Eclipses come in many forms | Science News for Students

Eclipses come in many forms

Here’s why solar and lunar eclipses happen, and how they differ from occultations and transits
Apr 26, 2017 — 7:10 am EST
solar eclipse

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


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.
Brocken Inaglory/Wikipedia Commons, [CC BY-SA 3.0]

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.
Qarnos/ Wikipedia Commons

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 happens because Earth is round. So part of Earth’s surface will fall inside the umbra halfway through the eclipse. People in this region are almost 13,000 kilometers (8,078 miles) closer to the moon than are those at the edge of the shadow’s path. And that difference in distance can sometimes be enough to bring that spot on Earth’s surface from the antumbra into the umbra.) 

Fewer than 5 in every 100 solar eclipses are hybrids. A bit more than one in three are partial eclipses. Somewhat more than one in three are annular eclipses. The rest, slightly more than one in every four, are total eclipses.

There are always between two and five solar eclipses every year. No more than two can be total eclipses — and in some years there will be none.

Why total solar eclipses excite scientists

Before scientists sent cameras and other instruments into space, total solar eclipses provided unique research opportunities to astronomers. For example, the sun is so bright that its glare normally blocks sight of its outer atmosphere, the corona. During a total solar eclipse in 1868, however, scientists collected data on the corona. They learned about the wavelengths — colors — of light it emits. (Such emissions helped identify the corona’s chemical make-up.)

eclipse corona
During a total solar eclipse, scientists can see the sun’s outer atmosphere (or corona, a pearly white aura around the sun). Also visible are large solar flares, or prominences (seen in pink).
Luc Viatour/Wikipedia Commons, (CC-BY-SA-3.0)

Among other things, the scientists spotted a weird yellow line. No one had seen it before. The line came from helium, which is created by reactions inside the sun and other stars. Similar studies have since identified many known elements in the solar atmosphere. But those elements exist in forms not seen on Earth — forms in which many electrons have been stripped away. These data have convinced astronomers that temperatures in the solar corona must reach millions of degrees.

Scientists also have used eclipses to look for potential planets. For instance, they’ve looked for planets that orbit the sun even closer than Mercury does. Again, the sun’s glare normally would block the ability to see anything that close to the sun, at least from Earth. (In some cases, astronomers thought they had seen such a planet. Later studies showed they had been wrong.)

In 1919, scientists gathered some of the most famous eclipse data. Astronomers took photos to see if distant stars looked out of place. If they were shifted slightly — compared to their normal positions (when the sun wasn’t in the way) — that would suggest that light zipping past the sun had been bent by its huge gravitational field. Specifically, that would provide evidence supporting Albert Einstein’s general theory of relativity. That theory had been proposed only a few years earlier. And indeed, the eclipse did provide such evidence for relativity.

Lunar eclipses

Sometimes the moon almost disappears for a short while as it falls into Earth’s shadow. Such lunar eclipses happen only at full moon, the phase when the moon is opposite the sun in our sky. It now appears as a completely lit disk. (From our vantage on Earth, it’s when the moon is rising as the sun is setting.) Just as with solar eclipses, not every full moon creates a lunar eclipse. But lunar eclipses happen more often than solar ones because Earth’s shadow is so much broader than the moon’s. In fact, Earth’s diameter is more than 3.5 times that of the moon. Being so much smaller than Earth, the moon can more easily fit completely within our planet’s umbra.

lunar eclipse
Even at the height of a total lunar eclipse, the moon is visible — if ruddy colored — because the sunlight that travels to it through Earth’s atmosphere.
Alfredo Garcia, Jr./Wikipedia Commons (CC BY-SA 4.0)

Although total solar eclipses temporarily black out only a narrow path on Earth’s surface, a total lunar eclipse can be seen from the entire nighttime half of the planet. And because Earth’s shadow is so wide, a total lunar eclipse can last up to 107 minutes. If you add in the time that the moon spends entering and leaving our planet’s penumbra, the entire event can last as much as 4 hours.

Unlike a total solar eclipse, even during a total lunar eclipse the moon remains visible. Sunlight travels through Earth’s atmosphere during the whole event, illuminating the moon in a ruddy hue.

Sometimes only a portion of the moon enters Earth’s umbra. In that case, there’s a partial lunar eclipse. That leaves a circular shadow on the moon, as if a chunk had been bitten away. And if the moon enters Earth’s penumbra but totally misses the umbra, the event is called a penumbral eclipse. This latter type of eclipse is often faint and hard to see. That’s because many portions of the penumbra are actually pretty well lit.

More than one-third of all lunar eclipses are penumbral. Some three in every 10 are partial eclipses. Total lunar eclipses make up the rest, more than one in every three.


An occultation (AH-kul-TAY-shun) is a sort of an eclipse. Again, these happen when three celestial bodies line up in space. But during occultations, a really large object (usually the moon) moves in front of one that appears much smaller (such as a distant star).

saturn occultation
This is an occultation of the planet Saturn (small object on right) by the moon (large object) that was photographed in November 2001.
Philipp Salzgeber/Wikimedia Commons (CC-BY-SA 2.0)

The moon has no real atmosphere to block light from behind it. That’s why some of the most scientifically interesting occultations occur when our moon moves in front of distant stars. Suddenly, the light from an object occulted by the moon disappears. It’s almost as if a light switch flicked off.

This sudden absence of light has helped scientists in many ways. First, it has let astronomers discover that what they first thought was a single star might actually be two. (They would have orbited each other so closely the scientists couldn’t separate the stars visually.) Occultations also have helped researchers better pin down distant sources of some radio waves. (Because radio waves have a long wavelength, it can be hard to tell their source by looking at that radiation alone.)

Finally, planetary scientists have used occultations to learn more about lunar topography — landscape features, such as mountains and valleys. When the ragged edge of the moon barely blocks a star, light can briefly peek through as it emerges from behind mountains and ridges. But it shine unimpeded through deep valleys that are pointed toward Earth.

On rare occasions, other planets in our solar system can pass in front of a distant star. Most such occultations don’t yield much new information. But big surprises occasionally turn up. Take 1977, when Uranus passed in front of a distant star. Scientists who meant to study the atmosphere of this gas planet noticed something weird. Light from the star flickered 5 times before the planet passed in front of the star. It flickered another five times as it was leaving the star behind. Those flickers suggested the presence of five small rings around the planet. But no one could confirm they existed until NASA’s Voyager 2 spacecraft flew by the planet nine years later, in 1986.

Even asteroids can occult the light from distant stars. Those events let astronomers measure the diameter of asteroids more accurately than with other methods. The longer that light from a star is blocked, the larger the asteroid must be. By combining observations taken from several different spots on Earth, researchers can map out the form of even oddly shaped asteroids.

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venus transit
In this composite image from June 5, 2012, the planet Venus (small black dot) transits, or passes in front of, the sun as seen from the space-based Solar Dynamics Observatory.
NASA/Goddard Space Flight Center/SDO


Like an occultation, a transit is a type of eclipse. Here, a small object moves in front of a distant object that appears much larger. In our solar system, only the planets Mercury and Venus can transit across the sun from Earth’s viewpoint. (That’s because the other planets are farther than us from the sun and thus can never come between us.) Some asteroids and comets, however, can transit the sun from our point of view.

Scientists have always been interested in transits. In 1639, astronomers used observations of a transit of Venus — and simple geometry — to come up with their best estimate until that time of the distance between the Earth and the sun. In 1769, British astronomers sailed halfway around the world to New Zealand to see a transit of Mercury. That event couldn’t be seen in England. From data the astronomers collected, they were able to tell that Mercury has no atmosphere.

transit gif
When an exoplanet passes in front of its parent star, it blocks light in a regular pattern that tells scientists how big the planet it, as well as how often it orbits the star.
Silver Spoon/Wikipedia Commons (CC-BY-SA-3.0)

When an object passes in front of the sun, it blocks a little bit of light. Usually, because the sun is so large, much less than 1 percent of the light will be blocked. But that small change in light can be measured by ultra-sensitive instruments. In fact, a regular and repeated pattern of slight dimming is one technique that some astronomers have used to detect exoplanets — ones orbiting distant stars. The method doesn’t work for all distant solar systems, however. For transits to occur, such solar systems have to be oriented so that they appear edge-on as seen from Earth.

Power Words

(more about Power Words)

annulus    (adj. annular) A ring-shaped object or opening.

antumbra    That part of the moon's shadow during an eclipose that continues on beyond its umbra. As with a penumbra, the moon's only partly blocks the sun. For someone in the antumbra, the sun appears bigger than the moon, which will appear in silhouette. An annular eclipse occurs when someone in the moon's shadow on Earth passes through the antumbra.

asteroid     A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.

atmosphere     The envelope of gases surrounding Earth or another planet.

average     (in science) A term for the arithmetic mean, which is the sum of a group of numbers that is then divided by the size of the group.

black hole     A region of space having a gravitational field so intense that no matter or radiation (including light) can escape.

celestial     (in astronomy) Of or relating to the sky, or outer space.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical can also be used as an adjective to describe properties of materials that are the result of various reactions between different compounds.

comet     A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”

corona     The envelope of the sun (and other stars). The sun’s corona is normally visible only during a total solar eclipse, when it is seen as an irregularly shaped, pearly glow surrounding the darkened disk of the moon.

diameter     The length of a straight line that runs through the center of a circle or spherical object, starting at the edge on one side and ending at the edge on the far side.

eclipse     This occurs when two celestial bodies line up in space so that one totally or partially obscures the other. In a solar eclipse, the sun, moon and Earth line up in that order. The moon casts its shadow on the Earth. From Earth, it looks like the moon is blocking out the sun. In a lunar eclipse, the three bodies line up in a different order — sun, Earth, moon — and the Earth casts its shadow on the moon, turning the moon a deep red.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

element     (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.

ellipse     An oval curve that is geometrically a flattened circle.

excite     (in chemistry and physics) To transfer energy to one or more outer electrons in an atom. They remain in this higher energy state until they shed the extra energy through the emission of some type of radiation, such as light.

exoplanet     A planet that orbits a star outside the solar system. Also called an extrasolar planet.

field     (in physics) A region in space where certain physical effects operate, such as magnetism (created by a magnetic field), gravity (by a gravitational field), mass (by a Higgs field) or electricity (by an electrical field).

galaxy     A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.

geometry     The mathematical study of shapes, especially points, lines, planes, curves and surfaces.

helium     An inert gas that is the lightest member of the noble gas series. Helium can become a solid at -272 degrees Celsius (-458 degrees Fahrenheit).

hybrid     An organism produced by interbreeding of two animals or plants of different species or of genetically distinct populations within a species. Such offspring often possess genes passed on by each parent, yielding a combination of traits not known in previous generations. The term is also used in reference to any object that is a mix of two or more things.

lunar     Of or relating to Earth’s moon.

Milky Way     The galaxy in which Earth’s solar system resides.

moon     The natural satellite of any planet.

NASA     Short for the National Aeronautics and Space Administration. Created in 1958, this U.S. agency has become a leader in space research and in stimulating public interest in space exploration. It was through NASA that the United States sent people into orbit and ultimately to the moon. It also has sent research craft to study planets and other celestial objects in our solar system.

new moon     The phase of the moon that appears fully dark, when viewed from Earth. At this time, the moon will sit between the earth and sun. So the lunar face lit by the sun is turned away from us. 

New Zealand     An island nation in the southwest Pacific Ocean, roughly 1,500 kilometers (some 900 miles) east of Australia. Its “mainland” — consisting of a North and South Island — is quite volcanically active. In addition, the country includes many far smaller offshore islands.

occultation     A celestial eclipse-like event in which an object that appears large from Earth, such as the moon, obscures a smaller-seeming object, such as a distant star.

orbit     The curved path of a celestial object or spacecraft around a star, planet or moon. One complete circuit around a celestial body.

penumbra     The outer edges of the moon's shadow, a zone that is not completely dark. During a solar eclipse, people within the moon's penumbra will see only a partial blockage of the sun's light. 

phenomena     Events or developments that are surprising or unusual.

planet     A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, the object must be big enough to have pulled neighboring objects into the planet itself or to have slung them around the planet and off into outer space. Astronomers of the International Astronomical Union (IAU) created this three-part scientific definition of a planet in August 2006 to determine Pluto’s status. Based on that definition, IAU ruled that Pluto did not qualify. The solar system now includes eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

radio     To send and receive radio waves, or the device that receives these transmissions.

radio waves     Waves in a part of the electromagnetic spectrum. They are a type that people now use for long-distance communication. Longer than the waves of visible light, radio waves are used to transmit radio and television signals. They also are used in radar.

relativity     (in physics) A theory developed by physicist Albert Einstein showing that neither space nor time are constant, but instead affected by one’s velocity and the mass of things in your vicinity.

solar eclipse     An event in which the moon passes between the Earth and sun and obscures the sun, at least partially. In a total solar eclipse, the moon appears to cover the entire sun, revealing on the outer layer, the corona. If you were to view an eclipse from space, you would see the moon’s shadow traveling in a line across the surface of the Earth.

solar system     The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets.

star     The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

supernova     (plural: supernovae or supernovas) A massive star that suddenly increases greatly in brightness because of a catastrophic explosion that ejects most of its mass.

theory     (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.

transit     (in astronomy) The passing of a planet, asteroid or comet across the face of a star, or of a moon across the face of a planet.

umbra    The darkest part of the moon's shadow during a solar eclipse. For people on Earth passing within the umbra, the moon will appear to totally cover the sun, briefly blacking out its light.

unique     Something that is unlike anything else; the only one of its kind.

Venus     The second planet out from the sun, it has a rocky core, just as Earth does. Venus lost most of its water long ago. The sun’s ultraviolet radiation broke apart those water molecules, allowing their hydrogen atoms to escape into space. Volcanoes on the planet’s surface spewed high levels of carbon dioxide, which built up in the planet’s atmosphere. Today the air pressure at the planet’s surface is 100 times greater than on Earth, and the atmosphere now keeps the surface of Venus a brutal 460° Celsius (860° Fahrenheit).

wave     A disturbance or variation that travels through space and matter in a regular, oscillating fashion.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.

Further Reading

NASA. “Lunar eclipses and the moon’s orbit.” April 10, 2014.

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