This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

Eclipses (Part 2)

by Dr Jamie Love Creative Commons Licence 1997 - 2011

Annular and partial eclipses are dull and disappointing events (in my opinion) but a total eclipse is amazing!

During totality the entire photosphere is covered (occulated) by the Moon. You will recall that the photosphere is the "surface" of the Sun, or another star, from which the light in emitted. Therefore, when you are in totality, you are completely hidden from the Sun's rays. The sky turns from day-blue to night-black and all the stars come out. The temperature drops quickly, the birds stop singing and, well, it's amazing! During even the best annular or partial eclipse part of the photosphere shines through and the result is far from spectacular. It gets a little dark and cool, kind of like a cloud rolling in, but the sky stays blue and the stars stay hidden. (Boring. ) Also, YOU CAN GO BLIND STARING AT A PARTIAL OR ANNULAR SOLAR ECLIPSE!

Is a lunar eclipse "boring"?

Well, it isn't as amazing as a solar eclipse but it's pretty interesting and a lunar eclipse is easier to observe for several reasons. First, you don't have to be in such a particular location to see a lunar eclipse. All you have to do is be on the right side (Moon side) of the Earth. As long as you can see the Moon during its eclipse, you can see a lunar eclipse. You will see the Full Moon slowly pass into the Earth's shadow. The Earth casts one heck of a big shadow and that brings us to the second reason it is easy to see a lunar eclipse - the geometry need not be so precise. Another reason it is so easy to see a lunar eclipse is because they can last for hours!

The Moon doesn't completely disappear during a lunar eclipse because some sunlight is refracted through the Earth's atmosphere. The elliptical nature of the Moon's orbit means that sometimes it's in a deeper shadow than at other times, depending upon whether the Moon is in the Earth's umbra or penumbra. All these complications mean that a lunar eclipse can be highly variable. Some lunar eclipses are bright with the "shadowed" Moon turning wonderful colors like cooper or orange. There are historical reports of the Moon disappearing completely during a lunar eclipse but this may have been due to dust or volcanic ash in the Earth's atmosphere at the time. This dust and ash might also contribute to the Moon's color during a lunar eclipse.

Well, it sounds like a lunar eclipse is pretty cool. What's special about solar eclipses?

During a total solar eclipse three of the Sun's outermost features become visible and, until modern astronomical equipment was invented, total eclipses were the only way to see these important features.
[My lack of artistic abilities would make a mockery of these beautiful features so I will not try to draw them. Instead, I will simply describe them and you can use your imagination.]

Immediately outside the photosphere is the chromosphere. I mentioned this layer in a previous lesson when I told you how spectroscopy can tell us what the Sun is made of. If we imagine the photosphere to be the Sun's "surface" we can imagine the chromosphere to be the Sun's "atmosphere". The photosphere is only 300 kilometers deep but the chromosphere is about 10,000 kilometers thick. If you were to travel from the surface of the Sun to the top of the chromosphere you would find that the temperature rises from 5500oC at the photosphere to a whopping 50,000oC at the top of the chromosphere!

But that doesn't make sense! How can it get warmer farther from the Sun?

The most recent "explanation" (flavor of the month ) is that the energy is transferred from down below by a continuous "rumble" of microflares. These are tiny solar flares that appear to feed magnetic energy up into this part of the Sun.

Like any "atmosphere" surrounding a "world", it's hard to tell where it ends but eventually the chromosphere merges with the corona. The corona is made of extremely thin (tenuous) gas.
The inner corona, called the K-corona, contains particles with "temperatures" as high as 2 millionoC and it rises above the chromosphere to a height of about 75,000 kilometers. If the current theory is correct (and if I understand the explanation) the microflares actually heat the K-corona and the warmth at the top of the chromosphere is caused by the heat from the K-corona above it.
The outer corona, called the F-corona, extends for several million kilometers into space and it is considerably cooler than the layers below it.

Neither the chromosphere nor the corona can be seen without special equipment - except during a solar eclipse.

What's the third important feature visible during a solar eclipse?

Yes, I was just coming to the third. Prominences are masses of glowing gas (mostly hydrogen) that rise up from the surface of the Sun.
A quiescent prominence hangs in the chromosphere sometimes for weeks. Of course, you need one of those special astronomy devices to watch a quiescent prominence for that long. For the purposes of a solar eclipse observation, a quiescent prominence is one limited by its location in the chromosphere.
An explosive prominence is a short-lived but violent explosion from the Sun's surface that rises up thousands of kilometers and may even hurl the material through the corona and completely away from the Sun! An explosive prominence is an amazing and beautiful phenomenon (unless one is headed your way and you don't have the Earth's atmosphere to protect you - so astronauts might have a different opinion).

There are other features of the Sun (spicles, flares, spots, etc.) but I won't go into them here. Solar astronomy is as big as astronomy gets in this Solar System and the subject is very complicated.

To the casual observer of a solar eclipse only the corona is obvious.

Before I conclude my description of the highlights of a solar eclipse, I must tell you about one other spectacular feature. Baily's Beads are bright bits of the chromosphere seen along the edge of the Moon just before totality and just after totality. They are caused by the Sun shining through the topography of the Moon's surface.

Interesting.
If eclipses are caused by specific alignments and these are caused by orbits, does that mean eclipses happen regularly?

Yes, but not as "regularly" as you might think. This brings us to the word "Saros". Ancient Babylonians used the word "saros" to mean "repetition" and the English astronomer Sir Edmond Halley borrowed the word to describe the repetition of eclipses.

Eventually a certain alignment starts and that initiates a series of eclipses called a Saros series. A Saros series begins with a barely noticeable partial eclipse at one of the Poles (North or South). The eclipse produced by the next cycle will be a little bit further towards the equator and usually lasts a little longer but it will still be only partial. With each cycle, called a Saros cycle, each eclipse occurs closer to the equator and lasts longer. Eventually the partial eclipses are replaced by total eclipse. After about 30 cycles the eclipse occurs close to the equator and its totality lasts for its maximum duration. The next eclipse of that series will be slightly on the other side of the equator and usually slightly shorter in duration. With each cycle the reoccurring eclipses move further away from the equator as they migrate to the other pole. Eventually the series "fades" into several partial eclipses and the series ends with a final, short partial eclipse near the pole opposite the pole from which the series started.

Note and warning - some astronomers, books and websites call a Saros cycle simply a "Saros" and that might confuse you because you may want to know if they are talking about a series or a cycle. When talking of a series they will (should) call it a "Saros series" and leave the shortened "Saros" for cycles only. (But sometimes they aren't too clear. I will make a point to be clear here.)

How many cycles are in a series and how long is a Saros cycle?

Each Saros series has between 70 and 85 Saros cycles. (That's between 71 and 86 eclipses.) The number varies depending upon the specifics. I'll spare you the details. On the other hand, each Saros cycle is exactly 223 lunar cycles. Let's get specific here about what I mean by a "lunar cycle". I've been using that word in a rather undefined way in order to avoid the complexity that we will now explore.

You may recall that a synodic month is the length of time it takes the Moon to return to its position with respect to the Sun. (Another name for this length of time is a lunation.) Of course, you have to count time in synodic months, not "normal" months, because eclipsing requires the Moon to be in a very particular position with respect to the Sun as seen from the Earth. (It must be a Full Moon for a lunar eclipse and a New Moon for a solar eclipse. Right?) One synodic month is about 29.5 days but synodic months are not the whole story. Remember, for an eclipse to occur the Moon must also be at a node. These two nodes move slowly but in a regular and predictable manner.
The Moon returns to its (new) node every 27.2 days and this length of time is called a Draconic month. To repeat a Saros cycle in the series you have to plot both the Draconic and synodic months until they meet again. When you do all the math it turns out that a Saros cycle occurs every 223 synodic months (or 242 Draconic months, if you want to think of it that way).
Specifically, one Saros cycle works out to be 6,585.3 days or 18 years, 11 days, and 8 hours if that time includes four Leap Years. [If that time includes five leap years the next eclipse of that series will occur in 18 years, 10 days, and 8 hours.]
Phew!

By keeping track of the Saros cycles in a Saros series you can calculate the next cycle and predict when the next eclipse of that series will occur - 18 years, 11 days, and 8 hours later. Easy. Unfortunately, it will not occur where you last saw it. That's because of the "8 hours" part of the calculation. The Earth will have rotated 1/3 of a rotation in eight hours so the next eclipse of the series strikes the Earth 120o west of where it did during the last cycle. So you have to wait for three Saros cycles (about 54.1 years) to expect that series to return to your neighborhood. And when it does return to your neighborhood, the eclipse will occur slightly north or south of its original position because each cycle moves the eclipse slightly closer towards the opposite pole from which it started!

The Ancients had a rough idea about the Saros cycles. The Babylonians, 2500 years ago, used it to predict lunar eclipses - which are easier to observe as explained earlier. According to history (legend?) Thales, a Greek philosopher, predicted the return of a Saros event. His prediction of an eclipse for 25 May 585 BC is said to have put an end to a war between King Alyattes of Lydians and King Cryaxes of Medes.

This is complicated and I'm confused! Give me an example of a series.

OK. According to computer models, Saros series (#) 130 started with a partial eclipse on the 20th of August 1096 (AD) near the South Pole. Like the first eclipse of any Saros series, it was very brief and partial. Exactly 223 synodic months later, on the first of September 1114, this eclipse "returned". This too was a very brief and partial eclipse but it was a wee bit longer than the previous one and a little further north of the South Pole. Each cycle produced a path for the eclipse that was farther north and lasted longer. [There are exceptions to this "trend". Generally speaking, each eclipse gets longer as it approaches the equator and the trend reverses as it moves away from the equator but the specifics of the geometry can add a hiccup to the trend. Therefore, this is a "trend" not a "rule".] On the 5th of April 1457, on its 21st cycle, this eclipse produced its first total eclipse which lasted 2 minutes and 8 seconds. Each subsequent total eclipse of this series was a little further north of the other (and a third of a planet away).

On February 16, 1980 Saros cycle 41 of Saros series 130 swept across Africa just south of the equator. At its "best" location, where there was maximum totality, the New Moon covered the Sun for a total of 4 minutes and 8 seconds. On February 26, 1998 (there had been five leap years) cycle 42 swept from the Pacific across Central America and into the Caribbean blocking out the Sun for 4 minutes and 9 seconds at its maximum position. When this series returns for cycle 42 in 2016 (March 9th) its path of totality will be restricted to the western Pacific Ocean. Notice that 1/3 global rotation effect.

This Saros series will continue to produce an eclipse each cycle (now in the Northern Hemisphere) every 18 years and 11.3 days. On July 30th in the year 2250 Saros cycle 56 of this series will be so far north that it will produce its first partial eclipse in nearly 800 years. After that it will produce nothing but shorter and shorter partial eclipses, farther and farther north until its last Saros cycle (#72) produces it last eclipse (#73) on the 11th of November in the year 2430. That will be the end of Saros series 130. That series will have lasted over 13 centuries which is about the average "life span" of a Saros series.

Hey, does all this mean we can only have one eclipse every 18 years?

Yes, from a single Saros series, but there are many of these series and they overlap. There are currently 42 Saros series running so within 18 years and 11.3 days there should be 42 eclipses. However, many of them are partial eclipses due to being early or late in the series and some are annular eclipses because they occur during the Moon's apogee. On average, there are about six total solar eclipses per decade somewhere on the Earth.

There have been some eclipses since the Saros cycle 42 of series 130 (on the 26th of February 1998). There was an annular eclipse on the 22nd of August 1998 from Saros series 135. There was another annual eclipse from Saros series 140, on the 16th of February 1999. And I experienced a total eclipse (my second) from Saros series 145!

The first eclipse of Saros series 145 was on the 4th of January 1639 near the North Pole. This series will produce 77 eclipses, the last one near the South Pole on the 17th of April 3009.

On the 11th of August (1999) the 21st eclipse of this series occurred and the folks over here in Europe were very excited about it! Here's a description of the path of totality for that eclipse. Imagine (or use a map to follow) the eastward progression of this eclipse. All solar eclipses move across the Earth from west to east. This motion across the Earth's surface is NOT what you would expect if it were caused by the Earth's rotation. The Earth's rotation causes events to moves in the opposite direction - from east to west. (Think about the Sun's path or motion of the stars. Those motions are all caused by the Earth's rotation.) The motion of an eclipse is caused by the motion of the Moon in its orbit! Recall that the Moon is orbiting the Earth in the same direction as the Earth's spin. It may not appear obvious to you but throughout the day the Moon moves slightly eastward, ahead of the Earth. Think about the Moon's orbit around the Earth and its direction, and you will understand why solar eclipses move across the Earth from west to east.

The Moon's umbra first touched the surface of the Earth in the Atlantic Ocean at 09:31 UT. At that point its shadow was only 49 kilometers wide and totality lasted only 47 seconds. (Hardly worth a boat trip.) This shadow zoomed eastward across the Atlantic at about a kilometer per second until at 10:10 UT it made landfall on the southwest coast of England. During the intervening time the shadow had grown to 103 kilometers wide and the duration of the eclipse along the centerline was then about 2 minutes. At 10:11 UT the umbra moved onto the Cornish Peninsula. And we were there!

Feel free to skip this part but you might enjoy the "flavor" of an eclipse this way.

Our eclipse trip to Cornwall

We arrived in Wadebridge, a village in Cornwall along the path of totality, on August 9th. We were expecting huge crowds so we had booked our Bed&Breakfast months ago (from the 9th to the 11th) but the huge crowds were not there. Oh, there was some increase in the population in Cornwall but it was not at all like the numbers expected. The poor turnout was due to bad weather and the news (media) complaining that the crowds would be so huge that no one would enjoy it.

Well, they were right about the weather. On the morning of the eclipse (August 11th) the sky was wall to wall clouds with a rare hole of sunshine. We prepared ourselves psychologically for the worst and decided to go west, to the coast, because we like the beach and thought it would be a nice place to be regardless of the eclipse.

We drove to a tiny seaside village called Perranporth, walked the beach and settled on a high, grassy knoll overlooking the sea. It didn't look promising and I felt badly for my friend, Christine. I had seen an eclipse before but this was to be her first and it looked like it was going to be a wash out. By the time the eclipse started, around 9:30 UT, the cloud cover was so thick that it was already plenty dark. As the minutes went by it grew darker but nothing obvious and we still could not see the Sun. Then, about two minutes before totality, we heard cheers and whoops from a crowd of folks to the south of us. We looked up and saw a hole in the clouds moving toward us from their direction! As the edge of the clouds' hole moved towards the Sun, we saw the Sun start to shine through. We put on our solar-shades (special glasses that protect the eyes from the harmful rays of the Sun) and watched as the clouds parted and revealed the Sun now nearly eclipsed!

It was amazing.

We watched the Moon work its way over the last remaining edge of the Sun and saw a beautiful diamond ring effect. The timing on this is always critical because you don't want to hurt your eyes by removing your protective shades too early but the ever-dimming image makes you want to remove the shades to get a better view and the thin clouds moving by also made it tricky. We were doing a lot of "glances" without the shades as the diamond ring appeared. Then, suddenly, totality occurred! Now, with our naked (unprotected) eyes we enjoyed the full beauty of the Sun's ghostly corona. Small, thin clouds made it difficult to tell where the corona ended and the clouds began but the solar prominences were obvious. We used our binoculars to get a better view. Bright pink prominences were sticking out from lots of places. One particularly numerous group of prominences (at around 2 o'clock - using a clock face on the Sun) included a flare that had a great deal of length and structure. Obviously an explosive prominence. Beautiful.

During the minute and a half of totality, a few thin clouds drifted across the hole but our view was not seriously obstructed. Of course, the cloud cover made observations of any other sky event a "non-event". I had planned to see Mercury and Venus and even was going to try a quick sweep with my binoculars to find Comet Lee (which was passing near GEMINI at the time) but Sirius was the only object bright enough to shine, occasionally, through the overcast sky.

Totality ended with a final diamond ring and we put our shades back on to watch the Moon move away from the Sun. However, a minute or so after totality the clouds moved in and we never saw the Sun again until the eclipse was over.

We were among the luck few who actually saw totality and I was very happy that Christine got to see it. Folks just a few kilometers away saw nothing! We didn't see Bailey's beads (clearly) but that's OK. We didn't see a "curtain" of shadow or sunlight as totality swept over and then away from us because the rest of the clouds diffused the effect. Oh, well. We did notice just before and after totality that shadows (from the high grass on our hill) produced shaper, "weird" shadows. Cool.

We had a good eclipse.
Many thanks to all those who wished us "Clear skies!"

At 10:16 UT the umbra left England, rushed across the English Channel and landed in northern France. The shadow swept 30 miles north of Paris and then visited Belgium, Luxembourg and then Germany. At 10:41 UT the umbra left Germany and made its way across the Alps! The moment of the greatest eclipse, when all the geometry was right for maximum duration, occurred at 11:03:04 UT in south central Romania. Here the umbra's width was 112 kilometers and the eclipse lasted 2 minutes and 23 seconds. At 11:07 UT Bucharest was covered by the shadow and then the umbra moved out over the Black Sea. At 11:21 UT the umbra landed on northern Turkey, cut across it, nipped across a small bit of Syria and then hit Iran. At 12:22 UT the umbra crossed into Pakistan but by this time the path had shrunk to 85 kilometers and totality there lasted only 1 minute and 13 seconds. By then the umbra was moving at about 2 kilometers per second. Eventually, at 12:28 UT this eclipse reached India but 12 minutes later it moved into the Bay of Bengal and then the umbra fell into space at 12:36:23 UT.

That was the end of Saros cycle 21 of Saros series 145. For about three and a half hours the Moon's shadow had touched the Earth. (By the way, if the motion of the shadow had been caused by the Earth's rotation it would have taken a lot longer to go between the Atlantic and Indian Oceans - and it would have been in the opposite direction. Right? Think about it.)

Have you ever seen a total eclipse of the Sun in good skies?

Yes!

Feel free to skip this part too, or not .

On the 26th of February 1979 I enjoyed Saros series 120 cycle 55. I was living in Minnesota (USA) at the time, earning my Master's degree. We (I and my best friend - Roger) drove a couple hundred miles to get to Winnipeg - the closest major city along the path of totality. We found a spot in the parking lot of the Winnipeg Zoo and climbed to the top of a snow pile in order to get a good view. We had a very clear, blue sky with just a few high thin clouds. Great! At about quarter to eleven AM (local time) the Moon started to move in front of the Sun and we watched the image projected onto a screen (using a technique involving a pinhole through a box). As totality approached exciting things happened in quick succession. The event I recall most clearly was how a huge shadow approached from the western horizon at the speed of a locomotive! It was so "unreal". The horizon seemed to wiggle. I later learned that this effect is called shadow bands and is probably due to the way light is refracted along the edge of this quickly cooling "cold front" caused by the umbra. Suddenly we were in darkness. All the stars came out and the critters in the zoo behind us went dead quiet. On the other hand, all the people in the parking lot went nuts! It was the astronomical equivalent of a rock concert. The temperature plummeted (from cold to very cold). I enjoyed the view, wasted some film and it was all over far too soon. Totality lasted a little over a minute and a half. This time a bright curtain of light hurtled towards us from the west and suddenly we were in the daylight. Wow!

On March 9th 1998 the next cycle (#56) of that series (120) occurred but that eclipse was only visible in northeast Asia (mostly Siberia) so I missed it. But there is usually a good eclipse every year or two. The trick is to be there. In spite of the brevity, a total eclipse is an exciting event and I encourage folks to make the effort to be in the path of totality. If you are off the path you really don't see much of interest.

Hmm, can you have two eclipses from a single Saros cycle? After all, there are two nodes.

Good question.

There are 27.2 days in a Draconic month (652.8 hours) so half a Draconian month is 326.4 hours or 13.6 days. That's when we would expect the Moon to reach the other node. But by that time there cannot be the same phase of the Moon so you have to think about the "opposite" eclipse - a lunar eclipse! You don't always get a pair (lunar and solar) of eclipses each time but it does happen pretty frequently.

However, we must have a Full Moon for a lunar eclipse and that is half a synodic month away from the New Moon that was needed for the solar eclipse. A synodic month is 708 hours so half of that is 354 hours or 14.75 hours.

So which time is the one to use? 13.6 days or 14.75 days?

That's another good question.

The answer lies in the details of the geometry but it works out to about 14 days. [Nice compromise, isn't it? It's about halfway between the two times.] Sometimes the geometry isn't quite right for a "double eclipse" but we had just such an opportunity with the Saros series #145 cycle 21. (The one Christine and I saw in Cornwall.) A partial lunar eclipse occurred before that solar eclipse.
[Important point - technically this lunar eclipse was part of Saros series #119 because there is an interval of 18 years and 11.3 days between cycles in each series. So I just lied! Here I'm trying to make a point that the geometry was doubly good for this double event. However, a professional astronomy would object to me calling this lunar eclipse part of the Saros series 145. I hope you understand.]
On July 28th 1999 at 11:36 UT the Earth's shadow covered about 40% of the Moon. This was only a partial lunar eclipse but it was still worth a look.

Hey, 11:36 UT was the middle of the day! You cannot see a Full Moon in the middle of the day!

You're right. (Very good!) This lunar eclipse could not be seen by folks along the UT line (in Great Britain, for example). Indeed, no one in Europe saw it. However, folks on the other side of the planet had an opportunity to watch the Earth's shadow cover part of the Moon. Unfortunately for most folks, this lunar eclipse occurred at a most unfortunate time (and place). The eclipse began at 10:22 UT and that was early in the morning for folks on the east coast of North America. They got only a teasing glimpse as the eclipse started just before the Moon set and as the Sun roses. [By know you should understand that a Full Moon sets around sunrise because of the geometry of the situation.] On the other hand, people living on the west coast saw the eclipse start at 3:22 Pacific Daylight Time (PDT) [that's 10:22 UT] so it was worth while for them to get up early to see it. (Or to stay up late waiting for it. Hey, we're talking about Californians here! ) By 4:36 PDT [that's 11:36 UT] those west-coasters saw 40% of the Moon hidden in the Earth's shadow and I suspect there was some nice coloring of the Moon. By 5:46 PDT [that's 12:46 UT] the show was over as the last whisper of Earth-shadow left the Moon. If you are were in the Pacific Ocean (Hawaii, Japan, New Zealand, Australia, etc.) the local times could have allowed you to see this lunar eclipse at a more comfortable hour (late at night instead of early morning).

This lesson has a lot of detail you could probably do without! I went into those details to exercise your mind. I hope you have imagined how the shadow of the Moon traveled across the surface of the Earth, changing its speed and size as it ran along.

Each eclipse has timed events all of its own. However, I think you should be able to understand how an eclipse occurs (both solar and lunar) and appreciate that the complexity of predicting them is not all that complicated, except in the details, and not at all mysterious. On the other hand, it takes a lot of time for even an expert to accurately make these predictions. It's beyond me!

Hey, how come you looked at the Sun during the eclipses?

Excellent question.

If you stare at the Sun's photosphere, YOU WILL GO BLIND!
The Sun's corona will not harm your eyes. It is ONLY safe to stare at the Sun's CORONA. The "trick" is to look at the corona without looking at the photosphere. That can ONLY be done SAFELY during TOTALITY. It is NOT SAFE to watch a partial eclipse (without eye protection) because the photosphere is still visible. (Even just a small amount of photosphere light will blind you.) Similarly, it is NOT SAFE to watch the Sun entering or leaving totality UNTIL the photosphere is covered.

If you are planning on watching a solar eclipse event, contact your local university for advice on how best to watch it and where to buy safe eye protection.




This work was created by Dr Jamie Love and Creative Commons Licence licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.