Tuesday, July 04, 2017

I Can Name That Eclipse in Five Notes

Total solar eclipses have always captivated our attention. They have even made their way into our music. See how many of these songs you recognize.

March 7, 1970 total solar eclipse. Image Credit: NSO/AURA/NSF


You’re So Vain, composed and performed by Carly Simon

. . . you flew your Lear jet up to Nova Scotia
To see a total eclipse of the sun

This is the only known recorded song with lyrics that mention a specific eclipse. But which one? The recording was released November 1972. And Nova Scotia had recently experienced two total solar eclipses, one on March 7, 1970 and another on July 10, 1972. Simon has reported that she wrote the song in 1971. If Simon was reflecting on recent events, then she probably meant the total solar eclipse of March 7, 1970.

Eclipse, by Pink Floyd from ‘Dark side of the Moon’

… and everything under the sun is in tune
but the sun is eclipsed by the moon.

Total Eclipse of the Heart, sung by  Bonnie Tyler.

…Once upon a time there was light in my life
But now there's only love in the dark
Nothing I can say
A total eclipse of the heart…

If we go farther back in time, we come across a smattering of sheet music published in the 1800s and early-1900s.

The Total Eclipse Gallop, composed by E. Mack and published in 1919 by Lee & Walker. This song commemorates the August 7, 1869 total solar eclipse, which was visible across the continental United States.

Eclipse, composed by Herman Darewski (1883-1947) was published in 1919.

Other memorable dance tunes that are now long forgotten include:

Eclipse March, 1899 published by Troedel & Co., National Library of Australia.

Eclipse Polka,  by Giuseppe Bistolfi. Published by Kansas City, MO: J.W. Jenkins Son, 1889. 

University of Missouri, Kansas City Sheet Music Collection

Eclipse Polka, 1853, Published by Wm. Vanderbeek and Son,

Eclipse Waltz, 1854, W. C. Peters and Sons

Eclipse Polka, 1874, Lee & Walker

Eclipse Quickstep, 1885, Richards, J. G

Eclipse Gallop, 1885, Spear & Dehnhoff

Eclipse Schottische, 1884, Stewart, S. S.


For more information on the August 21 total eclipse, and eclipses in general, visit:



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Monday, July 03, 2017

Solar Viewing Safety

When it comes to looking at the sun, safety comes first! Here are some important questions and answers about solar viewing safety.

Anatomy of the Human Eye. Image Credit: NASA

Why is it not safe to look at the sun even when only a small part of it is visible?

The rods and cones in the human retina are very sensitive to light. Even a thin sliver of the sun’s disk covers thousands of these light-sensitive cells. Normally during daylight conditions, the iris contracts so that only a small amount of light passes through the lens and then reaches the retina. This level of indirect sunlight is perfectly OK and the eye has evolved over millions of years to safely see the daylight world under most circumstances. The problem is that the sun’s surface is so bright that if you stare at any portion of it, no matter how small, it produces enough light to damage individual retinal cells.  It takes a few seconds for this to happen, but afterwards you will see a spot as big as the solar surface you glimpsed when you look away from the sun at some other scenery. Depending on how long you gazed at the sun and how badly the retinal cells were damaged,  this spot will either fade away in time or remain permanent.  You should never assume that you can look away quickly enough to avoid eye damage because every person is different in terms of their retinal sensitivity, and you do not want to risk being the one who damages their eyes just to try to look at the sun. If you want to see what the sun looks like, use a properly-equipped telescope. Or just go online and view thousands of pictures taken of the sun by telescopes and NASA spacecraft.

Is it true that you should not look at the sun even during a total solar eclipse?

There is a misunderstanding being circulated that during a total solar eclipse when the moon has fully blocked the light from the sun, that there are still harmful ‘rays’ that can injure your eyes.  This is completely false. When the bright photosphere (the visible surface) of the sun is completely covered, only the faint light from the corona is visible, and this radiation is too weak to have any harmful effects on the human retina.

The misunderstanding comes about because of using the general term ‘solar eclipse’ to describe both the total phase when the sun disk is completely blocked, and the minutes before and after totality when there is still some of the sun’s disk visible. It is harmful to view even a sliver of the sun disk because of its intensity, and so to simply say that you should not view a solar eclipse is rather inaccurate.

Do lunar and solar eclipses have any noticeable effect on humans?

There is no evidence that eclipses have any physical effect on humans. However, eclipses have always been capable of producing profound psychological effects. For millennia, solar eclipses have been interpreted as portents of doom by virtually every known civilization. These have stimulated responses that run the gamut from human sacrifices to feelings of awe and bewilderment. Although there are no direct physical effects involving known forces, the consequences of the induced human psychological states have indeed led to physical effects.

How are eyes damaged by staring at the sun?

Solar retinopathy is a result of too much ultraviolet light flooding the retina. In extreme cases this can cause blindness, but is so painful that it is rare for someone to be able to stare at the sun for that long. Typically, eye damage from staring at the sun results in blurred vision, dark or yellow spots, pain in bright light or loss of vision in the center of the eye (the fovea). Permanent damage to the retina has been shown to occur in approximately 100 seconds, but the exact time before damage occurs will vary with the intensity of the sun on a particular day  and with how much the viewer's pupil is dilated  from decongestants and other drugs they may be taking.  Even when 99% of the Sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn. Note, there are no pain receptors in the retina so your retina can be damaged even before you realize it, and by then it is too late to save your vision!

Where can I get the right kind of solar filter to view the eclipse?

Many people will obtain eclipse viewing glasses. To date, three manufacturers have certified that their eclipse glasses and hand-held solar viewers meet the ISO 12312-2 international standard for such products: Rainbow Symphony, American Paper Optics, and Thousand Oaks Optical. These companies may be found online and the glasses ordered, but you really need to order your glasses many months in advance because of the anticipated huge audience that could number in the hundreds of millions. If you are a photographer or amateur astronomer, you will want professional-grade solar filters to cover your binoculars, telescope or camera. Companies like Thousand Oaks Optical and others you can find by using the keyword ‘Solar filters’ have these filters for sale, but again due to the large number of likely customers along the path of totality, you need to order your filter many months in advance. You will also need some time to learn how to use the filter with your optical system, and if you are photographing the eclipse, take lots of test shots to get the right solar disk size and sharpness.

Is it only the bright light that is dangerous when viewing the sun?

Actually, although filters and glasses do safely block the intense sunlight that is known to damage retinas, the infrared ‘heat’ from the sun can also make viewing uncomfortable as it literally warms the eye.  This is why staring at the sun for minutes at a time even with proper filters can still over-heat the tissues and fluids in the eye, and the consequences of this heating can be dangerous as well. To avoid this problem before totality takes place, try not to use your filters without frequently looking-away to cool your eyes. During totality, there is no adverse heating of the eyeball since the solar disk is not visible.

Isn’t this ‘safety’ issue about eclipse viewing, a bit overblown?

Absolutely not!  You cannot look at the sun without suffering severe damage. We have many built-in reflexes to prevent this. The ONLY exception is in viewing solar eclipses. It is an inherently dangerous activity that you have to do very carefully in order not to suffer eye damage. There are specific steps you can take, based on the experience of  thousands of professionals, not only in astronomy but in medicine. So, bottom line: read the safety warnings and make sure you understand how to view the eclipse before  August 21, so that the only lasting impression you have is a wonderful memory of the event, not a damaged retina!

For more information on the eclipse, and how to safely view it, visit:



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Thursday, June 29, 2017

The Sun, a Primer

The Sun is a huge, glowing ball at the center of our solar system. The sun provides light, heat, and other energy to Earth. The sun is made up entirely of gas. Most of it is a type of gas that is sensitive to magnetism. This sensitivity makes this type of gas so special that scientists sometimes give it a special name: plasma. The planets and their moons, dwarf planets, tens of thousands of asteroids, and trillions of comets revolve around the sun. The sun and all these objects are in the solar system. Earth travels around the sun at an average distance of about 92,960,000 miles (149,600,000 kilometers) from it.

An Extreme Ultraviolet Imaging Telescope (EIT) image of the Sun and a huge, handle-shaped prominence, taken on September 14,1999, in the 304 angstrom wavelength. Prominences are huge clouds of relatively cool dense plasma suspended in the Sun's hot, thin corona. At times, they can erupt, escaping the Sun's atmosphere. Image credit: NASA/European Space Agency

The sun's radius (distance from its center to its surface) is about 432,000 miles (695,500 kilometers), approximately 109 times Earth's radius. The following example may help you picture the relative sizes of the sun and Earth and the distance between them: Suppose the radius of Earth were the width of an ordinary paper clip. The radius of the sun would be roughly the height of a desk, and the sun would be about 100 paces from Earth.

The part of the sun that we see has a temperature of about 5500 degrees C (10,000 degrees F). Astronomers measure star temperatures in a metric unit called the Kelvin (abbreviated K). One Kelvin equals exactly 1 Celsius degree (1.8 Fahrenheit degree), but the Kelvin and Celsius scales begin at different points. The Kelvin scale stars at absolute zero, which is -273.15 degrees C (-459.67 degrees F). Thus, the temperature of the solar surface is about 5800 K. Temperatures in the Sun's core reach over 15,000,000 K (27,000,000 degrees F).

The energy of the sun comes from nuclear fusion reactions that occur deep inside the sun's core. In a fusion reaction, two atomic nuclei join together, creating a new nucleus. Fusion produces energy by converting nuclear matter into energy.

The sun, like Earth, is magnetic. Scientists describe the magnetism of an object in terms of a magnetic field. This is a region that includes all the space occupied by the object and much of the surrounding space. Physicists define a magnetic field as the region in which a magnetic force could be detected—as with a compass. Physicists describe how magnetic an object is in terms of field strength. This is a measure of the force that the field would exert on a magnetic object, such as a compass needle. The typical strength of the sun's field is only about twice that of Earth's field.

But the sun's magnetic field becomes highly concentrated in small regions, with strengths up to 3,000 times as great as the typical strength. These regions shape solar matter to create a variety of features on the sun's surface and in its atmosphere, the part that we can see. These features range from relatively cool, dark structures known as sunspots to spectacular eruptions called flares and coronal mass ejections.

Flares are the most violent eruptions in the solar system. Coronal mass ejections, though less violent than flares, involve a tremendous mass (amount of matter). A single ejection can spew approximately 20 billion tons (18 billion metric tons) of matter into space. A cube of lead 3/4 mile (1.2 kilometers) on a side would have about the same mass.

The sun was born about 4.6 billion years ago. It has enough nuclear fuel to remain much as it is for another 5 billion years. Then it will grow to become a type of star called a red giant. Later in the sun's life, it will cast off its outer layers. The remaining core will collapse to become an object called a white dwarf, and will slowly fade. The sun will enter its final phase as a faint, cool object sometimes called a black dwarf.

To be continued...

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Wednesday, June 28, 2017

Are You an Umbraphile? You May Be After August 21!

um-bra-phile (ˈəm-brə-ˌfī(-ə)l) noun. 1. an avid observer of, or a person with a great interest in, eclipses. [from Latin umbra ‘shade’ and Greek philos ‘loving’]

An umbraphile is, literally, a "shadow lover." He or she is addicted to total solar eclipses. Those who have not stood in the moon’s shadow may not understand. But those who have, do. For many umbraphiles, it is way of life. These are the “solar eclipse chasers.” Once every 16 months, on average, umbraphiles drop what they are doing and travel, by whatever means necessary, to gather along a narrow strip in some remote corner of the globe defined by the laws of celestial mechanics.

The next total solar eclipse will occur August 21, crossing the continental United States from coast to coast. Are you an umbraphile? If you aren't, you may be very soon!

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Tuesday, June 27, 2017

Have You Made Your Reservations for the August 21 Total Solar Eclipse?

There are less than two months until the August 21 total solar eclipse. The path of totality will pass from coast to coast across the entire continental United States. The path of totality is about 70 miles wide. So, there are lots of great places in the U.S. you can see the event.

The Path of Totality for the August 21, 2017 Total Solar Eclipse. Image Credit: NASA

If you are coming from outside the path of totality and hope to stay overnight within the path August 20, before the August 21 event, you should make your plans now if you have not done so already. Hotels are filing up fast, and are already filled up in some locations.

Please do not wing this! The Department of Transportation is asking that people DO NOT pull off to the side of the interstate. Visit the DoT website where you can see how traffic may be affected by the eclipse. Visit the Fact Sheet link below.

2017 Solar Eclipse Transportation Fact Sheet for State and Local Departments of Transportation

Here are the states through which the center line of the path of totality will pass.

  • Oregon
  • Idaho
  • Wyoming
  • Nebraska
  • Kansas
  • Missouri
  • Kentucky
  • Tennessee
  • Georgia
  • South Carolina


To get more of an idea of the location of the path of totality, check out this list of major cities that will be in the path.

  • Corvallis, Albany and Lebanon, Oregon
  • Idaho Falls, Idaho
  • Casper, Wyoming
  • Grand Island, Lincoln Nebraska
  • St Joseph, Missouri
  • Kansas City, Kansas
  • St Louis, Missouri
  • Bowling Green, Kentucky
  • Nashville, Tennessee
  • Greenville, South Carolina
  • Columbia, South Carolina


The website Eclipse2017.org has a great page to see what cities will be in the path and how long totality will last at those locations. Follow the link below.

Cities that lie in the Path of Totality

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Sunday, June 25, 2017

Make a Pinhole Camera to View the Solar Eclipse

Pinhole Camera

You don't need a lot of money to observe a solar eclipse in complete safety. With just a few simple supplies, you can make a pinhole camera that lets you watch a solar eclipse safely and easily from anywhere. 

A pinhole camera is a simple camera without a lens. Instead of a lens, it has a tiny aperture, a pinhole. Light from a scene passes through the aperture and projects an inverted image, which is known as the camera obscura effect.

Remember: You should never look at the sun directly without equipment that's specifically designed for looking at the sun. Even using binoculars or a telescope, you could severely damage your eyes or even go blind! The totality portion of a solar eclipse is safe. But looking at anything as bright as the sun is NOT safe without proper protection. And no, sunglasses do NOT count. 

Now, let's make a pinhole camera! You will need the following materials.
  • 2 pieces of white card stock
  • 1 piece of unused, smooth, aluminum foil
  • 1 pair of scissors
  • 1 roll of tape
  • 1 pin or paper clip

Image Credit: NASA/JPL-Caltech


1. Cut a square hole into the middle of one of your pieces of card stock.
Image Credit: NASA/JPL-Caltech


2. Tape a piece of aluminum foil over the hole.
Image Credit: NASA/JPL-Caltech


3. Use your pin or paper clip to poke a small hole in the aluminum foil.
Image Credit: NASA/JPL-Caltech


4. Place your second piece of card stock on the ground and hold the piece with aluminum foil above it (foil facing up). Stand with the sun behind you and view the projected image on the card stock below! The farther away you hold your camera, the bigger your projected image will be.

To make your projection a bit more defined, try putting the bottom piece of card stock in a shadowed area while you hold the other piece in the sunlight.
Image Credit: NASA/JPL-Caltech


5. For extra fun, try poking multiple holes in your foil, making shapes, patterns and other designs. Each hole you create will turn into its own projection of the eclipse, making for some neat effects. Grab a helper to take photos of your designs for a stellar art project you can enjoy even after the eclipse has ended. 
Image Credit: NASA/JPL-Caltech

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Saturday, June 24, 2017

Ugarit Total Solar Eclipse

Trivia: The Ugarit Total Solar Eclipse

In 1948, a clay tablet was discovered in the port city of Ugarit in Northern Syria. In the text of the tablet, a Mesopotamian historian noted, "On the day of the new moon, in the month of Hiyar, the sun was put to shame, and went down in the daytime, with Mars in attendance." Scientists realized the text described a total solar eclipse in which the planet Mars was visible during totality.

Researchers originally dated the eclipse event as May 3, 1375 B.C. But further study suggested a different eclipse. Researchers considered the dating of the tablet, combined with the text's statement of the month in which the eclipse occurred and the fact that Mars was seen during totality. This evidence pointed to the total eclipse of March 5, 1223 B.C. The revised findings were first published in 1989 in the journal Nature.

The Ugarit eclipse is one of the earliest solar eclipses recorded. The path of totality began in the Atlantic Ocean, crossed north-western Afrca, Turkey, and central Asia.

The Accelerating Moon, the Decelerating Earth

The dating of ancient solar eclipses provides astronomers with reference points to determine long-term evolution of angular momentum in the Earth-Moon system--that is, it helps astronomers understand how the moon's orbit and Earth's rotation have changed over time. The revised date--March 5, 1223 B.C.--implies that the secular deceleration of Earth's rotation has changed very little during the past 3,000 years.

Secular What?

Ocean tides are caused by the gravitational pull of the moon (and, to a lesser extent, the Sun). The resulting tidal bulge in Earth's oceans is dragged ahead of the moon in its orbit due to the daily rotation of Earth. As a consequence, the ocean mass offset from the Earth-Moon line exerts a pull on the moon and accelerates it in its orbit. Conversely, the moon's gravitational tug on this mass exerts a torque that decelerates the rotation of Earth. The length of the day gradually increases as energy is transferred from Earth to the moon, causing the lunar orbit and period of revolution about Earth to increase.

You can learn more about secular acceleration from the following NASA article.

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Thursday, June 22, 2017

Total Solar Eclipse of January 1, 1889

Trivia: The Total Solar Eclipse of January 1, 1889, also known as the New Year’s Day Eclipse of 1889.

The path of totality began in the Bering Sea, crossed the North Pacific Ocean, passed through California to the north of San Francisco, through northern Nevada, Idaho, northwestern Wyoming, Montana, the northwestern part of North Dakota, and into central Canada, passing through southern Manitoba, and finishing on the western edge of Ontario.

The first photograph of a solar eclipse was taken during solar eclipse of July 28, 1851. But even with this technological advancement, most of the recorded observations of a total solar eclipse remained in the form of the written word and drawings. For the January 1, 1889 eclipse, a group of amateur and professional astronomers joined forces with a new photography society in San Francisco. They agreed to combine their resources to observe and record the eclipse. One example of the efforts was a photographic plate on which multiple exposures were made, showing many partial eclipse phases leading to totality.

On February 7, the group reunited in downtown San Francisco and presented their observations. The group enjoyed the experience so much that they agreed to form their own astronomical society, called the Astronomical Society of the Pacific (ASP). Originally a society of forty members, the ASP has grown to a national society dedicated to astronomy education and outreach. The ASP helps people of all ages learn astronomy and helps those people share their knowledge with others.

You can learn more about the ASP at the society’s official website:

www.astrosociety.org

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Wednesday, June 21, 2017

Two NASA Briefings on the Total Solar Eclipse of August 21, 2017

On June 21, representatives from NASA, other federal agencies, and science organizations, provided important viewing safety, travel and science information on the August 21, 2017 total solar eclipse. This eclipse will be the first in 99 years that will cross the entire continental United States.
Two briefings were held at the Newseum in Washington. The briefings aired live on NASA Television and streamed on the agency’s website.

Over the course of 100 minutes, 14 states across the United States will experience more than two minutes of darkness in the middle of the day. Additionally, a partial eclipse will be viewable across all of North America. The eclipse will provide a unique opportunity to study the sun, Earth, moon and their interaction because of the eclipse’s long path over land coast to coast. Scientists will be able to take ground-based and airborne observations over a period of an hour and a half to complement the wealth of data and images provided by space assets.

The June 21 briefings were:

Logistics Briefing
  • Thomas Zurbuchen, associate administrator of NASA’s Science Mission Directorate at the agency’s headquarters in Washington
  • Vanessa Griffin, director of the National Oceanic and Atmospheric Administration’s Office of Satellite and Product Operations in Suitland, Maryland
  • Brian Carlstrom, deputy associate director of Natural Resource Stewardship and Science at the National Park Service in Washington
  • Martin Knopp, associate administrator of the Office of Operations in the Federal Highway Administration at the U.S. Department of Transportation in Washington


Science Briefing
  • Thomas Zurbuchen
  • Angela Des Jardins, principal investigator of the Eclipse Ballooning Project at Montana State University, Bozeman
  • Angela Speck, professor of astrophysics and director of astronomy at the University of Missouri, Columbia
  • Dave Boboltz, program director of solar physics in the Division of Astronomical Sciences at the National Science Foundation in Arlington, Virginia
  • Linda Shore, executive director of the Astronomical Society of the Pacific in San Francisco
  • Matt Penn, astronomer at the National Solar Observatory in Tucson, Arizona


For more information on the eclipse, and how to safely view it, visit:



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Thursday, May 18, 2017

Saturn's Shortening Shadow

Saturn's shadow across the rings, as seen from the Cassini spacecraft. Image Credit: NASA/JPL-Caltech/Space Science Institute

The Cassini spacecraft continues sending back great images and data during the Grand Finale portion of its mission at Saturn. Here we see a sign of the changing seasons at Saturn, in that the planet’s shadow across the rings is shortening.

The above image was taken at a distance of about 760,000 miles (1.2 million kilometers) from Saturn. The image scale is 46 miles (73 kilometers) per pixel.

To learn more, read the full article here.

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Wednesday, April 26, 2017

Cassini's Big Finish Begins

Saturn eclipsing the sun, seen from behind by the Cassini orbiter. Earth can be seen as a small dot between the rings on the upper, left-hand side. Image Credit: NASA/JPL/Space Science Institute

(My apologies for the "big finish." But because the other phrase is so prevalent, I just had to find something to say other than "grand finale,” at least once.)

The NASA Cassini mission is winding down, quite literally. On April 26, the Cassini spacecraft became the first to dive between Saturn and its ring system. This begins the spacecraft’s “grand finale” in which it will make 22 amazing orbits and, on September 15, enter Saturn’s atmosphere and burn up. The spacecraft will be useful until the very last moment–it will be sending back data continuously, including measurements of the composition of Saturn’s atmosphere, rotation rate and interior structure.

Until that time, the instrument teams have several new observations to make. These include understanding a Saturn radiation belt, discovered inside the rings in 2004, and taking close-up pictures of the rings and other features.

Read the 2004 Article: New Radiation Belt

The spacecraft will also image Saturn’s cloud tops at close range, weigh its ring system (which will indicate just how old it is), sample the atmosphere of the planet and its rings, and measure Saturn’s internal structure.

Read the NASA PDF Resource on the Mission (published 1997)

Originally called Cassini-Huygens, the mission involves ESA, NASA and the Italian Space Agency. The idea of the mission began in 1982, when the European Science Foundation and the American National Academy of Sciences formed a working group to investigate future cooperative missions. Two European scientists suggested a paired Saturn Orbiter and Titan Probe as a possible joint mission. In 1983, NASA's Solar System Exploration Committee recommended the same Orbiter and Probe pair as a core NASA project. NASA and the European Space Agency (ESA) performed a joint study of the potential mission from 1984 to 1985. ESA continued with its own study in 1986, while American astronaut Sally Ride, in her 1987 report, also examined and approved of the Cassini mission.

In 1988, NASA Associate Administrator for Space Science and Applications Len Fisk wrote to his counterpart at ESA, Roger Bonnet, strongly suggesting that ESA choose the Cassini mission from the three candidates at hand and promising that NASA would commit to the mission when ESA did.

Read the Article: Launch of Cassini Spacecraft

On October 15, 1997, a Titan IVB/Centaur rocket launched from Cape Canaveral Air Force Station in Florida, sending the Cassini orbiter and its Huygens probe on a seven-year, 2.2-billion mile journey to the Saturn system.

Read About the Spacecraft

The mission arrived at Saturn on July 1, 2004. The mission was originally planned for four years. But Cassini-Huygens was so successful that the mission multiple times, eventually to 2017. It has flown past seven of the larger satellites, including giant Titan – which is larger than the planet Mercury. The orbiter passed Titan more than 70 times. Flying within 880 km of the moon, it studied Titan’s orange clouds and nitrogen-rich atmosphere. It also mapped its surface with an imaging radar.

On Christmas Day 2004, Huygens separated from Cassini. Three weeks later, it entered Titan’s thick atmosphere, becoming the first probe to land on the surface of a planetary satellite (other than Earth’s moon). Protected by a heat shield, the probe slowed from 18,000 to 1,400 km per hour in just three minutes. Soon after, a large parachute opened. At a height of about 160 km, the probe began to take pictures and study the atmosphere. For more than two hours, data from Huygens were received and stored on Cassini as it flew overhead.

Read More

To learn more about Cassini, Huygens, Saturn, Titan, the rest of the Saturn system, and the Grande Finale, check out these links.

Read About the Mission

See the Mission Timeline

Read the FAQ for Cassini – The Grand Finale

Read the 2017 Article: Bittersweet feeling as Cassini mission embarks on its ‘grand finale’ ahead of death plunge

Read the 2014 Article: Cassini 10 Years at Saturn Top 10 Discoveries

Read the 2017 April Article: Cassini Completes Final -- and Fateful -- Titan Flyby

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Tuesday, April 04, 2017

What is an Orbit?

or-bit (ˈȯr-bət) noun. 1. a. A path described by one body in its revolution about another (as by the earth about the sun or by an electron about an atomic nucleus); also, one complete revolution of a body describing such a path. b. A circular path[Middle English, from Medieval Latin orbita, from Latin, rut, track, probably from orbis]

An orbit is a regular, repeating path that one heavenly body takes around another body. A body which orbits another body is a satellite of the other body. So, the moon is a satellite of Earth, while at the same time, Earth is a satellite of the sun, along with the other planets, comets, asteroids, and many other bodies in the solar system. And remember that the sun orbits the center of our Milky Way galaxy. So, the sun is a satellite of the Milky Way.

A satellite can be classified as natural or artificial. The moon, Earth and other heavenly bodies are natural satellites. All orbiting objects made by humans are artificial satellites. The first artificial satellite was Sputnik 1, launched in 1957. A more recent example is the International Space Station, which was launched in pieces and assembled in orbit.


Most orbiting bodies move along or close to an imaginary flat surface. This imaginary surface is called the ecliptic plane.


What Shape Is an Orbit?


Orbits come in different shapes. All orbits are elliptical, which means they are the shape of an ellipse, similar to an oval. For the planets, the orbits are almost circular. The orbits of comets have a different shape. They are highly eccentric or "squashed." They look more like thin ellipses than circles.


Satellites that orbit Earth, including the moon, do not always stay the same distance from Earth. Sometimes they are closer, and at other times they are farther away. The closest point a satellite comes to Earth is called its perigee. The farthest point is the apogee. For planets and other bodies that orbit the sun, the point in their orbit closest to the sun is perihelion. The farthest point is called aphelion. Earth reaches its aphelion during summer in the Northern Hemisphere. The time it takes a satellite to make one full orbit is called its period. For example, Earth has an orbital period of one year. The inclination of an orbit is the angle the orbital plane when compared with Earth's equator.


This diagram of an Earth orbit demonstrates an elliptical orbit and shows apogee, perigee, aphelion, and perihelon. Image Credit: NOAA

How Do Objects Stay in Orbit?

An object in motion will stay in motion unless something pushes or pulls on it. This statement is called Newton's first law of motion. Without gravity, an Earth-orbiting satellite would go off into space along a straight line. With gravity, it is pulled back toward Earth. A constant tug-of-war takes place between the satellite's tendency to move in a straight line, or momentum, and the tug of gravity pulling the satellite back.

An object's momentum and the force of gravity have to be balanced for an orbit to happen. If the forward momentum of one object is too great, it will speed past and not enter into orbit. If momentum is too small, the object will be pulled down and crash. When these forces are balanced, the object is always falling toward the planet, but because it's moving sideways fast enough, it never hits the planet. Orbital velocity is the speed needed to stay in orbit. At an altitude of 150 miles (242 kilometers) above Earth, orbital velocity is about 17,000 miles per hour. Satellites that have higher orbits have slower orbital velocities.

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Sunday, April 02, 2017

What is an Eclipse?

e-clipse (i-ˈklips) noun. 1. a. The total or partial obscuring of one celestial body by another. b. The passing into the shadow of a celestial body. Compare OCCULTATION, TRANSIT. [Middle English, from Anglo-French, from Latin eclipsis, from Greek ekleipsis, from ekleipein to omit, fail, suffer eclipse, from ex- + leipein to leave]

An eclipse happens when one heavenly body, such as a moon or planet, moves into the shadow of another heavenly body. There are two types of eclipses as seen from Earth: an eclipse of the moon and an eclipse of the sun.

An eclipse of the moon...

The moon moves in an orbit around Earth, and at the same time, Earth orbits the sun. Sometimes Earth moves between the sun and the moon. When this happens, Earth blocks the sunlight that normally is reflected by the moon. Instead of light hitting the moon’s surface, Earth's shadow falls on it. This is an eclipse of the moon—a lunar eclipse. A lunar eclipse can happen only when the moon is full.

A lunar eclipse can be seen from Earth at night. There are two types of lunar eclipses: total lunar eclipses and partial lunar eclipses.

A total lunar eclipse happens when the moon and the sun are on exact opposite sides of Earth. Although the moon is in Earth's shadow, some sunlight reaches the moon. The sunlight passes through Earth's atmosphere, which causes Earth’s atmosphere to filter out most of the blue light. This makes the moon appear red to people watching from Earth.

A partial lunar eclipse happens when only a part of the moon enters Earth's shadow. In a partial eclipse, Earth's shadow appears very dark on the side of the moon facing Earth. What people see from Earth during a partial lunar eclipse depends on how the sun, Earth and moon are lined up.

A lunar eclipse usually lasts for a few hours. At least two partial lunar eclipses happen every year, but total lunar eclipses are rare. It is safe to look at a lunar eclipse.

A lunar eclipse. Image credit: NASA

An eclipse of the sun...

Sometimes when the moon orbits Earth, it moves between the sun and Earth. When this happens, the moon blocks the light of the sun from reaching Earth. This causes an eclipse of the sun, or solar eclipse. During a solar eclipse, the moon casts a shadow onto Earth.

There are three types of solar eclipses. The first is a total solar eclipse. A total solar eclipse is only visible from a small area on Earth. The people who see the total eclipse are in the center of the moon’s shadow when it hits Earth. The sky becomes very dark, as if it were night. For a total eclipse to take place, the sun, moon and Earth must be in a direct line.

The second type of solar eclipse is a partial solar eclipse. This happens when the sun, moon and Earth are not exactly lined up. The sun appears to have a dark shadow on only a small part of its surface.

The third type is an annular (pronounced “ANN-you-ler”) solar eclipse. An annular eclipse happens when the moon is farthest from Earth. Because the moon is farther away from Earth, it seems smaller. It does not block the entire view of the sun. The moon in front of the sun looks like a dark disk on top of a larger sun-colored disk. This creates what looks like a ring around the moon.

A solar eclipse. Image credit: NASA

During a solar eclipse, the moon casts two shadows on Earth. The first shadow is called the umbra (pronounced “UM-bruh”). This shadow gets smaller as it reaches Earth. It is the dark center of the moon’s shadow. The second shadow is called the penumbra (pronounced “pe-NUM-bruh”). The penumbra gets larger as it reaches Earth. People standing in the penumbra will see a partial eclipse. People standing in the umbra will see a total eclipse.

The umbra and penumbra. Image credit: NASA

Solar eclipses happen approximately once every 18 months. Unlike lunar eclipses, solar eclipses only last for a few minutes.

You should never look directly at the sun. It can permanently damage your eyes. You must use proper safety equipment to look at any type of solar eclipse. More on this later.

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Monday, March 27, 2017

Journey to the Shadow, No. 1

On August 21, millions of people will look skyward to witness a spectacle not seen in 38 years—a total eclipse over the continental United States. This time, the shadow will span the continental U.S. from coast to coast, the first to do so since 1918. Even those in the U.S. who will not be in the path of totality, will see a partial solar eclipse.

The August 21 eclipse might be the most viewed sky event in history. The Sun’s shadow will travel from Oregon to South Carolina. In addition, the shadow will pass over thousands of miles of the Pacific and Atlantic oceans. But the shadow will touch no other land.

This will be the first total solar eclipse to cross the continental United States in 38 years. One eclipse did pass over Hawaii in 1981. But the last for the continental U.S. was February 26, 1979. Sadly, few saw it because observers had to be in one of only five states in the northwest, and the wintry weather was bad along the path of totality.

Before the 1979 eclipse, there was the eclipse of March 7, 1970. That eclipse passed up the U.S. east coast. More people directly saw that eclipse than the 1979 eclipse, but because that was 47 years ago, few in the United States are still with us who saw the 1970 total eclipse with their own eyes.

For August 21 total eclipse, I plan to add my name to the list of those who have seen a total eclipse for themselves. I plan to stand in the path of totality and experience night in the daytime. Along the way, I will review the phenomenon which we call solar and lunar eclipses. I will recount the history, delve into the mythology, and try to separate the fact from fiction. This is just my first step. Join me on my journey to the shadow.

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