and brighter (by 20 percent) than an “average” full moon.
The moon changes size because its orbit around the Earth is elliptical, not circular, and therefore always changing distances. During each orbit, the moon’s distance varies between 250,000 miles and 220,000 miles.
Sunday’s moon just happens to be orbiting at its closest point (perigee) to Earth — and therefore appears larger than usual.
The term describing this coincidence of an eclipsed moon orbiting nearest to us is “perigee-syzygy.” Two great board-game words. Little wonder even astronomers prefer to say “supermoon.”
Sunday’s supermoon has been hyped to foretell catastrophic tidal surges, devastating volcanic eruptions, planet-crunching earthquakes and Earth-grinding asteroids.
Relax. Don’t sell any stocks. No apocalypse.
Supermoon eclipses have repeated themselves for eons, and throughout all the years humans have remained safe as they stand quietly to marvel at the beauty and peacefulness of a shadowed moon crossing the heavens.
Roy Gephart is a retired environmental scientist and avid amateur astronomer. He can be reached at firstname.lastname@example.org.