Solar Sonic Boom: Eclipses May Generate Atmospheric Shocks

Something strange happens during a solar eclipse. As the Moon’s shadow passes over the surface of the Earth, observers have noticed mysterious bands of shadow ripple ahead and behind the eclipse. It seemed possible that these bands were a result of constructive and destructive interference of sunlight around the limb of the Moon (an effect known as diffraction), or atmospheric turbulence may have had a part to play. However, a new theory has come to light. As the Moon’s shadow travels across the Earth’s surface, it may be possible that the shadow cools the atmosphere suddenly, creating a pressure difference. This gives rise to a sonic phenomenon: a shock front. This may refract the path of light from the lunar limb and through the atmosphere, creating the bands of light and dark. The solar eclipse may be a sonic phenomenon as well as an optical one…

If an object travels faster than the speed of sound, a shock will form. This shock is generated as a body passes through the atmosphere faster than sound can propagate. On Earth, at sea level, the speed of sound is approximately 1,225 kilometres per hour (or 761 miles per hour; i.e. the sound of an explosion would take an hour to travel a distance of 761 miles). Should an aircraft travel at 1,225 km/hr or beyond, the pressure waves it generates cannot keep up with the plane. In this case, a shock wave will form, more commonly known as a “sonic boom” for stationary observers.

So, back to the solar eclipse. How can the shadow of the Moon create a sonic boom? It’s only a shadow, it’s not a solid body moving inside the atmosphere; surely a shock isn’t possible? Actually, research carried out by astrophysicist Dr Stuart Eves who works with the Surrey Satellite Technology Limited (SSTL) suggests it may be possible, and the phenomenon produced is known as “infrasound”. He believes that as the lunar shadow passes over the Earth’s surface, there is intense, local cooling of the atmosphere after the leading and before trailing edge of the eclipse. This cooling sets up a sudden pressure difference.

As the eclipse shadow moves through the atmosphere, the sudden disappearance of the Sun changes the Earth’s temperature.” – Dr Eves.

If we consider that the eclipse shadow travels at supersonic velocities (1,100 miles per hour at the equator and up to 5,000 miles per hour near the poles), and the strong pressure gradient travels with the eclipse, a shock front is created in the atmosphere, generating infrasound waves. The sub-audible infrasound generated by this occurrence modifies the atmosphere to such an extent that it will deflect the path of light through the atmosphere. In this case, the light and dark bands around the eclipse shadow would be created by refraction.

Some scientists are sceptical about this new theory, but Eves thinks his explanation may also help to explain other phenomena during eclipses. Infrasound may be responsible for strange Foucault pendulum behaviour (the sensitive pendulums – used to demonstrate the rotation of the Earth – swing wildly during eclipses). The infrasound pulses may cause the ground to vibrate, interfering with the pendulum swing. Infrasound may also explain some bizarre animal behaviour during these events. Sub audible sound wave frequencies are known to distress or alarm birds, perhaps their strange behaviour during eclipses could be down to infrasound propagation.

Source: BBC

Your Eclipse Photos, Part II

Like I said, you buried me with photos of last week’s total lunar eclipse. So here’s another batch. Thanks to everyone who went outside and remembered to bring a camera.

If you’re a budding (or veteran) astrophotographer, I highly recommend you check out the Bad Astronomy/Universe Today forum. We have a section just for people to post their astrophotos.

This first photograph comes from Joe from Michigan State University with a digital camera and a 4″ telescope.


Beth Katz

Beth Katz


Brian Galka
Brian Galka – Saskatoon


Rob Ratkowski
Rob Ratkowski – Maui


John Lyder
John Lyder – Trinidad and Tobago


Simone Bolzoni
Simone Bolzoni – Italy


Rick E.
Rick E. – Toronto


Joseph Guzmán
Joseph Guzmán – Chicago

Your Photos of the Total Lunar Eclipse

I asked and you delivered. Here is just a fraction of the eclipse photos Universe Today readers sent in. Thanks to everyone who participated!

Up first, here’s a mosaic of six images of the Moon captured by Thomas Jacobs using a 4.5″ reflector from Woodstock, Georgia in the US. The photos were captured through a pretty heavy cloud cover.


Rick Stankiewicz
Rick Stankiewicz captured this photo of the eclipse from Thunder Bay at -24 degrees Celsius. Outside, watching the eclipse for almost 5 hours – now that’s dedication. You can see Saturn and Regulas in the image as well.


John Gianforte
An image of the eclipse captured by John Gianforte at the University of New Hampshire observatory. They had more than 100 people on site, visiting the observatory during the eclipse. And I’m jealous to report… they had perfect weather for viewing.


Edward Willett
Edward Willett captured this image from Regina, Saskatchewan. He says this was the best he could do under the frigid conditions, with a frozen 6-year old tugging at his arm to go back inside, but I think it’s pretty great.


Philip van Heerden
And from South Africa, here’s Philip van Heerden’s photograph, taken near twilight.


Julia Tchervova
Julia Tchervova


Dean and Betty Johnson
Dean and Betty Johnson

Send in Your Eclipse Pictures, Tell Your Stories

lunareclipse20080221-0331z.thumbnail.jpg

How was your view of this week’s lunar eclipse? The skies actually opened up here in Vancouver, and we were able to see good portions of the eclipse. The kids were really excited, and got to stay up late watching the eclipse – it was all they were talking about the next day.

So send me your eclipse pictures, and I’ll run a quick gallery. Email them to me at [email protected], and I’ll try to post them in the next few days.

And to tide you over, here’s an image captured by ESA astronomers from Spain.

Post your eclipse stories in the comments below.

Upcoming Total Lunar Eclipse on August 28th, 2007

finazzi1.thumbnail.jpg

By the end of August, you might be just cooling down from the excitement of the Perseid meteor show, but it’s time to gather the friends and family again for another fantastic sky show – a total lunar eclipse on Tuesday, August 28th, 2007. This is going to be one of most popular, visible from 5 continents, including most of North America. Got that, set your calendar right now.

If you live in Western North America like me, you know we’ve gotten the short end of the lunar eclipse stick for the past few years. Well, now’s our time to shine in the ruddy glow of a fully eclipsed moon. This is the one we’ve been waiting for.

The eclipse will begin just after midnight August 28th for folks in Pacific Daylight Time – 12:54 am PDT. It won’t look like much in the beginning, but then the Earth’s shadow will slowly start to darken the Moon. Around 2 hours later, at 2:52 am PDT, the eclipse will reach totality, when the Moon is fully in the Earth’s shadow. It will change from grey to red, and stay that way for another 90 minutes. Then it will exit the shadow again just before dawn.

The eclipse will be visible from Australia, Japan, parts of Asia and most of the Americas. Unfortunately, it won’t be visible to observers in Africa or Europe. Since the eclipse gets a late start, we’ll get a good view on the West Coast of North America; as long as you’re willing to stay up late. Eastern observers will need to bring coffee, since their view won’t wrap up until 7:22 am EDT.

Here’s a special web page from NASA on the eclipse, providing diagrams and starting times for this event. I’ll provide another reminder as we get closer to the 28th, I just wanted to make you aware, and encourage you to set that evening aside for a wonderful sky show.

Original Source:NASA Eclipse Information

March 29 Total Eclipse

Partial solar eclipse across the United States on May 10, 1994. Image credit: Sky & Telescope Click to enlarge
Those lucky enough to live in parts of Africa, Turkey and Central Asia will experience a total eclipse of the Sun on Wednesday, March 29, 2006. Even larger areas of the Earth will get to see the Sun dim in a partial solar eclipse, from Brazil to China. If you’re lucky enough to be watching from the thin path of totality, the Sun will be completely obscured by the Moon, making the corona visible like a fiery ring.

On Wednesday, March 29, 2006, a total eclipse of the Sun will sweep across parts of West and North Africa, Turkey, and Central Asia. The eclipse will be partial across a much wider region, including most of Africa, all of Europe, and much of western and southern Asia.

The accompanying maps provided by Sky & Telescope magazine tell the story. They will help skywatchers in those regions of the world to plan their activities for the big day.

A solar eclipse happens when the Moon crosses the face of the Sun as seen from your viewpoint on Earth. The globe map shows the entire region of Earth that will be touched by any part of this eclipse, including the complete path of totality: where the Sun will become completely covered by the Moon. As the map shows, the total eclipse starts at sunrise at the tip of Brazil, crosses the Atlantic in the morning, the Sahara Desert at midday, Turkey in the afternoon, and ends at sunset in Central Asia. Red lines on the globe map indicate how much of the Sun’s diameter is covered at maximum partial eclipse. Purple lines tell when this happens, in Universal Time (UT; also called Greenwich Mean Time or GMT).

The close-up map of Europe and the Middle East gives more detail. Here, blue and red lines tell the start and end times of partial eclipse, respectively, again in UT/GMT. The partial eclipse for any site is at its maximum halfway between these times. The black lines on the map tell the percentage of the Sun’s diameter that will be covered at maximum eclipse.

Although a partial solar eclipse can’t hold a candle to a total one, it’s a memorable celestial event in its own right. Can you see a change in the illumination of the landscape around you? A partial eclipse has to be surprisingly deep to alter the light visibly, because our eyes are very good at adjusting to ambient light levels. But when this does happen, the world seems to take on an odd, silvery feel like no other. Look for crescent-shaped dapplings on the ground where sunlight shines through leaves. In a safely solar-filtered telescope (see below), look for mountain silhouettes on the Moon’s dark edge. Look too for a difference between the Moon’s complete darkness and the not-so-complete darkness of any sunspots that the edge of the Moon approaches.

Warning: Never look at the bright surface of the Sun without proper eye protection! Examples are special “eclipse glasses” properly designed for the purpose, a #14 rectangular arc-welder’s filter, or special astronomers’ solar filters (see our list of solar filter suppliers). Staring at the bright Sun can burn your retina, leaving a permanent blind spot in the center of your vision. The only reason a partial eclipse poses a special danger is because it can prompt people to look directly at the Sun, something they wouldn’t normally do. See our complete descriptions of the various ways to watch safely, including the “projection method” using a pinhole, binoculars, or a telescope.

Looking while the Sun is totally eclipsed, on the other hand, is safe. At that time, of course, none of the Sun’s bright surface is in view.

People who’ve never tried photographing a solar eclipse before can get fine results by following Sky & Telescope’s tips for photographers.

More on this particular eclipse appears in the January and March 2006 issues of Sky & Telescope, the Essential Magazine of Astronomy.

For detailed local predictions at any given location, please see NASA’s Web site for this eclipse.

Original Source: Sky and Telescope

Penumbral Lunar Eclipse, April 24

Image credit: NASA
NASA is planning to send people back to the Moon. Target date: 2015 or so. Too bad they won’t be there this Sunday because, on April 24th, there’s going to be a solar eclipse, and you can only see it from the Moon.

On Earth, solar eclipses happen when the Moon covers the Sun. On the Moon, the roles are reversed. It’s Earth that covers the Sun. Such an eclipse is “a marvelous sight,” according to Apollo 12 astronaut Alan Bean, who saw one in 1969. He was flying home from the Moon along with crewmates Pete Conrad and Dick Gordon when their spaceship flew through Earth’s shadow. “Our home planet [eclipsed] our own star.”

No one will see the April 24th eclipse, but we can imagine what it would be like:

You’re standing on the Moon. It’s broad daylight, almost high noon. The Sun is creeping slowly across the sky. How slowly? A lunar day is about 29.5 Earth-days long. So the Sun moves 29.5 times slower than our Earth-sense tells us it should. At that leisurely pace, the Sun approaches a dark but faintly-glowing disk three times its own size.

The disk is Earth with its nightside facing the Moon. You can see moonlit clouds floating over Earth’s dark oceans and continents. You can also see a faintly glowing ring of light around the planet–that’s Earth’s atmosphere with sunlight trickling through it. A telescope would show you Earth’s city lights, too. Beautiful.

Then the eclipse begins.

Looking through dark-filtered glasses, you watch the Sun slip behind Earth. Earth’s atmosphere, lit from behind, glows red, then redder, a ring of fire the color of sunset, interrupted here and there by the tops of the highest clouds.

Ninety minutes later–patience is required!–only a little bit of the Sun remains poking out over the edge of the planet. Arranged just so, the pair remind you of a giant sparkling diamond ring.

The Sun never completely vanishes because this eclipse is partial, not total. During a total eclipse, Earth would hide the Sun completely, which has the odd effect of turning the Moon blood red. But that’s another story.

Partial eclipses, while not as eerie or dramatic as total eclipses, are still good. In fact, future space tourists will probably rocket to the Moon to see them. It’ll be an exclusive club, people who’ve witnessed Earth taking a bite out of the Sun. The membership in 2005 is only two: Alan Bean and Dick Gordon, the surviving crew of Apollo 12.

Stuck on Earth, what can you do? As a matter of fact, it is possible to observe this Sunday’s solar eclipse from Earth in a roundabout way:

During the eclipse, Earth’s shadow will fall across the Moon and we can see that happen. Our planet’s shadow has two parts, a dark inner core called the umbra and a pale outer fringe called the penumbra. (Aside: Step outside on a sunny day and look at your own shadow. It’s dark in the middle and pale-fuzzy around the edges. You have your own umbra and penumbra.) The Moon on April 24th will glide through Earth’s penumbra, producing what astronomers call a “penumbral lunar eclipse.”

Penumbral eclipses are not easy to see because the penumbra is so pale. If you’re enthusiastic about such things, however, it’s worth a look. A subtle but distinct shading should be visible across northern parts of the Moon during greatest eclipse around 09:55 UT on Sunday morning, April 24th. That’s 02:55 a.m. PDT or 05:55 a.m. EDT in North America. The best place to be is the Hawaiian Islands where the eclipse happens only 5 minutes before local midnight on Saturday, April 23rd. The Moon will be high in the sky, ideally placed.

Even in Hawaii the experience is subtle. Not impressed? You’re just on the wrong world.

Original Source: [email protected]

Share Your Eclipse Experience

Well, how did it go? Did you get a chance to see the eclipse from your part of the world? Did you share it with your friends and family, and maybe build a little astronomy enthusiasm in your loved ones? One eye on the sky and one eye on the game? Come to the forum and share your experiences from last night. I’ll let you know how my night went. If you got pictures from last night, join the forum and post your photos into the Astrophotography forum and enjoy the “ooohs and ahhhs” from jealous rained out (or geographically challenged) forum members from around the world. Next eclipse is in 2007.

Fraser Cain
Publisher
Universe Today