Saturday, February 12, 2005

NASA's returned to manned spaceflight; FUSE's kluge

Fox News is reporting that the space shuttle Discovery will go into space in May. It will carry several tile patch kits for in-flight repairs, if necessary.

I've been waiting for this for some time, now. Very happy to see us back in space. Now we just need to get Servicing Mission 4 up there to work on the Hubble Space Telescope... The Space Telescope Science Institute, which operates the Hubble, says we may have to go to "2-gyro mode" this year, since the gyroscopes are showing mechanical problems. You need three gyros to keep a spacecraft stabilized. Two-gyro mode won't be a total loss, but it will create some "jitter" in one direction, slightly blurring the images along that axis. It will also restrict the directions in which the Hubble can point, which means some targets might become unviewable.

At some point, I'll have to describe the amazing kluge put together by the FUSE observatory team, when their space telescope's guidance system lost control along one axis. OK, but briefly: they used the fact that the FUSE spacecraft is orbiting through the Earth's magnetic field, and FUSE is running an electrical current.'s a gigantic electric motor! They use the "torquer bars"--electromagnets in the spacecraft made to dump excess angular momentum--and run a certain current through them, creating a magnetic field for the spacecraft and letting it push off of the Earth's magnetic field. (This is a crude description of how forces work in electromagnetism, but bear with me.) One of the project scientists told me (after I asked), that they had not even planned on this before launch. It was an impromptu fix after one of the "reaction wheels" failed. If they had thought of it beforehand, he said, they'd have made the torquer bars bigger! And they continuously have to update the current running through the torquer bars as the Earth's magnetic field changes. It's a rapid change at that altitude (near Earth orbit)--noticeable from one orbit to another.

Now FUSE has had a second reaction wheel fail. They're down to just one left. As of last month, my colleague told me that they were meeting to figure out how to update the torquer bar solution. Let's hope it works!

Astronomer's night out

Up 'til the wee hours this week, trying to finish up a research paper I've been working on for three years. It's time to publish when you're sick of looking at the thing! It's amazing how long it can take to pretty-up the figures. Diagrams. That kind of figure. And I've learned never to do a "sanity check" on your mathematical results if you're afraid of what you might find...

On the other hand, we've had two straight nights of beautifully clear skies. I've gotten nice binocular views of the comet out, and as soon as my camera mounting plate comes in the mail, I can try some astrophotography with my new tripod and clock drive. Very eager to try this out!

Friday, February 11, 2005

CNN's Eason Jordan resigns

Bill, over at INDC Journal, is reporting that CNN's chief news executive, Eason Jordan, is resigning. (Full story on the AP Wire.) This, after he claimed at the World Economic Forum that the US military was targeting journalists in Iraq.

I'm actually not cheerleading the effort to have him fired. I'd prefer to have him publicly retract his comments (very publicly, which means somewhere other than on CNN) and explain why he made them in the first place. Either back up the claim, or take it back. Fully, honestly, and humbly. And apologize, too.

I've got in mind something like John Cleese's apology to Kevin Klein while being dangled out the window in A Fish Called Wanda:

"I'm really really sorry, I apologize unreservedly. [...] I do, I offer a complete and utter retraction. The imputation was totally without basis in fact, and was in no way fair comment, and was motivated purely by malice, and I deeply regret any distress that my comments may have caused you, or your family, and I hereby undertake not to repeat any such slander at any time in the future."

Again, I don't want the guy fired, but I want something along these lines. Heck, I don't want Ward Churchill fired! He's a jerk, and I would doubt the educational value of his classes, but speaking anti-American idiocy, in class or out, is and should not be a firing offense. The quality or veracity of his scholarship would be another thing, of course, but if you're going to fire him, do it for the right reasons.

Monday, February 07, 2005

A Culturally Significant Date

This year, 2005, Good Friday falls on March 25, during a full moon. According to tradition, Jesus died during a full moon, on Friday the 14th of Abib. Since Abib is a lunar month, it doesn't regularly correspond to the same date on the Roman calendar every year. During the days of the Roman Empire, this was a source of inconvenience, since the Romans used a solar calendar, and their Greek subjects used the Metonic calendar. So they had a convention by which they would translate Roman annual celebrations to the Greek calendar. Using that convention, April 6th would get translated to the 14th of Abib.

Of course, in only about 1 out of 19 years would April 6th and Abib 14th actually occur on the same day, but as I said, this was a convention. It was used for convenience, not exactness. So in the Christian east, the date of Jesus' death was assigned by convention to April 6.

But in the west, in the 3rd century, Tertullian, Hippolytus, and others believed that they knew the exact day on which Jesus died. According to this tradition, he died on a Friday March 25. Modern astronomers doubt whether this is possible, but no one doubts that this was widely believed in the west in the 3rd century.

March 25th was a very significant date to 3rd century Christians. For reasons I do not quite understand, they believed that Jesus was incarnate for an integral number of years, so that he was conceived on the same day of the year on which he died, i.e. March 25th. They also believed Friday March 25th to be the day God created Adam, 5198 years to the day before the conception of Jesus. Adam's later fall was assigned to a March 25th as well.

A century later, when Christmas was first celebrated in Rome, the date December 25th was chosen, probably because it followed 9 months after the conception of Jesus on March 25th. (In the east, Christmas was first celebrated on January 6th, nine months after April 6th.)

A Good Friday on the 14th of Abib occurs only about 1 year in 7. And a Good Friday on March 25th is even more rare. But this year we have a Good Friday that falls on both the 25th of March and the 14th of Abib. 2005 is only the third year when this has happened since the Gregorian calendar was instituted in 1582. The other two times were in 1633 and 1910. The next time it will happen will be in 2157.

For fun, you can run the Gregorian calendar backwards to the first century. The historical value of doing this is, of course, nil. Neither the Gregorian nor the Nicene nor the Hillel calendar reforms were yet instituted in the first century. But as a point of pure trivia, the Gregorian calendar yields exactly one Good Friday that falls on both 25 March and 14 Abib in the first century: 39 AD.

Sunday, February 06, 2005

First Order Approximations

Ash Wednesday is already upon us, directing our thoughts to the coming Easter. The usual expression for the formula of the date of Easter is that Easter is the first Sunday after the first full moon that occurs on or after the vernal equinox. This formula is true in a conventional sense. A more exact expression would be that Easter is the first Sunday after the 14th of Abib, where Abib is defined by the Nicene council as being the first Metonic month whose 14th day occurs on or after March 21st.

Calling the 14th of a Metonic month "the full moon" is kind of like calling 12 AM "midnight" or 12 PM "noon"; it's a first order approximation. If the earth and moon orbited in perfect circles, then the 14th of a Metonic month would be a full moon, at least in Europe, and 12 PM would be noon, at least in the center of your time zone, unless it's Daylight Savings Time. If the earth orbited in a perfect circle, then the seasons would be equal in length, and Spring would always start on March 21st. Of course, everyone has known for millennia that the heavenly bodies don't quite move in perfect circles, but it would be a hassle to make clocks that ran slower in Winter than in Summer just to get 12 PM right, and it would be a hassle to make a Metonic calendar that ran slower in Winter just to get the 14th of a Metonic month right. So for calendar purposes Meton ignored epicycles, and made his calendar based on cycles alone, a first-order calendar that matched the average lunation very well indeed, just as our 24-hour clock matches the mean solar day very well.

In the 4th century, the Nicene council simplified the Metonic calendar by linking it to March 21st, eliminating the need to calculate embolisms to keep it lined up with the sun. Independently in the 4th century, Rabbi Hillel II applied Meton's calendar for Jewish use, so that Jews dispersed over the world could celebrate their liturgies in solidarity on the same days. In the 16th century, Pope Gregory reformed the Julian calendar, realigning March 21 (to first order) to the vernal equinox. The Jewish and Orthodox calendars have yet to undergo such a reform, and so the Julian Abib sometimes occurs one lunation after the Gregorian Abib.

This year, 2005, in America and Europe, the full moon shall indeed fall on the 14th of the Gregorian Abib, but the Julian Abib shall align with the following lunation, making for an early Easter and a late Passover.