Tag Archives: solar eclipse

Total Solar Eclipse: The Physics of Light

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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:

Each of the three types of solar eclipse are caused by the moon blocking light from different parts of the sun.

Each of the three types of solar eclipse are caused by the moon blocking light from different parts of the sun.
Credit: Wikimedia Cmglee

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

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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.

When moon is too small to cover the entire sun’s disk, a ring or “annulus” of bright sunlight surrounds the moon.
Credit: NASA/Cruikshank

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.

Make sure to wear eye protection when you go out to look at the eclipse!
Credit: NASA/Bill Ingalls

 

NASA to Provide Live Coverage, Interviews, and Social Media for March 8 Solar Eclipse

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Editor’s note: You can watch both the solar eclipse program and the raw video feed on the Watch the March 8 Solar Eclipse Live page.

NASA, in partnership with the Exploratorium Science Center in San Francisco, will host activities around the March 8 total solar eclipse, including opportunities to talk with solar scientists and live coverage of the eclipse originating from Woleai island in Micronesia.

At 1 p.m. on Tuesday, March 8, solar scientists from NASA’s Goddard Space Flight Center in Greenbelt, Maryland, will participate in a Reddit Ask Me Anything.

NASA Television will begin coverage at 8 p.m. on March 8. The period of total eclipse, called totality, will occur from 8:38 to 8:42 p.m.

Twitter, Google+ and Facebook users will be able to join the conversation and ask questions using the hashtag #eclipse2016. The NASA Twitter account for the eclipse is @NASASunEarth. The public will be able to tag and share their images of the solar eclipse on the NASA Flickr group at: https://www.flickr.com/groups/eclipse2016/.

The total eclipse will be visible in parts of South East Asia and a partial eclipse will be visible in parts of Alaska, Hawaii, Guam, and America Samoa. An eclipse occurs when the moon passes directly between Earth and the sun. When the moon’s shadow falls on Earth, observers within that shadow see the moon block a portion of the sun’s light.

Information about eclipses is available online at:

https://www.nasa.gov/eclipse

For solar eclipse video resources, visit:

http://go.nasa.gov/1VR3Fvk

2013 total solar eclipse. Credits: Copyright Pavel Štarha, Kristián Molnár

2013 total solar eclipse.
Credits: Copyright Pavel Štarha, Kristián Molnár

Ask an Astronomer About the Shortest Lunar Eclipse of the Century on April 4

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On Saturday morning, April 4, 2015 not long before sunrise, the bright full moon over North America should turn a lovely shade of celestial red during a total lunar eclipse. Join NASA astronomer Mitzi Adams as she takes questions via Twitter @NASA_Marshall. For Twitter questions, use the hashtag #eclipse2015. The question and answer via Twitter will begin at 6 a.m. EDT and continue through the end of the eclipse (approximately 8:00 a.m. EDT on April 4).

The lunar eclipse will be visible from all parts of the United States.  Eastern North America and western South America can see beginning stages of the partial umbral eclipse low in the west before sunrise April 4, whereas middle Asia (India, western China, mid-Asian Russia) can view the ending stages of the partial umbral eclipse low in the east after sunset April 4. Greenland, Iceland, Europe, Africa and the Middle East won’t see this eclipse at all. A world map of eclipse visibility is available here. The total eclipse will last only five minutes. You can find more information  here.

This image shows the Dec. 20, 2012 total lunar eclipse, as seen from Sagamihara, Japan.

This image shows the Dec. 20, 2012 total lunar eclipse, as seen from Sagamihara, Japan.

Marshall Scientists to Take Questions via Twitter About the Partial Solar Eclipse

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On Thursday, October 23, 2014, from 5:00pm – 6:00pm CDT, Marshall scientists Mitzi Adams, Sabrina Savage and Alphonse Sterling will be taking questions about the partial solar eclipse on the NASA Marshall Twitter account: http://twitter.com/NASA_Marshall, using the hashtag #askNASA.

At approximately 4:54 p.m. CDT, the eclipse will begin, with maximum eclipse occurring at 5:54 p.m. The partial eclipse will end at 6:49 p.m. CDT, which is after 6:02 pm sunset in Huntsville.

The magnitude of this eclipse, that is the fraction of the Sun’s diameter covered by the moon, will be 44%.  The obscuration, or the fraction of the Sun’s area occulted by the moon, will be 32%.The Sun will be in the constellation Virgo, with Saturn low on the horizon after sunset, and Mars will be farther to the east.

A live Ustream feed of the partial solar eclipse will be available here.

Local Viewing Opportunity

Von Braun Astronomical Society (VBAS) is partnering with the U.S. Space & Rocket Center® on Thursday, October 23, 2014, from 5 p.m. until sundown, for the observance of the partial solar eclipse. Join astronomers in the Davidson Center for Space Exploration parking lot to discuss the phenomenon and observe the solar eclipse through the telescopes. There will be visible-light viewing telescopes to see any sunspots, and special telescopes with hydrogen-light viewing in order to see the prominences at the edge of the sun.

The telescopes are equipped with filters for safe viewing of the sun.  Never look at the Sun directly!  Attempting to look directly at the sun without such special filters is harmful to the eyes.
This event is free and open to the public.