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First blue moon since 2012 will cross sky on Friday

A blue moon (looking rather orange) was photographed on Dec. 31, 2009, in Nebraska. The second full moon in a calendar month is called a blue moon.
A blue moon (looking rather orange) was photographed on Dec. 31, 2009, in Nebraska. The second full moon in a calendar month is called a blue moon. Associated Press file

As rare as a blue moon? Not this week, when the first blue moon since August 2012 will grace us with its presence on Friday evening.

With typical summer clear skies forecast for the Sacramento region, nothing should get in the way of seeing the celestial event.

A blue moon can refer to the second full moon in any calendar month, and the astronomical rarity occurring at the end of the week was preceded by a full moon on July 2. The next blue moon won’t occur until 2018, when there will be the even rarer occurrence of two blue moons in a single year – one on Jan. 31 and one on March 31. There will also be a total lunar eclipse on Jan. 31. Although every total lunar eclipse requires a full moon, Dec. 30, 1982, was the last time a blue moon was involved.

The phases of the moon – from the invisible new moon through a full moon and back to invisible again – take an average of 28-29 days. If a full moon happens very early in a month, there’s enough time for a second one to follow in the same month.

The timing is the rarity, not the way the moon looks. On Friday night, “it’s going to look like a typical full moon,” said Jayce Pearson, flight director for the Challenger Learning Center at the Discovery Museum Science and Space Center.

He elaborated on a common lunar optical illusion: If you catch a glimpse of the moon – blue or otherwise – while it is low on the horizon, it may appear larger than how it looks when it is high in the sky. To test how your eyes are playing tricks on you, take a picture of the moon close to the horizon and at its highest point, compare photos and see that the moon is the same size at both times.

A moon can appear blue, although this is even rarer than the so-called blue moon like Friday’s. Debris in the sky, such as ash from a volcano or from a forest fire, can act as a filter and cause us to see the moon with a colored hue. A fire in the area and smoke in the atmosphere, common to Sacramento, can cause the moon to have a slight reddened appearance.

Rare blue or common, the full moon isn’t welcomed by all.

Walt Heiges, president of the Sacramento Valley Astronomical Society, says a full moon is the “worst time to go out and look” at the sky. If you want to go look at the “night sky to see deep-sky objects, you don’t want a shiny moon in your way,” he explained.

The best time to see the stars, he said, would be during the new moon phase when the moon is absent (Aug. 14).

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