Sequential images of the moon show the change inflicted by a penumbral eclipse. The images were taken April 25, 2013, by space enthusiast Stanislaus Ronny Terence of Chennai, India, during a trip to Frankfurt, Germany.
This September, the moon will pass through the outer edge of the Earth’s shadow in a celestial event that may not be visible to the naked eye.
During a total lunar eclipse, skywatchers can often see the moon turn an intense, blood-red color as it passes through the central shadow of the Earth. During a penumbral eclipse, like the one happening on Sept. 16, sunlight is only blocked by the outer shadow of the Earth, called the penumbra.
The September lunar eclipse is one of several minor lunar eclipses leading to the next total eclipse of the moon in early 2018. [Lunar Eclipses: What Are They & When Is the Next One? ]
Looking out for a penumbral eclipse
Penumbral eclipses can be difficult to see from Earth because the moon is merely passing through the Earth’s penumbral shadow, causing only subtle shading on the moon’s surface.
“If you didn’t know a penumbral eclipse was going on, you might not even notice it,” Noah Petro, a research scientist at the Goddard Space Flight Center in Maryland, told Space.com. “It’s somewhat challenging for people who don’t spend all of their time thinking about eclipses to actually picture.”
To an observer on the surface of the moon, however, during a penumbral eclipse the Earth would appear to be partially eclipsing the sun. This can have a more significant impact on scientific operations like NASA’s Lunar Reconnaissance Orbiter (LRO), a spacecraft that orbits the moon. Even a minor lunar eclipse is important for Petro (a member of the LRO team) and colleagues to keep tabs on.
“Because our spacecraft gets its power from the sun, when the sun disappears we have to be aware of that,” Petro said. “This is a relatively minor eclipse. I don’t think we are doing any special operations to the spacecraft, but during total eclipses we have to turn all the instruments off except for one that we leave on to measure the change in temperature of the lunar surface during an eclipse.
The Sept. 16 penumbral eclipse, though subtle, will best be visible to varying degrees across Europe, Africa, Asia, Australia and the Western Pacific. The point of maximum eclipse will take place at 18:54:20 Universal Time (UTC), or 2:54 a.m. EDT (1854 GMT).
During a solar eclipse , if you aren’t along the path of totality you might miss the event altogether. Unlike solar eclipses, lunar eclipses are visible from a much wider spectrum and for a slightly longer period of time — up to 10 minutes, where solar eclipses might last 7 minutes at most.
This NASA chart prepared by eclipse expert Fred Espenak shows the regions on Earth where the penumbral lunar eclipse of Sept. 16, 2016 will be visible. The primary visibility areas include Europe, Africa, Asia, Australia and the Western Pacific.
Credit: NASA/Fred Espenak
What do you need to see it?
It’s going to be somewhat of a challenge to see the penumbral eclipse without help from a telescope. But before bothering with equipment, skywatchers will first have to worry about geography.
“The first thing is to make sure that it’ll be visible wherever you are,” Petro said. “While these lunar eclipses are more forgiving and you don’t have to be along the path of totality, you do have to be in the right hemisphere to see it.”
For best viewing, rely on a basic telescope or even a good pair of binoculars. A standard DSLR camera should capture an image without a hitch. During partial and total eclipses in which the moon passes into the central shadow of the Earth (the umbra), you might not even need equipment to see the moon’s surface change to an orange or even deep-red color, something that Petro suggests everyone should make an effort to see.
This full chart of the Sept. 16, 2016 lunar eclipse, prepared by NASA eclipse expert Fred Espenak, shows both the visibility regions for the eclipses, as well as how the moon will dip through the outer shadow of Earth.
Credit: NASA/Fred Espenak
When is the next eclipse?
There will be another penumbral eclipse on Feb. 11, 2017, followed by a partial lunar eclipse on Aug. 7, 2017. The big event, the next total lunar eclipse, will take place Jan. 31, 2018, and will be visible across Asia, Australia, the Pacific, and parts of the Western U.S.
According to Petro, the 2018 total lunar eclipse will be a much more obvious skywatching event, and will require the LRO to turn off all but one of the instruments aboard the orbiter.
“We can get really good science out of what happens to the surface of the moon during total lunar eclipses but again, the cool thing is that the moon changes color ,” Petro said. “It’s something fun to see — it’s benign, but it’s a change. And anytime we see change in the skies it’s always kind of exciting.”
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