If you live in the Americas, you'll have to get up early on Saturday morning to catch a glimpse of a partial lunar eclipse - but this one should look bigger than you'd expect, thanks to a trick of the eye.
This lunar eclipse is actually the second event in this year's eclipse parade, and arguably the least spectacular blackout of the bunch. The really big events are coming up ... including an exotic total solar eclipse next month and a perfectly timed lunar eclipse in the midst of the December holiday season.
The heart of Saturday's eclipse won't even be visible from parts of the East Coast. But the fact that it takes place close to sunrise means the eclipsed moon will be low in western skies as seen from the rest of the United States - and that means the well-known "moon illusion" will come into play.
The moon always looks bigger when it's near the horizon, as compared with when it's high in the sky. But the reasons for that are still a matter of debate among psychologists: One factor is that the moon's proximity to the horizon leads the viewer to see it alongside tiny distant objects on the horizon. Our primate brains are programmed to perceive the moon as being even farther away, and much bigger than those distant objects.
But when the moon is hanging in a big empty sky, our brains don't make that perceptual connection quite as easily. Some researchers say we perceive the heavens as a shallow inverted bowl, with celestial objects high in the sky seeming to loom more closely, like a cloud or a bird directly overhead. Others say the "inverted bowl" theory is dead-wrong, and say the angular-size illusion involves something called oculomotor micropsia.
For more perspectives on the moon illusion, check out these archived explanations from NASA Science News, Space.com and Bad Astronomy - and ponder the mystery as you gaze at the morning's darkening moon.
Sky and Telescope
This chart shows the progression of Saturday's partial lunar eclipse, with times expressed as UTC (GMT). The peak of the event comes at 11:38 UT, which is 7:38 a.m. ET (after sunrise) or 4:38 a.m. PT.
Here are a few more tips to add some style to your moon observations:
• Because Earth's shadow covers only about half of the moon at the most, this eclipse won't be as spectacular as totality, but you may see a slight reddening of the dark half, due to the light refracted by Earth's atmosphere, as explained in this article.
• The peak of the eclipse comes at 7:38 a.m. ET - after the sun has risen - but for West Coast residents that's a doable 4:38 a.m. Prime time comes even earlier in the Pacific and east Asia - with Australia potentially the most pleasant place for eclipse-viewing. (Put a shrimp on the barbie for me Saturday evening, mate!) For the full details on viewing conditions, click here.
• This weekend is a good time to look for the International Space Station in the skies above. In fact, the shining orbital outpost is due to make a pass over the United States right around the time of the eclipse. Check out NASA's online sighting database to find out when you'll be able to spot it. Click onto SpaceWeather.com for more about space station sightings as well as pictures of the eclipse and this month's auroral displays.
• This eclipse is a mere warmup for the big South Pacific solar total eclipse that's due to occur on July 11. The bad news is that I won't be going to Easter Island to watch the show in person. The good news is that you can usually find a webcast to watch totality in real time, even if it's the middle of the night where you live. Stay tuned for a rundown of the viewing opportunities.