eclipse – Watch the Skies

The Solar Eclipse Experience

On Oct. 14, 2023, and April 8, 2024, the entire United States and millions around the world will be able to view a solar eclipse.

A map showing where the Moon’s shadow will cross the U.S. during the 2023 annular solar eclipse and 2024 total solar eclipse. (NASA SVS)

There are three different kinds of solar eclipses: total, annular, and partial. When the Moon is far from the Earth, its size is too small to completely cover the Sun, thus an annular eclipse is observed, like what is expected on Oct. 14, 2023. When the Moon is close to the Earth, its larger size completely covers the Sun, causing a total eclipse, which will occur on April 8, 2024. A partial eclipse occurs when the Earth, Moon, and Sun are not perfectly aligned so only a part of the Sun will appear to be covered, giving it a crescent shape. During a total or annular solar eclipse, people outside the totality/annularity paths will see a partial solar eclipse.

From left to right, this image shows a total solar eclipse, annular solar eclipse, and partial solar eclipse.

The Eclipse Experience

Mitzi Adams, NASA Marshall Space Flight Center Heliophysics and Planetary Science Branch Assistant Chief, shares her observations during the five total eclipses she has experienced. “It is like nothing you’ve ever experienced before. It’s sort of like somebody puts a bowl on top of Earth right above where you’re standing. In the middle of the day, it gets darker, but you can still see light around the rim.” Adams explains. “You can essentially observe a sunrise or sunset. The temperatures cool. The wind picks up. The birds may go to roost, or the coyotes may howl.”

During an annular eclipse like the one coming up on Oct. 14, even with the sun covered up to 90%, the sky remains fairly bright. Those in the path of annularity will have a chance to observe the famed “ring of fire” effect, but it is important to manage your expectations and to remember that solar viewing glasses will be needed during the event’s entirety.

Bill Cooke, NASA’s Meteoroid Environment Office Lead and eclipse enthusiast, says he is most looking forward to the 2024 total eclipse because totality, when the sun is covered 100%, will last much longer than the last total eclipse in 2017 – up to nearly four and a half minutes.

In any of the upcoming eclipse events, in our technology-fueled world, you may also experience some electronic changes as the moon moves across Earth and the ionosphere cools.

The ionosphere forms the boundary between the Earth’s lower atmosphere – where we live and breathe – and the vacuum of space. It is formed when particles are charged, or ionized, by solar radiation. A total solar eclipse effectively “turns off” the ionosphere’s primary charging mechanism, mimicking nighttime conditions, so the many communications signals passing through the ionosphere could be disrupted.

GPS signals could produce location errors. Radio waves could change, sometimes even allowing Ham Radio operators to send or receive transmissions over longer distances.

The ionosphere is also home to many NASA satellites, including the International Space Station.

Solar Eclipse Science

Experiencing an eclipse is one way that everyone can participate in NASA Science. Depending on your access to different types of technology (phones, laptops, telescopes), there are several NASA Citizen Science projects you can participate in that relate to the Sun’s corona and the effects of the Moon’s shadow on Earth’s upper atmosphere.

GLOBE Observer https://observer.globe.gov/do-globe-observer
- Help monitor the conditions of clouds, water (especially as a habitat for mosquitoes), plants (trees and other land cover), and see change over time.
Solar Jet Hunter https://www.zooniverse.org/projects/sophiemu/solar-jet-hunter
- Join the hunt for solar jets — enigmatic bursts of energy from our own star — the Sun!
Planet Hunters Transiting Exoplanet Survey Satellite (TESS) https://www.zooniverse.org/projects/nora-dot-eisner/planet-hunters-tess
- Find planets that will help us understand how these extrasolar systems form and evolve over time
Eclipse Soundscapes https://eclipsesoundscapes.org/
- Provide multi-sensory observations and recorded sound data to study how solar eclipses affect life on Earth!
HamSCI https://hamsci.org/
- Advance scientific research and understanding through amateur radio activities.
Radio JOVE: https://radiojove.gsfc.nasa.gov/
- Observe and analyze natural radio emissions of Jupiter, the Sun, and our galaxy using their own easy to construct radio telescopes.
Sungrazer Project https://sungrazer.nrl.navy.mil/
- Become a “Comet Hunter”, and immediately begin looking for new comets in the spacecraft data.

Regardless of how you plan to experience a solar eclipse, or any solar viewing for that matter, remember to always do so safely. https://solarsystem.nasa.gov/eclipses/safety/

Lauren Perkins
NASA Marshall Space Flight Center

Coming Soon: A “Ring of Fire” in the Sky

On Oct. 14, 2023, an annular total solar eclipse will be visible to millions across the globe as it sweeps through the skies of the northwestern United States through Mexico and Central America and into South America, exiting the continent in Brazil. Even if you are not in the path of annularity, you will still be able to see a partial solar eclipse if weather conditions are right.

An animation of the Earth that shows where the annular eclipse path will be in 2023. — Animated map showing the 2023 annular eclipse path (red dot) and partial eclipse visibility (shadowed area). Credit: Eclipse Predictions by Fred Espenak, NASA’s GSFC

A map developed using data from a variety of NASA sources shows a detailed eclipse path and what observers across the States can expect to see at their local time.

The Moon’s distance from Earth is not constant, sometimes it is a little closer, sometimes a little farther away. When the Moon passes between the Sun and Earth while it is close to or at its farthest point from Earth, an annular eclipse happens. Because the Moon is farther away from Earth than on average, it appears smaller than the Sun, creating a “ring of fire” effect in the sky, and since the Sun is never completely covered, observers must wear proper eye protection at all times while watching an annular eclipse.

During a total solar eclipse, like the upcoming 2024 Solar Eclipse, the Moon is close enough to Earth to be sufficiently large to completely cover the bright face of the Sun. During the few minutes of totality, there will be darkness around midday.

A black circle is in the middle with a bright orange glow around it shows the 2022 annular solar eclipse. — An annular solar eclipse creates a “ring of fire” around the Moon, similar to that seen in this image taken by the Hinode spacecraft on January 4, 2011. Credit: JAXA/NASA/SAO/NAOJ

What you can see during an annular eclipse depends on the weather and your location.

- - You need a clear sky to see the eclipse. However, even with cloud cover, the eerie daytime darkness associated with eclipses is still noticeable to human animals as well as the four-footed ones and the flying ones. Birds go to roost, bees return to the hive, and even turtles come out of ponds.
  - To see all phases of an annular eclipse, including the “ring of fire,” you must view it from somewhere within the path of annularity.

An animation video of what the annual eclipse looks like. A black circle moves across the bright orange circle that represents the moon. — This conceptual animation is an example of what you might expect to see through certified solar-viewing glasses or a handheld solar filter during an annular solar eclipse, like the one happening over the United States on October 14, 2023. Annular eclipses are famous for the “ring of fire” effect that appears around the edge of the Moon. This happens because the Moon is slightly farther from Earth and appears too small to block out the Sun completely. Credit: NASA

If you are not within the path of annularity, watching the eclipse from a virtual location is a great option. Join NASA for conversations with scientists and telescope views from across the country on NASA’s YouTube beginning at 10:30 a.m. CT on Oct. 14, 2023.

By Lauren Perkins
NASA Marshall Space Flight Center

May’s Full Moon Comes with Supermoon Eclipse

As we approach month’s end, there is not one, not two, but three celestial events happening with our Moon!

The Moon will be located on Earth’s opposite side from the Sun and fully illuminated May 26, 2021, at 6:13 a.m. CDT. This Full Moon was known by early Native American tribes as the Flower Moon because this was the time of year when spring flowers appeared in abundance.

Compared to other Full Moons in 2021, the Flower Moon will have the nearest approach to Earth, making it appear as the closet and largest Full Moon of the year. This is what is commonly referred to as a “supermoon”. Yet, it’s not just bringing brightness and size. May’s supermoon is also bringing a “super power” to change its color, and the color is red!

A telescopic visualization of the 2021 total lunar eclipse.
Credits: NASA’s Scientifc Visualization Studio

Mars is most commonly known as the Red Planet. But have you ever witnessed our own planet’s Moon turn red? If you haven’t, you’ll get your chance with this year’s only total lunar eclipse also happening May 26! It’s been nearly two and a half years since the last one.

A total lunar eclipse occurs when the Moon passes completely through the Earth’s dark shadow, or umbra. During this type of eclipse, the Moon will gradually get darker, taking on a rusty or blood-red color. The color is so striking that lunar eclipses are sometimes called Blood Moons.

The total eclipse phase will be visible near moonset in the western United States and Canada, all of Mexico, most of Central America and Ecuador, western Peru, and southern Chile and Argentina. The eclipse can be seen in its entirety in eastern Australia, New Zealand, and the Pacific Islands, including Hawaii. Unlike a solar eclipse, you won’t need special glasses to view this lunar eclipse, just go outside and keep your head to the sky!

“Folks in Hawaii and the Aleutian Islands will get to see the entirety of this eclipse – it will be quite a show for them,” said Bill Cooke, Lead, NASA Meteoroid Environments Office.

The eclipse is set to begin May 26 at 1:46 a.m. PDT, with the Moon entering the darkest part of the Earth’s shadow at 2:45 a.m. Part of it will remain in the umbra until 5:53 a.m. To catch totality – the period when all of the Moon’s surface is blanketed by the Earth’s dark shadow – look up between 4:11 and 4:26 a.m.

We haven’t had a total lunar eclipse occur with a supermoon in almost six years, and the next total lunar eclipse won’t happen over North America until May 2022.

Enjoy this spectacle of the sky!

by Lance D. Davis

Total Solar Eclipse to Cast Shadow on South America

by Lauren Lambert

What is a Solar Eclipse?

A solar eclipse is a natural phenomenon that occurs when the Moon passes between the Sun and Earth. This event happens when the Moon completely blocks the Sun and the Moon’s shadow falls onto a portion of the Earth’s surface.

There are three types of solar eclipses: total, partial and annular. During a total solar eclipse, observers can witness daytime twilight because the disk of the Moon blocks 100% of the Sun. During a partial solar eclipse, the Moon is not entirely covering the Sun and you will likely not notice any difference in light intensity. You may only notice a subtle difference if the partial eclipse is close to total and you go outside at maximum eclipse. Lastly, an annular eclipse can be observed when the Moon is at apogee, or the farthest from Earth within its elliptical orbit. This causes a ring of light, or annulus, to be visible around the Moon, which is sometimes referred to as the “ring of fire.”

total solar eclipse image — During the total solar eclipse, the Sun’s visible-light corona (meaning crown), only visible at maximum eclipse from within the path of totality, is seen here as a crown of white light extending from around the edge of the eclipsing Moon. The red loops of material also seen around the edge, are called prominences, in which magnetic fields enclose hot solar material. Credit: NASA/Armstrong’s Gulfstream III.

Total eclipses are of particular interest to solar scientists, because with the Moon blocking the bright light of the Sun, you can see the Sun’s atmosphere from the ground. Solar scientists at Marshall Space Flight Center, and around NASA, make use of telescopes called coronagraphs that block the Sun so they can see the dim atmosphere, the corona, around it. But — given how perfectly the Moon lines up with the Sun — you can see the atmosphere closer to the surface of the Sun than we even can with our telescopes in space.

The shadow of the Moon on a planet during an eclipse can be described using three terms: umbra, antumbra and penumbra. The umbra is the shadow that is cast when the Moon completely covers the Sun and is where the path of totality falls. If the Moon is further away from the Earth, it is unable to block the Sun entirely. The Sun appears as a ring of light around the Moon. In this case, the shadow is known as the antumbra, or path of annularity, and occurs during an annular eclipse. Similarly, a partial solar eclipse can be observed when only a portion of the Moon blocks the Sun and creates a shadow referred to as the penumbra. The penumbra also occurs surrounding the umbra during a total eclipse, effectively covering those regions on the planet that only have a view of a partial eclipse.

Crescents of light from solar eclipse — Crescents of light are projected onto the ground during the partial phases of a solar eclipse due to light from the Sun passing through gaps between the leaves of trees, a pinhole effect. This is a safe and indirect way to view a solar eclipse. Credit: NASA/Johnson Space Center

Solar eclipses happen at least twice per calendar year, with total solar eclipses occurring about once every year and a half. But the possibility of seeing them is rare if you’re not in the right place at the right time. Additionally, since Earth is made up of mostly water, the path of totality, or the area receiving total blockage of the Sun, may not necessarily fall on land.

The year of 2020 sees two solar eclipses. The first occurred on June 21 and was an annular solar eclipse, visible from the continents of Africa and Asia. The second will be a total solar eclipse, occurring on Dec. 14, visible from South America. The path of totality crosses over Chile and Argentina, but some of their areas outside of the path of totality will experience a partial solar eclipse. The total eclipse will also be visible in Antarctica, South Africa, as well as the Pacific, Atlantic, and Indian Oceans. Observers will be able to witness the total solar eclipse for about 2 minutes.

If you are not within the path of totality, watching the total solar eclipse from a virtual location is an option as well. You can view it on NASA TV and the agency’s website, beginning at 10:30 a.m. EST on Dec. 14. Be sure to check it out, as the next total solar eclipse won’t be happening until Dec. 4, 2021.

Top 5 Solar Eclipse Viewing Tips:

Do not stare directly at the Sun. Wear safety approved, protective solar eclipse-viewing glasses to directly view the event or use some indirect means (see below). For more information here are some NASA Safety tips.
To indirectly view the eclipse, create a pinhole camera or box projector. Learn how to build your own here.
Stand under a tree and look at the ground. The trees act as pinhole projectors and will project hundreds of crescent shapes right at your feet.
To capture an eclipse with binoculars, a telescope, or a camera, you must use a safety-approved, protective solar filter on your lens.
Keeping with the theme of 2020 – Observe the eclipse virtually! It will be streamed live here.

Ask an Astronomer: What’s a Supermoon?

“The second supermoon of 2019 happened Feb. 19. The third of 2019 will happen March 19. But what’s a supermoon? We asked NASA astronomer Mitzi Adams what’s really going on here. Here’s her answer!”

Like the orbits of all bodies in the solar system, the Moon’s orbit around Earth is not circular, it has an oval or elliptical shape, with Earth slightly offset from the center. As a result, there are two distance extremes of each orbit: closest approach, known as perigee, and the farthest, or apogee. When the Moon is at closest approach and within a day or so of being full, it is called a supermoon because the Moon will be at its brightest and largest.

For the supermoon on Feb.19, the Moon will be full only six hours after it reaches the perigee distance of its orbit, making it the brightest and largest full Moon of the year. A supermoon also occurred in January with a slightly more distant perigee, a mere 362 miles (583 kilometers) farther away, but 14 hours after the full Moon. However, January’s supermoon included a total lunar eclipse seen in all of North and South America. The third and last supermoon of the year will happen March 19, when the perigee distance will be reached a day and five hours before the full Moon (see the table below for details).

Date	Perigee Distance	Time Before or After Full Moon
Jan. 21	222,043 miles (357,344 km)	15 hours after
Feb. 19	221,681 miles (356,761 km)	6 hours before
March 19	223,308 miles (359,380 km)	1 day, 5 hours before

A total lunar eclipse accompanied the first in a trilogy of supermoons in 2019. Credit: NASA/MSFC/Joe Matus

To watch tonight’s supermoon, or any full Moon, simply look for the Moon to rise in the east as the Sun sets in the west. The Moon will look extremely large when it rises and sets. This “Moon illusion” happens when the Moon is close to the horizon and there are objects within our line of sight such as trees or buildings. Because these relatively close objects are in front of the Moon, our brain is tricked into thinking the Moon is much closer to the objects that are in our line of sight. At Moon rise or set, it only appears larger than when it is directly overhead because there are no nearby objects with which to compare it. You can check this. When the Moon rises, hold a coin at arm’s length so that the coin covers the Moon. Repeat this throughout the evening and you will see that the Moon’s size does not change.

As it rises on Feb. 19, the Moon will be in the constellation of Leo. However, since the Moon is so bright, you may have trouble seeing the bright star Regulus, which is at the end of the “backwards question mark” that makes Leo easy to spot.

Looking more or less directly overhead, you could see the famous constellation Orion the Hunter with bright stars Betelgeuse, a reddish star, and Rigel, a bluish star. With a telescope or binoculars, you might be able to pick out the Orion nebula just below the belt stars of Orion, Alnitak, Alnilam, and Mintaka.

Great Nebula in Orion — Credit: Stellarium

To the west of Orion you should be able to spot reddish Mars.

As we observe this supermoon, keep in mind that 2019 marks the 50th anniversary of a great technological feat – humans travelled to the Moon, walked on its surface and returned safely to Earth. Twelve people walked on the Moon. Neil Armstrong and Buzz Aldrin were the first two, but let us not forget the other ten: Alan Bean, Charles “Pete” Conrad, Edgar D. Mitchell, Alan Shepard, Dave Scott, James Irwin, John Young, Charles Duke, Eugene “Gene” Cernan and Harrison Schmitt. These men, along with the command module pilots Michael Collins, Dick Gordon, Stu Roosa, Al Worden, Ken Mattingly, Ron Evans and the multitudes of support staff back on Earth, fulfilled a dream of exploring our nearest neighbor in space. As NASA and its commercial and international partners plan to return the Moon over the next decade with a long-term continued presence, the list of Moon walkers will surely include women, as well.

A good resource for more information on supermoons may be found here: https://solarsystem.nasa.gov/moons/earths-moon/what-is-a-supermoon/.

Constellation screenshots are from Stellarium, a planetarium software package that is accompanied by a GNU General Public License

Mitzi Adams is a solar scientist in the Heliophysics and Planetary Science Branch at NASA’s Marshall Space Flight Center.

Total Lunar Eclipse

By Mitzi Adams, NASA Marshall solar scientist

Last August, citizens and visitors to the United States of America had a rare opportunity to see a total solar eclipse, because the path of totality ranged from Oregon to South Carolina, essentially bisecting the country. But alas, the total lunar eclipse happening on Friday, July 27, will totally miss the United States. Being able to observe the Moon totally immersed in Earth’s shadow depends mostly on whether it is dark at the time the eclipse happens, so about half the Earth would be in the right place to see the eclipse, weather permitting of course. This time, residents of Europe, Africa, Asia, Australia, and parts of South America will be so lucky. In contrast, totality for a solar eclipse is very narrow and only a very small portion of Earth is in the shadow of the Moon. For the August 2017 eclipse, only those within an approximately 100 km (63 miles) wide path saw the Sun totally eclipsed.

So what happens when there is a lunar eclipse? Unlike the solar variety, Earth blocks the Sun for a lunar eclipse. For the lunar eclipse to happen, the Moon’s phase must be “full”, which means that the orbiting Moon is opposite the Sun, with Earth in between. When the Sun sets in the west, the Moon rises in the east — and this event happens once a “moonth” (or month). But a lunar eclipse does not happen every month. Why is that?

The Moon is seen here during the January 2018 lunar eclipse, setting in the western horizon, not yet in totality.
Image credit: NASA/Alphonse Sterling

Well, now we get into more tricky territory. Let’s try a thought experiment. Draw a line between the centers of the Sun, Earth, and Moon. This line is part of a plane that describes how Earth orbits the Sun, called the plane of the ecliptic. The Moon orbits Earth, only its orbit is tilted with respect to the plane of the ecliptic, sometimes the Moon is above the plane, sometimes it is below the plane. Only when the Moon’s orbit lines up with the ecliptic plane do we have a chance for an eclipse. If the phase of the Moon is “full” when this happens, we have a lunar eclipse. If the phase of the Moon is “new,” we have a solar eclipse. Sometimes the orbital planes do not line up exactly, in those cases, we would have partial eclipses.

Fred Espenak, click here for more info on Lunar Eclipse Geometry.

The July 27 eclipse is somewhat special because the length of totality will be the longest of this century at one hour, 43 minutes. Why? Several reasons. The Moon will be at apogee, or at the farthest distance from Earth (406,000 km or 252,000 mi) possible for our Moon. Objects in orbit around Earth move slower the farther away they are, which means it will take longer for the Moon to traverse the width of Earth’s shadow. In addition, the Moon will be almost exactly on that line that connects Sun, Earth, Moon, also increasing the length of time the Moon will spend in the umbral (darkest) part of Earth’s shadow. Finally, Earth reached its greatest distance from the Sun (aphelion) quite recently (July 6), meaning that Earth’s shadow on July 27 will be close to the largest it can be, adding even more distance (and time) to the Moon’s shadowy traverse.

This image is of the full Moon before the January 2018 lunar eclipse.
Image credit: NASA Marshall/Alphonse Sterling

The partial phase of the eclipse will begin at 18:24 UT, with totality beginning at 19:30 UT (see the NASA time zone page for help with conversion to your local time and official U.S. time). Totality will be over at 21:13 UT and the partial phase ends at 22:19 UT. Viewing a lunar eclipse does not require a telescope or even special glasses; however, while waiting for totality to begin, which is marked by a reddish-brown color to the Moon, a telescope could be used to view two planets that are in the evening sky. Mars will be visible, and should be pretty bright since there is currently a dust storm covering the entire planet. So the telescope will not see any surface detail here, but the redness of the planet will contrast well with the reddish hue of a totally eclipsed Moon. Saturn will be visible to the west of Mars — and even binoculars will resolve the rings, but a telescope could provide more detail. For all observers, find the full Moon in the night sky, Mars will be close to and below (south of) the Moon, a bright reddish “star-like” object. For detailed information about this eclipse, click here.

NASA Marshall Experts to Share Total Solar Eclipse In-person, on TV

On Monday, Aug. 21, for the first time in almost 100 years, all of North America will be treated to an eclipse of the sun. Those in the path of totality, running from Oregon to South Carolina, will experience one of nature’s most awe-inspiring events — a total solar eclipse.

Scientists, researchers and experts from NASA’s Marshall Space Flight Center in Huntsville, Alabama, will mobilize to experience the eclipse and share it with others. They will join participants from across the agency for a multi-hour broadcast, titled Eclipse Across America: Through the Eyes of NASA, to offer unprecedented live video of the celestial event, along with coverage of activities in parks, libraries, stadiums, festivals and museums across the nation, and on social media.

“It’s going to be a spectacular event,” said Marshall Chief Scientist James Spann. “We’ll be sharing our research and work with people and letting them know how to safely view the eclipse, not only at the events in the path of totality, but also worldwide online and on NASA Television. Excited doesn’t begin to describe how our team feels right now. It truly will be breath-taking, and we can’t wait.”

Marshall experts will be located at two of the broadcast’s 15 locations — Hopkinsville, Kentucky, and Austin Peay State University in Clarksville, Tennessee.

What Does a 97% Eclipse Mean Anyway?

Marshall’s Meteoroid Environment Office’s very own Dr. Bill Cooke, created this graphic showing the idealized view through a telescope with an H-alhpa filter at maximum eclipse for 4 locations: Birmingham, which will experience a 93% eclipse, Atlanta, which will have 97% of the Sun covered, the 97% eclipse in Huntsville, and the 99.6% eclipse in Chattanooga, which shows only the tiniest sliver of Sun down on the bottom.

Total Solar Eclipse: The Physics of Light

By Kevin Matyi

The motion of the moon is what causes eclipses, but the dramatic change in sunlight is what makes them so impressive to observers. But what exactly is happening when the moon passes in front of the sun?

The moon is blocking the sun’s light from reaching Earth, but there is more to the situation than just that. Their relative distance to Earth is one of the most important factors.

The sun is about 400 times farther from Earth than the moon and has a diameter about 400 times larger than the moon. As a result, both the sun and moon (near perigee) appear to be the same size in the sky, allowing the moon to perfectly block out the sun and cast a shadow on Earth during a total eclipse.

The shadow we see while in the path of totality is called the umbra, and the shadow of the surrounding partial eclipse is a penumbra. The shadow from an annular eclipse (when the moon appears smaller than the sun during an eclipse, and so a ring of light is visible around it) is called an anteumbra.

The physics of how each type of shadow is formed is difficult to explain but easy to visualize, so before I tell you about them, here is a picture (technically a ray diagram) of what happens during an eclipse:

For a total eclipse, the moon has to block out all of the sun’s light. To put the moon in the best position, imagine that a person on Earth is standing under the exact middle of the moon, the centerline of a total solar eclipse.

In this case, light coming from the middle of the sun is clearly going to be blocked by the moon, since it is directly in the way and visible light cannot penetrate rock. The most difficult light to block will be coming from the top and bottom of the sun.

To figure out whether the light will be blocked, a bit of drawing can help. If the light is coming from the exact bottom of the sun and you are wondering if a person can see the light while under the exact center of the moon, draw a line between where the light starts and the person’s eyes.

Does the moon get in the way of the line? If yes, then the person is experiencing a total solar eclipse. None of the sun’s light can get past the moon, so the sun is fully blocked.

If the answer is no, but the person is still standing under the center of the moon, then they are in an annular eclipse. The moon is in the perfect position to block all of the sun’s light, but it still fails to do so. In this case, it will appear to be a large black circle with a ring of sunlight called an annulus around it.

A partial eclipse is the most difficult to explain, since it has the most variability. All but a sliver of the sun may be blocked, or the moon can barely cover any of the sun. In general though, a partial solar eclipse happens when the moon is not quite directly between the observer and sun, but is still in the way of some sunlight.

You can use the same process for determining whether a person is experiencing a total solar eclipse to figure out if they are in the penumbral shadow of the moon. A slight complication is that the moon is off center, so it matters more where the origin point of the light is.

If the person is standing a little north of the moon’s center, then the line from origin to person should start from the sun’s southernmost point, the bottom, since the northern light is less likely to be blocked due to the moon being a bit more to the south from the person’s perspective.

If any of the sun’s light is blocked by the moon, then the person is experiencing a partial solar eclipse. The limit of this blockage, where only the slightest amount of sunlight is blocked, is the edge of the penumbra shadow.

If the moon is not blocking any light, then the moon may be close to the sun but there is no eclipse happening on that spot of Earth.

When the Earth, Moon and Sun Align

By Kevin Matyi

On Aug. 21, 2017, a total solar eclipse will cross the full continental United States along a narrow, 70-mile-wide path from Oregon to South Carolina.

The last total eclipse in the U.S. was in 1979. And the last total solar eclipse that crossed the entire continental U.S. happened in 1918. But why? Why has it been 99 years, and why have the intervening partial and even total eclipses caught only parts of the country?

In short, celestial geometry is complicated but predictable. Much like many other aspects of the cosmos, it is cyclic.

Need a minute to catch up? Go ahead. We’ll wait. Credit: NASA

Eclipse cycles arise from a natural harmony between three motions of the moon’s orbit. We call them “months” due to their repetitive nature.

The synodic month governs the moon’s phases. It’s measured by the time it takes to go from one new moon to the next, which takes about 29 ½ days. In that time, the moon rotates once around its own axis and goes around Earth once.

From the perspective of a solar eclipse, the new moon phase is important. It’s the point in the moon’s orbit when it passes between Earth and the sun. A total solar eclipse can only happen at a new moon, and only when the other types of movement line up as well.

When the moon, on its orbit around Earth, reaches the point closest to the sun we can’t see the moon reflecting sunlight, so it appears dark. This is the new moon.
Credit: NASA/Genna Duberstein

New moons happen once a month, but we don’t see eclipses every month because the moon’s orbit is tipped by about five degrees from Earth’s orbit around the sun. On most months, the new moon casts its shadow either above or below Earth, making a solar eclipse a rare treat.

The moon’s tilted orbit meets the sun-Earth plane at two points called nodes. A draconic month is the time it takes the moon to return to the same node. The moon’s orbital nodes drift over time, which is why a single location on Earth’s surface might wait hundreds of years between total eclipses.

As the moon orbits Earth, it also wobbles up and down, making total eclipses rarer than they otherwise would be. Credit: NASA

The moon’s path around Earth is not a perfect circle, which means the distance between us and the moon changes all the time. When the moon is closest to Earth in its orbit we call it perigee, and apogee when it’s farthest. This change in distance gives rise to the anomalistic month, the time from perigee to perigee.

The farther away the moon is from Earth, the smaller it appears. When the moon blocks all of the sun’s light, a total eclipse occurs, but when the moon is farther away — making it appear smaller from our vantage point on Earth — it blocks most, but not all of the sun. This is called an annular eclipse, which leaves a ring of the sun’s light still visible from around the moon. This alignment usually occurs every year or two, but is only visible from a small area on Earth.

A total solar eclipse requires the alignment of all three cycles — the synodic, anomalistic, and draconic months. This happens every 18 years 11 days and 8 hours, a period known as a saros.

One saros period after an eclipse, the sun, moon and Earth return to approximately the same relative geometry, a near straight line, and a nearly identical eclipse will occur. The moon will have the same phase and be at the same node and the same distance from Earth. Earth will be nearly the same distance from the sun, and tilted to it in nearly the same orientation.

The extra eight hours is the reason why successive eclipses in the same saros cycle happen over different parts of Earth. Earth rotates an extra third of the way around its axis. Each total solar eclipse track looks similar to the previous one, but it’s shifted 120 degrees westward.

Earth turning on its axis impacts where total solar eclipses occur.
Credit: Espenak & Meeus

During this year’s total solar eclipse, anyone within the path of totality will be able to see one of nature’s most awe-inspiring sights. This path, where the moon will completely cover the sun and the sun’s tenuous atmosphere — the corona — can be seen, will stretch from Salem, Oregon to Charleston, South Carolina. Observers outside this path will still see a partial solar eclipse where the moon covers part of the sun’s disk. A total solar eclipse presents the rare opportunity to observe the corona and chromosphere, and eclipse observations are important for understanding why sun’s atmosphere is 1 million degrees hotter than its surface.

For more information on the eclipse, where to view it and how to view it safely (wear eye protection!), visit https://eclipse2017.nasa.gov/sun.