27 Jul 2008
Wow. August is a big month for China. Not only is the enormous country hosting the biggest sporting event in the world, but it is also putting on one of the most spectacular sky events, as well - a total solar eclipse.
On August 1/2, the Moon's shadow, like an Olympic anti-torch, will begin a run starting actually in northernmost Canada, moving across Russia, into Mongolia, finishing up in China.
Those few, those lucky few, on the swath will see the sun completely covered by the Moon as it passes in front.
But why so few? Why not everyone? Why that day and not the next? Why not the next month?
The size of the Moon and its distance from Earth account for the limited viewing problem. The moon's shadow is actually a huge cone stretching for over 200,000 miles. That happens to be the distance from the moon to our planet. Meaning, by the time the shadow reaches us it is a small shadow of its former self, sometimes just tens of miles across.
If one happens to be directly in it - ta da! - total solar eclipse. If not, out of luck. Which is why the total solar eclipse is such an elitist event. Just the privileged among us who happen to live at, or make a trip to the streak of shadow blazing across the planet get to see it.
It's not like its counterpart, that lunar eclipse thing, which more than half the planet gets to experience merely by looking up when it happens. Moreover, our shadow on the moon is a hefty one and it takes hours for our satellite to travel through it. A lunar eclipse has a huge viewing audience.
The solar eclipse happens specifically next week because, of course, it is the time in the moon's orbit when it crosses in front of the sun. But it won't happen the next time it passes in front, nor the next time, nor the next time. Why?
The moon's orbit is a little off-kilter. It isn't in the same plane as our orbit around the sun so it doesn't always pass directly in front of the sun. Most of the time it is either a little higher, in which case the shadow misses us over the north pole of our planet, or below, in which case the shadow goes way south. On this present trip, it manages to slash earth across the northern hemisphere.
You may have noticed that these eclipses, both solar and lunar, happen at regular intervals, about every six months or so during "eclipse seasons." Our last eclipse set - lunar/solar - was in February, this one is in August, and the next is next February. Time to put on the Thinking Cap.
These "seasons" are when the moon's path takes it through the imaginary plane of earth's orbit around the sun and, at the same time, all three of us happened to be lined up nicely. Because of the Moon's awkward orbit this only happens about twice a year. It is then that we either block the sun's light to the moon or the Moon pushes itself between us and the sun, an event we will witness next week.
In a couple weeks, when the Moon moves around to the other side of the earth, we will have a lunar eclipse, just as expected. But, sadly, the whole other side of the planet gets to see this one - not we. This is a lose-lose eclipse month for us.
In fact, watching the solar eclipse next week for us in the States will be like watching the Olympics. We will need to see it via satellite, or internet. It will occur during the wee hours of the morning here. If you want to experience it live there will be several sites, including San Francisco's Exploratorium, that will carry live webcasts. Check them out.
We won't get to see a total solar here live and in color until one strikes the Great Northwest in 2017. Make plans now.
Until next time, clear skies!
13 Jul 2008
Let's go planet hunting! There are several planets out now and visible to the naked eye. Probably only one is worth getting out the scope for, namely that überbright guy climbing higher in the southeastern skies lately. But we'll get to him in a moment.
First under the glass is a pair of wonders, Mars and Saturn. They are setting in the western skies together after sunset. In fact, at this very moment they are less than two degrees away from each other in the constellation Leo, near the bright star Regulus. If you can, catch them in the darkening dusk, around 8:30, due west, about 25 degrees up.
But that apparent closeness is just an illusion; they are considerably more than just a breath away from each other. Really they are separated by more than 724 million spacious miles, nearly eight times the distance we are from the sun.
What's kind of interesting trivia here is this: Although we see the two next to each other, we are actually seeing Mars earlier than we see Saturn. We aren't seeing them at the same "time." How's that?
Remember that light is finite in speed. It travels at a ripping 186,282 miles a second, but it is not infinitely fast. Using that as a reference, Mars is 18 "light minutes" away from us - it took that long for its reflected light to get to our eyes. But Saturn is another 730 million miles farther out from Mars, more than a 'light hour" more distant.
Bottom line: If a huge comet smacked into both Mars and Saturn at exactly the same time, we would see the explosion on Saturn a full hour after the one we see on Mars. Crazy things can happen in space.
Speaking of giant rocks hitting planets, it is this week that we celebrate the momentous occasion in 1994 when Comet Shoemaker-Levy 9 (SL9) smacked into our other planetary star of the month, Jupiter.
It was back in March 1993 when Carolyn and Gene Shoemaker together with David Levy discovered a kooky comet with the 0.4-meter Schmidt telescope on our own Mt Palomar. It was kooky because it wasn't going around the sun, it was going around Jupiter!
Apparently our Big Brother with its scary big gravity had snatched this comet right out of its normal solar orbit years earlier and claimed it as its own.
Fine with us Earthlings! Jupiter has no doubt been stealing things out of the sky since the beginning, protecting us from incoming - and devastating - invaders bent on terrestrial destruction.
To make matters worse for the little comet, not only had Jupiter trapped it, but it had more recently chewed it up, as well. Coming too close to giant Jupiter, a small, weakly-held-together body like a comet can get broken up by Jupiter's gravity. SL9 got torn up into a couple dozen icy pieces.
But Jupiter wasn't content on merely trapping and then tearing apart the little guy. As a final insult, Jupiter ate it.
That's what happened in July 1994, fourteen years ago this week, starting on the 16th, when the orbit of the star-crossed, broken-up little comet took it too close to Jupiter, and its remnants slammed into the big guy over a period of several days.
Bam, bam, bam - one nasty impact after another struck hard, all of which released probably 1000 times the energy of all the nuclear weapons we have on Earth. It was an exciting time, a great week in astronomy.
Jupiter has more than recovered from what was nothing more than a bruise on its massive atmosphere. Take a look up there during the next couple months with a nice backyard telescope and you will see nothing but the parallel weather patterns, the Galilean satellites, and if you're lucky, the great Red Spot.
But the remnants of the only substantial solar system collision we have ever seen will be gone.
Go out this week if you can and catch a glimpse of all three of the most popular planets out there.
Until next time, clear skies!