Eclipses come in many forms

solar eclipse
solar eclipse

A solar eclipse is one of Nature’s most awesome spectacles. It occurs when the moon passes in front of the sun (as seen from Earth).

Amazing things happen in the heavens. In the hearts of distant galaxies, black holes swallow stars. Once every 20 years or so, on average, a star somewhere in our Milky Way galaxy explodes. For a few days, that supernova will outshine entire galaxies in our night sky. Near our solar system, things are thankfully quiet.

Nevertheless, awesome events happen in our neighborhood too.

Eclipse means to overshadow. And that’s exactly what happens during a solar or lunar eclipse. These celestial events take place when the sun, moon and Earth briefly make a straight (or very nearly straight) line in space. Then one of them will be fully or partially shrouded by another’s shadow. Similar events, called occultations and transits, occur when stars, planets, and moons line up in much the same way.

Scientists have a good handle on how planets and moons move through the sky. So these events are very predictable. If the weather cooperates, these events easily can be seen with the unaided eye or simple instruments. Eclipses and related phenomena are fun to watch. They also provide scientists with rare opportunities to make important observations. For instance, they can help to measure objects in our solar system and observe the sun’s atmosphere.

Solar eclipses

Our moon is, on average, about 3,476 kilometers (2,160 miles) in diameter. The sun is a whopping 400 times that diameter. But because the sun is also about 400 times further from Earth than the moon is, both the sun and moon appear to be about the same size. That means that at some points in its orbit, the moon can entirely block the sun’s light from reaching Earth. That’s known as a total solar eclipse.

This can happen only when there is a new moon, the phase that appears fully dark to us on Earth as it moves across the sky. This happens about once per month. Actually, the average time between new moons is 29 days, 12 hours, 44 minutes and 3 seconds. Maybe you’re thinking: That’s an awfully precise number. But it’s that precision that let’s astronomers predict when an eclipse will occur, even many years ahead of time.

So why doesn’t a total solar eclipse occur each and every full moon? It has to do with the moon’s orbit. It is slightly tilted, compared to Earth’s. Most new moons trace a path through the sky that passes near to — but not over — the sun.

Sometimes the new moon eclipses only part of the sun.

The moon creates a cone-shaped shadow. The totally dark part of that cone is known as the umbra. And sometimes that umbra doesn’t quite reach Earth’s surface. In that case, people along the center of the path of that shadow don’t see a totally darkened sun. Instead, a ring of light surrounds the moon. This ring of light is called an annulus (AN-yu-luss). Scientists call these events annular eclipses.

annular eclipse
Ring-like annular eclipses (lower right) occur when the moon is too far from the Earth to completely block the sun. In the early phases of this eclipse (proceeding from upper left), it is possible to see sunspots on the face of the sun.

Not all people, of course, will be directly in the center path of an annular eclipse. Those in line with only a portion of the shadow, its antumbra, will see a partially lit moon. The antumbra is also shaped like a cone in space. The umbra and antumbra are lined up in space but point in opposite directions, and their tips meet at a single point.

Why won’t the umbra reach Earth every time there’s a solar eclipse? Again, it’s due to the moon’s orbit. Its path around Earth isn’t a perfect circle. It’s a somewhat squished circle, known as an ellipse. At the closest point in its orbit, the moon is about 362,600 kilometers (225,300 miles) from Earth. At its furthest, the moon is some 400,000 kilometers away. That difference is enough to make how big the moon looks from Earth vary. So, when the new moon passes in front of the sun and is also located in a distant part of its orbit, it’s won’t be quite big enough to completely block the sun.

These orbital variations also explain why some total solar eclipses last longer than others. When the moon is farther from Earth, the point of its shadow can create an eclipse lasting less than 1 second. But when the moon passes in front of the sun and is also at its closest to Earth, the moon’s shadow is up to 267 kilometers (166 miles) wide. In that case, the total eclipse, as seen from any one spot along the shadow’s path, lasts a little more than 7 minutes.

The moon is round, so its shadow creates a dark circle or oval on Earth’s surface. Where someone is within that shadow also affects how long their solar blackout lasts. People in the center of the shadow’s path get a longer eclipse than do people near the edge of the path.

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shadow diagram
Partially lit portions of Earth’s shadow are known as the penumbra and antumbra. The cone-shaped umbra is completely dark. The shadows of all celestial objects, including the moon, are divided into similar regions.

Partial eclipses

People completely outside the path of the moon’s shadow, but within a few thousand kilometers on either side of it, can see what’s known as a partial solar eclipse. That’s because they’re within the partially lit portion of the moon’s shadow, the penumbra. For them, only a fraction of the sun’s light will be blocked.

Sometimes the umbra completely misses the Earth but the penumbra, which is wider, doesn’t. In these cases, no one on Earth sees a total eclipse. But people in a few regions can witness a partial one.

eclipse shadow
The moon’s shadow on Earth’s surface during a total solar eclipse, as seen from the International Space Station on March 29, 2006.

On rare occasions, a solar eclipse will start and end as an annular eclipse. But in the middle of the event, a total blackout occurs. These are known as hybrid eclipses. (The change from annular to total and then back to annular…

Stamping the Moon’s Craters onto a Leather Notebook Cover

If you are looking for something to hold your small writing pad, there are plenty of options available. However, you will not find many that feature celestial objects, and even fewer that feature the actual moon itself. In fact, judging by Devin Montgomery’s post on Imgur, it would seem that his notebook (one that features actual map features of the moon), may be a one of a kind design.

When asked why he would use the surface of the moon as the inspiration for the cover of his book, Montgomery, the head of a custom leather fabrication shop called Fabnik, said, “I’ve always loved space and have been listening to a lot of science fiction lately. The surface of the moon seemed like it would be hard to do, but neat if it worked.”

Montgomery first checked Astropedia to find an image of his favorite moon landscape (he really loves space), a crater called Tycho. After getting this raw data, he then imported the image into Inkscape to convert everything into vectors. He said that creating the texture for the stamp was the most difficult part of the build, as it was hard to accurately portray craters (normally seen in grayscale) on stamped leather.

Montgomery decided on black vegetable-tanned…

On This Day in 1972, Apollo 16 Blasted Off

When the Lunar Module (LM) reached the moon, astronauts John Young and Charlie Duke spent just shy of 72 hours on the surface. They spent more than 20 hours on EVAs, driving the lunar rover, setting up surface experiments, and gathering samples. In one poignant moment, Duke left a photo of his family on the lunar surface. He also named several locations for them: “Cat Crater” was named for his sons Charles and Thomas…

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How Earth got its moon

full moon
full moon

The story of our moon’s origin does not add up. Most scientists think that that the moon formed in the earliest days of our solar system. That would have been back around 4.5 billion years ago. At that time, some scientists suspect, a Mars-sized rocky object — what they call a protoplanet — smacked into the young Earth. This collision would have sent debris from both worlds hurling into orbit. Some of the rubble eventually would have stuck together, creating our moon.

Or maybe not.

Astronomers refer to that protoplanet as Theia (THAY-ah). Named for the Greek goddess of sight, no one knows if this big rock ever existed — because if it did, it would have died in that violent collision with Earth.

moon Theia
Early Earth and a smaller protoplanet called Theia may have collided long ago, many scientists think. That would have hurled debris from both into space. In this simulation, red particles escaped the system, yellow formed the moon and blue fell to Earth.

And here’s why some astronomers have come to doubt Theia was real: If it smashed into Earth and helped form the moon, then the moon should look like a hybrid of Earth and Theia. Yet studies of lunar rocks show that the chemical composition of Earth and its moon are exactly the same. So that planet-on-planet impact story appears to have some holes in it.

That has prompted some researchers to look for other moon-forming scenarios. One proposal: A string of impacts created mini moons largely from Earth material. Over time, they might have merged to form one big moon.

“Multiple impacts just make more sense,” says Raluca Rufu. She’s a planetary scientist at the Weizmann Institute of Science in Rehovot, Israel. “You don’t need this one special impactor to form the moon.”

But Theia shouldn’t be left on the cutting room floor — at least not yet. Earth and Theia could have been built largely from the same type of material, new research suggests. Then they would have had a similar chemical recipe. There is no sign of “other” material on the moon, this explanation argues, because nothing about Theia was different.

“I’m absolutely on the fence between these two opposing ideas,” says Edward Young. He studies cosmochemistry — the chemistry of the universe — at the University of California, Los Angeles. Determining which story is correct is going to take more research. But the answer could offer profound insights into the evolution of the early solar system, Young says.

Mother of the moon

Earth’s moon is an oddball. Most other moons in our solar system live way out among the gas giants, such as Saturn and Jupiter. The only other terrestrial planet with orbiting moons is Mars. Its moons, Phobos and Deimos, are small. The leading explanation for them is that likely were once asteroids. At some point, they were captured by the Red Planet’s gravity. Earth’s moon is too big for that scenario. If the moon had come in from elsewhere, it probably would have crashed into Earth or escaped and fled into space.

An alternate explanation dates from the 1800s. It suggests that moon-forming material flew off of a fast-spinning young Earth. (Imagine children tossed from an out-of-control merry-go-round.) That idea fell out of favor, though, when scientists calculated the spin speeds required. They were impossibly fast.

In the mid-1970s, planetary scientists proposed the giant-impact hypothesis. (Later, in 2000, they named that mysterious planet-sized body as Theia.) The notion of a big rocky collision made sense. After all, the early solar system was like a game of cosmic billiards. Giant space-rock smash-ups were common.

But a 2001 study of rocks collected during NASA’s Apollo missions to the moon cast doubt on the giant-impact hypothesis. Research showed that Earth and its moon were surprisingly alike. To figure out a rock’s origin, scientists measure the relative abundance of different forms of oxygen. Called isotopes (EYE-so-toaps), they are forms of an element with different masses. (The reason they differ: Although each has the same number of protons in its nuclei, they have different numbers of neutrons.)

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isotope moon
The mix of oxygen isotopes inside moon material and meteorites called enstatite chondrites (shaded yellow) is surprisingly similar to that of Earth rocks (blue line). Other solar system materials are largely composed of different isotopic mixes.

Scientists can use amounts of various isotopes as something like fingerprints at a crime scene. Rocks from Earth and its moon, the scientists found, had seemingly identical mixes of oxygen isotopes. That didn’t make sense if much of the moon’s material came from Theia, not Earth. Rufu and her colleagues modeled the impact on a computer. From that they calculated the chance of a Theia collision yielding a moon with an Earthlike composition. And it was very slim.

Studies have been done of other elements in moon rocks, such as titanium and zirconium. They, too, suggest that Earth and its moon…

It’s time to redefine what qualifies as a planet

PLANET OR NOT? A group of planetary scientists label Pluto and many other orbs in the solar system as planets, despite the definition set down by the International Astronomical Union in 2006.

Pluto is a planet. It always has been, and it always will be, says Will Grundy of Lowell Observatory in Flagstaff, Arizona. Now he just has to convince the world of that.

For centuries, the word planet meant “wanderer” and included the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. Eventually the moon and sun were dropped from the definition, but Pluto was included, after its discovery in 1930. That idea of a planet as a rocky or gaseous body that orbited the sun stuck, all the way up until 2006.

Then, the International Astronomical Union narrowed the definition, describing a planet as any round object that orbits the sun and has moved any pesky neighbors out of its way, either by consuming them or flinging them off into space. Pluto failed to meet the last criterion (SN: 9/2/06, p. 149), so it was demoted to a dwarf planet.

Almost overnight, the solar system was down to eight planets. “The public took notice,” Grundy says. It latched onto the IAU’s definition — perhaps a bit prematurely. The definition has flaws, he and other planetary scientists argue. First, it discounts the thousands of…

The Planetary Society Has a Few Tips for the President

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As the CEO of The Planetary Society, Bill Nye has written an open letter to President Trump, highlighting the need for prioritizing space research, and for supporting NASA’s space exploration efforts in particular. Bill Nye offers the president a comprehensive five-point plan for steering NASA’s objectives and orientation during his tenure:

1. Keep Mars as the Goal
Mars became the central objective of NASA’s efforts during the previous administration, and Bill Nye urges a continued focus on the same path. It is also important to insist on Mars because there are voices in the Trump administration that want to divert resources away from the Red Planet and focus on exploration of the moon. A vocal example of this is Newt Gingrich who has advocated “a permanent moon base”. For Bill Nye and The Planetary Society, a diversion of efforts to the moon would mean that a manned mission to Mars might be delayed by a generation.

2. Orbit Mars First
This point has emerged from earlier work at The Planetary Society, such as workshops on solving problems of inhabiting Mars, which have found that orbital engagement should precede a full landing on the Red Planet. This was the original strategy for the moon landing, where Apollo VIII orbited the moon and Apollo XI landed on it. The Planetary Society has suggested that in an orbital-first strategy, humans can be stationed in Mars’ orbit by 2033, and thereafter landed on the Mars surface by 2039.

3. Expand NASA’s Scientific Programs
Bill Nye draws attention to the ‘jobs’ element of NASA’s contribution, pointing out that there are tens of thousands of high-skilled jobs in engineering, manufacturing, and the pure sciences, that exist specifically thanks to NASA’s scientific programs. The report recommends that “at least 30 percent of NASA’s total budget be committed to its Science Mission Directorate,” and that we don’t forget two things: our curiosity and safety. A budget commitment to the science mission “will help humanity better understand its origins, protect us from solar storms, search for life beyond Earth, as well as understand our changing climate,” says the report. NASA’s acting administrator, Robert Lightfoot, has already stated that because of Trump’s proposed budget, the agency “will not pursue the Asteroid Redirect Mission (ARM).” Here’s why that matters. Trump signed the ‘NASA Transition Authorization Act of 2017’ on March 21, which seems to favor stability over progress, leaving…

In new Cassini portraits, Saturn’s moon Pan looks like pasta

Pan in Cassini image
This week, Cassini captured the closest images ever taken of Pan, a small moon that orbits amid Saturn’s rings.

Saturn serves up the closest thing to space pasta, the latest round of images from NASA’s Cassini probe, released March 9, show.

On March 7, the spacecraft snapped a series of portraits of Pan, Saturn’s small moon that orbits within a 325-kilometer gap in one of the planet’s rings. Taken at a distance of 24,572 kilometers from the moon, these are the closest images of Pan to date.

The close-ups could help refine astronomers’ understanding of the mini moon’s geology and…