Late sunrise
Published on Dec 22, 2009 at 2:10 pm.
6 Comments.
Filed under astronomy.
Yesterday, December 21, was the Winter Solstice here in the northern hemisphere. On that day, the Earth was in the part of its orbit where its north pole was tilted as far away from the Sun as it will get this year. The Sun appeared as far south in the sky as it will. That means that those of us in the northern hemisphere had out shortest day of the year. Where I live, in Texas, the Sun rose at 07:28 and set at 17:27 (Central Standard Time). That make the day just under ten hours long. The night, of course, was about fourteen hours long. After the equinox, the days begin to get longer. Granted, today we will get only seconds longer sunlight, but eventually that will add up. By the time that the Summer Solstice arrives in June, we will be getting about fourteen hours of daylight and ten hours of night (the reverse of the Winter Solstice). Of course, with the north pole tilted away from the Sun, the south pole it tilted towards the Sun. That means that the southern hemisphere is having their longest days and shortest nights right now, just like we will at the end of June.
Here in the United States, we declare that winter starts at the Winter Solstice and ends at the Vernal Equinox. Autumn starts at the autumnal equinox and ends at the winter solstice. I’ve written about the seasons before, and I think that this is perhaps not the best way to declare the seasons, but I suppose that it works well enough. Given that the solstices and equinoxes are well defined points in time, they make convenient dates to mark on the calendar. That gives the television weather people something to talk about. Really, of course, the weather is not significantly different on average on December 20 from what it is on December 22. But, as I said, these are convenient dates to mark. But, there is one point that they often get wrong. Almost every weather forecast on the Winter Solstice says that the solstice marks the shortest day of the year. This year, that was actually true, because the solstice occurred at 17:45 UT (that is 11:45 AM Central Standard Time). Sometimes, though, when the solstice occurs during the night, early morning, or late afternoon, it is actually the calendar day before or the day after the date of solstice that is actually the shortest day. The calendar date of the solstice is the shortest if the actual moment of the solstice is near the middle of the day. If you want to determine how long the day is at your location, the US Naval Observatory has a really nice online utility to give sunrise and sunset data for one day at a time or even for one year. If you look at the data using that utility, one finds that for where I live, the Sun rose on December 21 at 7:28 AM and set at 5:27 PM. But, you find the same sunrise and sunset times for today, December 22. But, at this time of year, the rate at which the length of daylight changes is very slow. In fact, the slowest changes in the length of daylight occur near the solstices, and the fastest changes in the length of daylight occur near the equinoxes.
Frequently, when the weather reporters are talking about the solstices, they make a mistake. Almost every year I hear one of them saying that the solstice marks the shortest day of the year and the latest sunrise. Hey, that would just seem to make sense, right? After all, if it is the shortest day of the year, that would seam mean that the sun rises latest and sets earliest. Unfortunately, that is not correct. If you look at the sunrise/sunset utility that I mentioned before, you’ll find that the sun rises tomorrow at 7:29 AM and sets at 5:28 PM. The day is still just about as long, but the sun is rising later than today or the day of the solstice! In fact, you find that time of sunrise slips a bit until it is rising at 7:33 AM on January 7. Sunset on January 7 occurs at 5:38 PM, so that means that the day is longer on January 7 than it is now, but the sun still rises later. What gives?
The answer to this mystery resides in the motion of the Earth around the Sun.
In the diagram above, the Earth is rotating as it moves around the Sun. Imagine the Sun at some point well below the bottom of the computer screen. The Earth moves a little less than one degree around its orbit each day. That means that it must rotate a little more than one degree in order for a point on the Earth facing the Sun to face the Sun once again. This is the difference between what we call the sidereal day (the time that it takes to make one complete rotation) and the synodic day (the time that it takes to go from the Sun highest in the sky until the Sun is again at its highest in the sky). The sidereal day is just over 23 hours 56 minutes long. The synodic day is closer to 24 hours. So, the Earth must rotate almost 361 degrees in order for one solar day to occur. The stars, on the other hand, rise every 23 hours 56 minutes. That means that the stars will appear to rise and set about 4 minutes earlier every day.
But, it gets a little more complicated than that. Earth’s orbit is a little bit elliptical. Earth was farthest from the Sun on July 4 of this year (2009), and it will be closest to the Sun January 3, 2010. Note that it is the tilt of the Earth’s axis, not its distance from the Sun that causes the seasons! But, as a planet moves around the Sun in an elliptical orbit, it speeds up and slows down. The planet moves quickest at perihelion (closest to the Sun) and slowest at aphelion (farthest from the Sun). So, Earth is currently moving a bit faster in its orbit than average. That means, however, that in order to complete one synodic day, or solar day (a point facing the Sun to once again face the Sun), the Earth will have to turn a little more near perihelion than it would near aphelion. That makes the days longer. Indeed, the length of the synodic day changes over the year, being longest near perihelion and shortest near aphelion. The synodic day (solar day) can range from about 23 hours 59 minutes 38 seconds to about 24 hours 29 seconds. That isn’t much difference, but it adds up. It would be a bit confusing if the clocks had to run at different speeds at different times of the year. Thus, clocks run at a constant rate set by the average length of the solar day over the year: 24 hours. The actual day isn’t far off of that, but with a difference of nearly half a minute per day, the discrepancy adds up. After a couple days, the clock will be a minute off. That is one reason that the sun doesn’t always appear highest in the sky (local noon) at the same time of day every day. Clock time and solar time are off a bit. To correct between solar time and clock time, you need to know the equation of time, which will tell you how much to add or subtract from a sundial’s time to find the clock time. With the solar (synodic) days out of sync with clock time, then solar events like sunrise, sunset, solar noon, etc, tend to be out of sync with the clock. They slip a little each day at this time of year. That is the reason that the latest sunrise does not occur at the Winter Solstice. If the Earth had a perfectly circular orbit, then the latest sunrise and the solstice would occur at the same time, but Earth’s elliptical orbit skews things a bit.
-Astroprof
Christmas Star Parties
Published on Dec 20, 2009 at 1:25 pm.
4 Comments.
Filed under amateur astronomy.
I do several public star parties each year. Overall, it probably amounts to a little more than one every two months. Typically, I give a public talk about some topic in astronomy, and then we do viewing through telescopes that I bring. Usually, there are a few amateur astronomers who come and set up their telescopes, too. Once in a while I schedule star parties in December. Over the years, I’ve done several December star parties. Frequently, given the holiday season, people ask me about the Star of Bethlehem. So, I worked up a Christmas presentation a few years ago about some possible celestial events that could have been the Star. I make it clear that this is all speculation. I don’t know for sure what was seen at that time, but it is fun to think about any actual celestial events that were occurring in the sky about the time of Christ’s birth. I’ve given this presentation several times in the last few years. This year, I gave it twice, last night and the week before, at different state parks in the area. Attendance was very light. In fact, the park rangers outnumbered the park guests!
In years past, my Christmas presentation has sometimes been well attended, but most of the time the attendance is light. Those who do attend it seem to be pleased, so I guess that I’ll continue doing it. I don’t have to have a huge crowd for me to think that it was a successful event.
But, for anyone else planning star parties this time of year, don’t be surprised if you don’t get a huge attendance. There are a lot of competing factors. For one thing, this was the weekend before Christmas. For me, I would think that it would be the ideal time for this program, but I tend to be a bit out of sync with a lot of people. Most other people are pretty busy with last minute Christmas shopping. I have found star parties the weekend before Christmas to frequently be very poorly attended. Adding to the timing, it was clear and cold. For astronomers, that is perfect weather! But, for the public, cold is a deterrent. Most people don’t know how to dress for the cold. Another factor that bit into our attendance is that last night the Dallas Cowboys played an important football game. They are a big deal locally, so attendance at anything tends to drop when they are playing a game, and attendance drops a lot when they play a big game.
But, it doesn’t matter to me if there were a lot of people or not. I enjoyed getting out of town and seeing a dark sky for myself. And, the rangers seemed to find my presentation interesting. As long as someone got something from the event, then it was worthwhile.
Here’s a big “Merry Christmas!” to everyone from Astroprof’s Page!
-Astroprof
LHC sets record, and we are still here.
Published on Nov 30, 2009 at 3:27 pm.
2 Comments.
Filed under physics.
According to a report on Science Daily, the Large Hadron Collider has set a new energy record. The twin beams were at an energy of 1.18 TeV, beating the 0.98 TeV energy of Fermilab’s Tevatron. That now makes the LHC the worlds most energetic particle collider. Despite all of the cries to the contrary, we are still here after this milestone. However, don’t expect all of the people crying that it is the end of the world to quiet down just yet. Next year, the LHC will work its way up to an energy of 3.5 TeV per beam, for a total collision energy of 7.0 TeV. Until it reaches that milestone, then the end-of-the-worlders will simply be telling us that the end is postponed. Then, once 7 TeV collisions become common, they’ll come up with some other explanation for why we are still here. The vast majority of physicists don’t have a problem with this collider. In fact, there are collisions at FAR higher energies than this occuring all of the time in the air over our heads. Galactic cosmic rays rain down on us at far higher energies than the LHC will ever be able to produce. In fact, it is not unheard of for galactic cosmic rays to have energies in excess of 100000000 TeV. Granted, the average cosmic ray energy is far lower than that, but the very high energy ones still occur, and they have done so for billions of years. None have destroyed Earth yet, so I don’t see any compelling reason to expect the LHC to do so, either.
Part of what makes this particle collider, and high energy physics in general, so confusing to the general public is the terminology. For one thing, the beam energy is often given in terms of electron volts (eV). An electron volt is the energy that it takes to move a charged particle having charge e (the fundamental charge, 1.602×10-19 coulombs) through a potential of one volt. That turns out to be equal to 1.602×10-19 Joules, a very tiny bit of energy. But, we are talking TeV here. What is that? TeV stands for tera electron volt. That is one trillion (1012) electron volts. That’s got to be a lot, right? Well for a subatomic particle, one TeV really is a lot. But, to give you the sort of idea of what kind of energy this is, imagine figuring out the amount of energy that it takes to lift a penny. If you lift the penny a distance of 0.045 mm (about 1/100 of an inch), then you have used about 7 TeV of energy.
It is hard to imagine how energy so small could scare anyone. Granted, some are worried about possibly the energy being confined to a small enough volume to create a miniature black hole. However, black holes that small would not be stable, so there would be little to worry about. I am not going to go into all of the reasons why such things are nothing to worry about. That’s been done before by many other writers on the internet. Black holes themselves are grossly misunderstood, and are not nearly as scary as science fiction writer portray them. Indeed, if miniature black holes really were a hazard to us, then we’d be doomed long before now due to the collisions from much higher energy cosmic rays. So, I am not worried that the LHC is going to destroy the world when it reaches full power.
-Astroprof
Image Credit: CERN
CoS 128
Published on Nov 11, 2009 at 12:13 pm.
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Filed under blogging.
I’ve been busy catching up on my classes, so I haven’t posted anything lately. But, you can catch up on lots of great space-related blog posts at the 128th Carnival of Space, being hosted this week at the AARTScope Blog.
-Astroprof
Ares I-X Launch Photos
Published on Nov 4, 2009 at 12:21 pm.
3 Comments.
Filed under NASA, rockets.
It’s been a week since the Ares I-X launch, so you are probably wondering where the pictures are. After the launch, I tried to clean up as best I could in the bathroom at the space center, and I headed straight to the airport to catch my plane back to DFW. Upon returning, I have been busy trying to play catchup. I needed to rescale the images so that they would not be too large. But finally, they are here! Click on the pictures for a larger view. Don’t rag on me that the pictures are not as good as the press photographers get. They’ve got more expensive camera equipment and a lot more experience than I do!
The first picture, above, is of the rocket sitting on the launch pad on Monday night. Below is a picture of me standing next to the countdown clock. (Hey, I had to get a tourist-type photo, OK?).
But, alas, the countdown only proceeded to T minus 4 minutes. It then held. On Tuesday, they said that they were just a bit behind on tasks. However, the weather played a factor, then a ship got in the way, and then more weather was a problem. Future flights will not have the same constraint on triboelectrification that this flight had. Part of the issue was that this was a developmental flight, so there were a lot of sensors on board looking at just how much triboelectrification this rocket will have anyway, so they needed pretty tight weather constraints. So, the next day, we were back at the space center at 5:30am again, and the countdown ticked down to T minus 4 minutes again. The picture above was during that second day (I got one of me on the first day, too). The initial problem on the second day was checking out the rocket after lightning strikes in the vicinity overnight. Everything checked out, but the weather was still not good. The following picture was what we kept seeing, for nearly seven hours over the two days.
Eventually, though, the weather was within tolerances for a few minutes. Fortunately, it was just long enough of a window for the rocket to get off the ground before the weather deteriorated. I have a lot of pictures of that, but the next three images show the rocket right after engine ignition, shortly after clearing the pad structures, and in flight.
It was quite impressive. I realize that there is some controversy over the Ares rocket design, but I am glad that I got to be there for the launch. I would very much like to see other launches. I’ll be investigating some more about the Ares rockets, including some of the criticisms, and I’ll be writing more about it later. Whether the Ares project goes forward or not, the data collected on this launch may be useful for engineers designing future rockets. Already, we know that there was a problem with the parachutes on the rocket. Finding what doesn’t work right, of course, is one of the reasons for test flights. This rocket was not the final Ares rocket, but it should provide useful data for those engineers building that rocket. Think of this as a huge wind tunnel test and a test of the recovery system.
I feel quite privileged to have asked to see the launch. No matter what happens to the Ares project, there will only be one first launch of the configuration (granted, the Ares I-Y launch in several years will more closely match the final configuration, but this is a rocket of about the same size and shape), and I got to see it! I would very much like to see some of the other launches, too. I would particularly like to be present at some of the launches of the commercial rockets that are being developed. This is a very exciting time that we are living in. Decades ago, the only people launching rockets were government space agencies and military of those governments. Now, there are a number of private companies that have gotten into the space launch business. This is very important. Aviation did not really take off as a major business, with all of the benefits that it provides, until private industry began to become heavily involved. I can envision a future in which private companies are almost continually launching rockets carrying satellites, lifting people to space stations, supplying those space stations, and perhaps even sending missions to the Moon and beyond. We are not there, yet. And, there is still a role for the government (NASA) in space exploration. But, these are exciting times.
-Astroprof
(Images of this post copyright Astroprof’s Page)
Triboelectrification trouble
Published on Oct 27, 2009 at 2:40 pm.
4 Comments.
Filed under rockets.
Well, the launch did not go as hoped for this morning. Last night, the forecast was for only about a 40% chance of the launch getting off. This morning, when I got up to head over to the space center, I saw stars! Things were definitely looking good! Then, as we headed to the launch site, I saw some clouds. Weather was moving in. Still, it looked like we’d get the thing off if everything went according to schedule. Well, it didn’t.
First of all, they were running a bit late. The announcer said that there were no particular problems. However, running late delayed the launch until clouds arrived. Weather aircraft were studying the clouds and balloons took a look at higher levels. Then, there was a delay due to high altitude winds. Next, a cover over a sensor near the top of the rocket got stuck. Finally, that came free. Then, the aircraft found the clouds just a bit too much. Finally, the weather cooperated. The winds were OK, there was a break in the clouds. The countdown started again. Suddenly, with about two and a half minutes to launch, the countdown stopped. The call was that a freighter had entered the exclusion zone offshore! (I may have my delays confused. I wrote this off of memory. See Ed’s comment.) At first, they were saying that it would take 90 minutes to clear the zone. It took far less time than that, but they had to reset the countdown. By the time they were ready to try again, the clouds had come back. Then, there was a window in the clouds but it was too windy. Finally, the wind died down, but the clouds were back. Finally, they scrubbed the launc for the day. We’ll go back in the morning to try again. I would definitely like to see the launch, but it appears that trying to change travel plans to stay another day would push the cost of the trip far beyond my travel allowance.
So, what’s the deal with the clouds? Naturally, those of us observing would like to have few clouds so that we could see the rocket arch across the sky. But, the bigger problem is something called triboelectrification. That’s a really fancy word that comes from Greek roots meaning electrification from rubbing. You may be familiar with it as “static electricity.” In fact, that is more like what they would have said years ago. Of course, it isn’t really static, since the rocket is moving, and it is the motion through the clouds that causes the problem.
What happens is that the rocket, as it passes through the clouds, pushes water droplets out of the way. The interaction between the rocket and the droplets results in charge being transfered between the two, leaving one positive and the other negative. It is like rubbing your feet along the carpet on dry days. You become electrically charged. I remember always being told that it was friction doing the work to build the static charge. But, as I understand it, chemical reactions, rather than friction, do the dirty work. The rubbing past one another simply exposes more surface area to the action.
The end result, of course, is that the rocket becomes eletrically charged, and so does a tube of vapor along the path of the rocket. That electric charge can interfere with signals between the rocket and the ground. For an unmanned test flight like this, where the whole idea of the flight is to provide flight data for analysis, then interfering with telemetry is bad. But, the interference also goes the other way. It can interfere with signals from the ground to the rocket. If the rocket were to go out of control, then the range safety officer has to be able to send a signal to destroy the rocket. So, if there are too thick of clouds over the launch pad, then the launch doesn’t happen. That was the problem most of this morning.
I got some good photos of the rocket on the pad, but my netbook lacks the tools to resize the to upload, so you’ll have to wait until I get home. Hopefully, I’ll have photos of the launch tomorrow!
Astroprof
Ares I-X
Published on Oct 25, 2009 at 11:07 am.
6 Comments.
Filed under rockets.
If everything works out OK, then NASA will launch a new rocket this coming Tuesday: the Ares I-X. And, if everything works right, I’ll be there to witness it!
The Ares rockets and Orion crew module are supposed to eventually replace the Space Shuttle in NASA’s inventory of craft to ferry astronauts to and from space. The Ares has been the target of quite a lot of criticism, too. There are calls for the project to be scrapped. However, most of those calling for scrapping the Ares project are outside of the space community, and almost all are outside of the field of aviation and rocketry. The most common complaint is, “Why can’t we just use one of the other big rockets that already exist?” Well, the reason is that those rockets are not designed for manned missions. In order to carry humans, rockets and aircraft have to go through a rigorous test procedure, and each and every part has to be separately certified as compliant with those tests. Going back and doing that for an existing rocket would be at least as expensive as building a new one from already certified parts. That is what the Ares is: a rocket built mostly from already certified parts. So, I am not convinced that it would really save money to scrap Ares, and it might even cost more money and more time in the end. Already, we are facing a period of time in which NASA will have no vehicle capable of getting astronauts to and from space. We’ll have to either purchase flights from other nations (Russia) or hope that private companies come up with a space taxi of some sort to get to and from the space station. That is why Ares is important. I only wish that the Constellation Program (the overall program that includes Ares, Orion, the Altair lunar lander, and more) had more money and resources to develop the program faster.
Two Ares rockets are planned. The smaller rocket (and even it is huge!) is the Ares I. The Ares V rocket will be taller, but also much wider. If built, it will be the largest operational rocket ever constructed. The Ares I will stand over 300 feet high. The Ares 1-X currently sitting at Launch Pad 39B at the Kennedy Space Center is 327 feet high. That makes it the tallest rocket launched from the Cape since the early 1970s, when the last Saturn V rocket launched. Incidentally, the last Saturn V rocket was launched in May 1973 from Launch Pad 39A. Over a week later, a Saturn I-B rocket lifted off from Launch Pad 39B with a crew of three astronauts to man the space station. I think that may have been the last time that two different rockets sat at the launch pads at Launch Complex 39 (Kennedy Space Center). Now, over 36 years later, two different rockets are sitting at launch pads at Launch Complex 39 once again: Ares I-X at Launch Pad 39B and the Space Shuttle Atlantis on Launch Pad 39A for the STS-129 mission. The following photograph catches this historic moment. Click on the photo to get a larger version.
The Ares I-X is the first test flight of the Ares configuration. The first stage of the Ares uses the same type of solid rocket motors that the Space Shuttle uses for its Solid Rocket Boosters. The Space Shuttle uses a stack of 4 solid rocket segments. The Ares uses a stack of 5 solid rocket segments. The second stage uses a J-2X engine, derived from the famous J-2 engines used on the upper stages of the Saturn V rockets. The body of the upper stage is derived from the Space Shuttle external tank. This is a new stage, and though it is derived from existing technology, there are modifications that need to be make, so it is not simply a matter of sticking an engine onto the end of a shuttle external tank. Atop the second stage would be the completely new components: the Orion Crew Exploration Vehicle and its service module and the new Launch Abort System, designed to pull the crew capsule away from the rocket in the event of a catastrophic failure of the rocket. These upper stages are not yet ready for flight.
So, why is the rocket flying on Tuesday if the upper stages are not ready? This first developmental flight of the Ares stack is primarily a test of the first stage and the design of the stack. Remember, the first stage is derived from the shuttle’s solid rocket boosters. These rockets are strapped onto the side of the shuttle’s external tanks. They were not originally designed to fly alone. So, one of the goals of this rocket flight is to test the solid rocket first stage of the stack. Also, the shuttle’s solid rockets only have a nose cone on them. The first stage of the Ares has another rocket on top of it (the second stage) and even more hardware on top of that! So, there is a very real engineering concern here. There should be no serious problem. The rocket should be easy to control, and the rocket segments should have the strength to hold all of this extra weight. Remember, the shuttle’s rockets have helped lift the much heavier Space Shuttle into orbit, and that was strapped to the side of the rockets (an even tougher problem from the point of view of engineering). Still, the rockets have not flown in this configuration before, and so there is always the potential for unforeseen problems. That is why we need this test.
For the Ares I-X flight, the first stage will be a bit scaled down from the full five segment first stage of later Ares flights. This will be essentially a four stage rocket that is a modified shuttle solid rocket booster with an inert fifth segment. The upper stages will consist of dummy stages (though there will be active guidance and thrusters on the dummy second stage). The rocket will take off, reach an altitude of about 130,000 feet and the second stage will deploy as if on an actual orbital mission. Since the second stage rocket will not fire, the stage will then fall into the Atlantic Ocean. After stage separation, momentum will carry the rocket about another 20,000 feet higher before it falls back to Earth. Parachutes will deploy, and the first stage will splash down into the Atlantic Ocean, where a surface ship will retrieve it. There will be sensors on the rocket recording every facet of the rocket operation. The Ares developmental flights will have far more sensors than will fly on the operational missions. Data collected on the flight (upper stage sensors as well as first stage sensors) will then be studied over the next couple of years. With any new engineering design, even a modification of an existing design, there will almost always be something unexpected to turn up. Having the first operational test so early in the project (the full unmanned test stack won’t likely fly for at least five years) allows engineers to study the data from the launch in order to modify the upper stages as needed before they are finished being built. That is the smart way of doing things: test out each part as it is ready. The next Ares launch, the Ares I-Y, is expected in four years, around November of 2013. The Ares I-Y will test the high altitude emergency Launch Abort System. In the mean time, there will be separate tests of the upper components, just not on an Ares stack. A full blown Ares/Orion launch, called the Orion 1 mission, may launch sometime early 2014. That would be the first test including a fully functioning (but unmanned) Orion capsule atop the stack.
Though there will be only a four segment first stage in the Ares I-X flight, the experience for those of us watching the launch should be the same as for a full five segment first stage. The biggest difference will be in how far and high the vehicle flies. I am hoping that everything goes well. I will be leaving for the Cape in a few hours. The rocket has passed its test review, and everything seems go for launch so far from the engineering aspect. Weather, though, may be a problem. The forecast is for clouds. Since this is a test flight, they must have clear weather to observe all aspects of the mission. However, they only need about 15 minutes of clear weather, so there is a decent change of getting that. Yesterday, the mission team estimated Tuesday’s weather as only 40% go. However, Tuesday is still several days away, and things might look up. As I look at the weather, it looks like it may be a bit better, perhaps 50%, but I am not the one making the call. Still, I’ll be there Tuesday and Wednesday to see the launch.
-Astroprof
Images courtesy NASA
