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A Total Eclipse of the Sun -- on the Moon!

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A Total Eclipse of the Sun-- on the Moon!

On January 9th sky watchers across some parts of Earth will enjoy a total lunar eclipse. But what would they see if they lived, instead, on the Moon?

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see captionJanuary 8, 2001 -- Tuesday night there will be a dramatic hour-long total eclipse of the Sun. Glowing tendrils of the solar corona will fill the sky and the planet Mercury will appear as a blazing -1st magnitude star less than 8 degrees from the blocked-out Sun.

It surely sounds like a once-in-a-lifetime sight, but don't bother packing your bags for a last-minute trip to catch the show. The path of totality is 384 thousand kilometers away -- on the Moon!

Solar eclipses on the Moon happen when our planet lies directly between the Moon and the Sun. As the full Moon glides into Earth's shadow, the blinding disk of the Sun temporarily vanishes and the Sun's faint corona emerges -- much like a solar eclipse on Earth.

This image (above) captured by a coronagraph on board the ESA/NASA Solar and Heliospheric Observatory shows what a solar eclipse might look like from the perspective of a Moon-dweller. The coronagraph blocks the Sun's blinding glare using an opaque disk 1.85-degrees across -- that's three and a half times wider than the Sun and, coincidentally, almost exactly the size of our planet viewed from the Moon. The Sun's inner corona is hidden from view in this picture, but the billowing outer corona is striking.

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Above: A Jan. 5, 2001, SOHOcoronagraph image. The solid blue circle is the coronagraph's occulting disk; the white circle is an outline of the Sun. The ghostly halo around the Sun is a coronal mass ejection and the bright object to the left is Mercury.

Unlike solar eclipses on Earth, which last just a few minutes and trace a narrow path of totality across our planet, solar eclipses on the Moon are widespread and long-lasting. Tomorrow night, for example, the entire Moon will be immersed in Earth's shadow for more than an hour.

Fortunately, the Moon's solar eclipse on Jan. 9th is not a total loss for Earth-bound sky watchers. That's because a solar eclipse on the Moon is also a lunar eclipse here on Earth! Residents of Africa, Europe, and Asia can watch our planet's shadow creep across the face of the Moon after nightfall on Tuesday. [Lunar eclipse viewing tips are available courtesy of Sky & Telescope. See also the NASA/GSFC press release.]

geometry of Earth's shadowEarth's shadow has a conical shape with two parts: The umbra (on the inside) is very dark and the penumbra (on the outside) is weaker. The Moon will first enter the penumbra at 17:43 UT and then the umbra an hour later. By 19:49 UT, the Moon will be completely inside the boundaries of the umbral shadow. The total lunar eclipse will persist for 62 minutes after that.

"You might think that the Moon would be completely dark at totality, but it's not," says George Lebo, a professor of astronomy at the University of Florida. "Our planet's atmosphere refracts sunlight into the umbral shadow, so even at maximum eclipse the Moon is weakly illuminated."

How the eclipsed Moon looks depends on how much dust and clouds are present in Earth's atmosphere. Total eclipses tend to be very dark after major volcanic eruptions, which expel large quantities of ash into the atmosphere. During the total lunar eclipse of December 1992, for example, volcanic dust from Mount Pinatubo in the Philippines rendered the Moon nearly invisible. Experienced eclipse-watchers expect this Tuesday's full Moon to take on a bright coppery hue during the hour-long total phase. 

see captionSky watchers in eastern North America can catch a glimpse of the lunar eclipse shortly after sunset on Tuesday, but only as the event is nearing its end. The Moon will be heading out of Earth's faint penumbral shadow at 5 pm Eastern Standard Time. By 6 o'clock the eclipse will be over.

Right: Three exposures of a lunar eclipse on January 20, 2000, captured by photographer Stephen Barnes. [more information]  

Tuesday's show will mark last total lunar eclipse until May 16, 2003. It seems unlikely that anyone will be on the Moon three years hence to see the corresponding solar eclipse. But one day, perhaps, lunar colonists will don pressure suits and stride out of their space domes to enjoy a sight that Earth-bound eclipse chasers will surely envy: an hour-long total eclipse of the Sun. Until then we can still see the Earth's shadow envelop the Moon from time to time and imagine what the view must be like...looking back from up there.

Tune in to SpaceWeather.com later this week for photos of the January 9th lunar eclipse.

Editor's Note: Solar eclipses on Earth are notable because the Moon and the Sun are almost exactly the same apparent size: 0.5 degrees as viewed from our planet. (The Sun is really 400 times larger than the Moon, but it's also 400 times farther away.) This coincidence leads to dazzling phenomena like Baily's Beads and the Diamond Ring effect that eclipse watchers on the Moon would miss. On the other hand, Moon-based observers could see something that their Earth-based cousins could not: the sight of our planet's dark disk surrounded by a thin ring of red sunlight refracting through Earth's atmosphere. "In effect," notes reader Paul Norris, "they would be bathed by the combined light of every sunset across the whole of Earth!"

lunar eclipse diagrams

Web Links

More information about the Jan. 9, 2001 eclipse - courtesy of Fred Espenak at the NASA Goddard Space Flight Center

Eclipse Photography Tips - from MrEclipse.com

5000 year Catalog of Lunar Eclipses -During the 50 century period, 2000 B.C. to 3000 A.D., Earth experiences 12186 lunar eclipses.

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