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Supermoon
An image of the moon taken by NASA’s Lunar Reconnaissance Orbiter is shown in two halves to illustrate the difference between the apparent size of a supermoon (left) and a “micromoon” (right).

Supermoon Sunday is at hand, and although some may scoff, the supermoon concept provides a good excuse to take a close look at a celestial sight we often take for granted.

By some measures, Sunday night’s full moon is the only supermoon of 2017. The liberal definition would be a full or new moon that’s at or near its closest approach to Earth in its orbit. My definition is stricter: There’s only one supermoon in a given year, reserved for the full moon with the biggest apparent size.

Supermoons are squishy because there’s no official astronomical definition of the term. The idea merely reflects the fact that the moon’s apparent size varies depending on how close it gets in its elliptical orbit around Earth. At its closest, it looks 14 percent larger and 30 percent brighter than it does at its farthest.

The difference is noticeable for close observers — particularly if the moon is near the horizon, where it can be compared with terrestrial landmarks. But for not-so-close observers? Not so much. That has led some astronomers such as Neil deGrasse Tyson to pooh-pooh the phenomenon.

“The supermoon is a 16-inch pizza compared with a 15-inch pizza,” he says. “It’s a slightly bigger moon; I ain’t using the adjective ‘supermoon.'”

Fortunately, Tyson’s word isn’t gospel, whether we’re talking about supermoons, dwarf planets or adjectives vs. nouns. If you want to celebrate the year’s most super supermoon as a holiday for moongazers, I don’t see anything wrong with that.

The bigger question is whether we’ll be able to see the supermoon in Seattle. The weather forecast calls for partly cloudy skies on Sunday night, and you can check the National Weather Service’s graphical forecast for a more specific prediction. (Click on the link for “Sky Cover.”)

If you’re clouded out on Sunday, you can still get into the supermoon spirit on Sunday with streaming video from the Virtual Telescope Project or the Slooh online observatory. You’ll have another chance to see a supermoon on Jan. 1, which marks the brightest full moon of 2018. And get ready for a total lunar eclipse on Jan. 31. That’ll be a super moon, even by Neil deGrasse Tyson’s standards.

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