I was just looking around the internet about the Astronomical Unit (AU) thinking about our class lab, and I ran across this internet gem. Please note the subtitle of the page, a very reassuring "Science in Ancient Artwork."
It started out looking okay (probably because I skipped the first few paragraphs), and the numbers for his argument made sense in the first thing he was saying about Mercury/Sun distance as the AU. Then the arrows came up about some "patterns," if a sane person can really call them that--I feel like he was in a room lined with pictures and clippings on the walls with yarn or string or something sturdily holding his argument together.
Also, ratios don't have units Mr. Charles William Johnson, so I don't really know why you threw that bit in there. I get that the numbers vary, but it's not at all because of the units they're measured it. It's either an imprecise conversion being used, or just different numbers.
And one last word, I think you should go meet with my high school freshman science teacher Mr. McKenna about significant figures. Don't worry, he's a nice guy and won't judge you too much for what you're trying to do with them.
Sunday, October 16, 2011
Tuesday, October 11, 2011
Oh, that is what you're calling the Transmission function...
Just a quickie: in class the other day we were talking about a slit experiment with an infinite number of infinitesimal slits in a space day. I didn't understand that the "transmission function" described meant the light right after the slit screen, I assumed it meant the light transmitted to the back screen where it was being measured. Knowing this, I now have no issues with that conversation because a finite width slit clearly displays interference with itself and it appears in the form of a sinc function.
Monday, October 10, 2011
Summer research and distant amino acids
My research this summer was based on terahertz (THz) spectroscopy of molecules in solution. While my work was not directly relatable to astronomy work at this point, maybe some day it will be as instrument sensitivity goes up. However, THz spectroscopy in different forms can be used for identification of molecules in space and research towards that took place in the lab along side me. THz frequency radiation roughly includes light from 300 to 3 micrometers in wavelength (1-100 THz), and it is in this region that many molecules have distinctive absorption bands. Water first of all has a huge, strong absorption in this region and thus water can be detected by looking at distant objects, checking the spectrum of radiation we receive from them, and comparing it to the known absorption of water. While this is more planetary science than straight astronomy or astrophysics, it is closer to what I am interested in, academically. The research that was happening in the Blake lab this summer with me involved taking samples of amino acids and similar interesting compounds, making them with a simple salt known to not absorb in this spectral range, forming a pellet from this mixture using high pressure, then looking at the absorption of THz through it. The idea here being that if specific absorptions can be identified to uniquely correspond to certain molecules, then this analysis can be applied to radiation from distant objects, with the hope of identifying amino acids in the atmospheres of distant planets.
Just for my own sake, I want to talk briefly about generation of THz frequency waves, and I suppose you get to read along as I do so. In our lab, the easiest way to generate THz waves is shining our ultrafast pulsed laser on a crystal of ZnTe. In a simplification of the process, the laser excites electrons in the semiconductor crystal and the consequent motion of these electrons creates THz frequency radiation because of Maxwell's principles. In other words, I got to play with big, powerful, and very fast (pulses on the order of 10^-14 s) lasers. My research was essentially doing the same thing except instead of using crystals to generate the THz, we were using solutions of inorganic molecules and trying to detect and interpret terahertz waves given by these samples.
So not exactly related to astronomy, but interesting nonetheless in my opinion.
Just for my own sake, I want to talk briefly about generation of THz frequency waves, and I suppose you get to read along as I do so. In our lab, the easiest way to generate THz waves is shining our ultrafast pulsed laser on a crystal of ZnTe. In a simplification of the process, the laser excites electrons in the semiconductor crystal and the consequent motion of these electrons creates THz frequency radiation because of Maxwell's principles. In other words, I got to play with big, powerful, and very fast (pulses on the order of 10^-14 s) lasers. My research was essentially doing the same thing except instead of using crystals to generate the THz, we were using solutions of inorganic molecules and trying to detect and interpret terahertz waves given by these samples.
So not exactly related to astronomy, but interesting nonetheless in my opinion.
Sunday, October 9, 2011
To be or not to LST, the question is how long is a year.
In class the other day, I got to talking with a few people about local apparent sidereal time (LST or LAST) and its relation to solar time. Our discussion came to an interesting place when we started talking about a sidereal year compared to a solar year. I seemed to be the one who was causing a discussion because I was interested in thinking about what time LST the new sidereal year would start at each year. Would an LST clock be at 11:59 before turning to 12:00 the next year? Having since looked up more about this, I should probably go back and talk about some definitions and such.
The way we had had LST defined for us was that the sidereal year started at noon on the vernal equinox and that noon LST would occur each day following when a star that was on the meridian (a great circle drawn imaginarily north to south passing through the zenith, which is a point in the sky directly above the observer) at noon the previous day again passed the observer's meridian. As a solar day is defined so that at noon each day the sun should pass through the meridian, the same is true of a sidereal day except that the star is not the sun, but instead a more distance star that seems to not move relative to our motion in the solar system. A solar day and a sidereal day are different because the earth moves in its yearly path around the sun which changes the background stars' position relative to the sun in our daily rotation. That may sound complicated, but it really is not. For a nice diagram, see this picture from wikipedia here. As can be seen from the diagram, because the earth moves relative to the sun along its orbit each day, but the earth does not seem to move relative to distant stars, causing noon LST to occur earlier each day relative to noon solar time, about 4 minutes earlier each day.
So, on to my original question, what time does the new sidereal year start in LST? Based on the definition above, an LST clock would not go smoothly across the change in year because the equinox moves relative to the background stars very slightly. According to the wikipedia article, LST is really based on the local of the vernal equinox relative to the meridian and the background stars relative to the meridian. While these are not very different, it does still make a difference how LST is defined. Though, looking back to what I was thinking in class, I think I was wrong on the magnitude of the difference I was thinking of at the time. I think my problem then was that I didn't think of the fact that in a sidereal year, there are approximately 366.24 sidereal days, one more day than a solar year because we orbit around the sun.
I think most interesting though was what I found when I was looking for more information on this. As before, using wikipedia and just clicking random links that I wanted to know more about, I've found that (as really makes sense when you think about it) pretty much nothing about earth's motion is very nice. We don't travel in a circle around the sun, it's more of an ellipse, but then that isn't even true. Because the earth/moon orbit the sun together, that gives the earth an even more irregular motion. Then of course the earth's axis of rotation is precessing, which is also affected by the sun and moon. Then, lest we forget the other planets in the solar system, they also affect earth's orbit and its precession.
One last fact: the change in the precession of the earth is called nutation and seems to be mostly affected by the sun and the moon, but wikipedia also lists ocean currents, wind, and even the motions of the earth's molten core as affecting this motion.
I'm just glad that my watch fairly accurately tells me what time of the day it is despite all these perturbing factors.
For these thoughts, I'd like to acknowledge some helpful conversations with Prof. Johnson, Jackie, and whoever I was working with in class that day, I believe David, Dan, and Mee.
The way we had had LST defined for us was that the sidereal year started at noon on the vernal equinox and that noon LST would occur each day following when a star that was on the meridian (a great circle drawn imaginarily north to south passing through the zenith, which is a point in the sky directly above the observer) at noon the previous day again passed the observer's meridian. As a solar day is defined so that at noon each day the sun should pass through the meridian, the same is true of a sidereal day except that the star is not the sun, but instead a more distance star that seems to not move relative to our motion in the solar system. A solar day and a sidereal day are different because the earth moves in its yearly path around the sun which changes the background stars' position relative to the sun in our daily rotation. That may sound complicated, but it really is not. For a nice diagram, see this picture from wikipedia here. As can be seen from the diagram, because the earth moves relative to the sun along its orbit each day, but the earth does not seem to move relative to distant stars, causing noon LST to occur earlier each day relative to noon solar time, about 4 minutes earlier each day.
So, on to my original question, what time does the new sidereal year start in LST? Based on the definition above, an LST clock would not go smoothly across the change in year because the equinox moves relative to the background stars very slightly. According to the wikipedia article, LST is really based on the local of the vernal equinox relative to the meridian and the background stars relative to the meridian. While these are not very different, it does still make a difference how LST is defined. Though, looking back to what I was thinking in class, I think I was wrong on the magnitude of the difference I was thinking of at the time. I think my problem then was that I didn't think of the fact that in a sidereal year, there are approximately 366.24 sidereal days, one more day than a solar year because we orbit around the sun.
I think most interesting though was what I found when I was looking for more information on this. As before, using wikipedia and just clicking random links that I wanted to know more about, I've found that (as really makes sense when you think about it) pretty much nothing about earth's motion is very nice. We don't travel in a circle around the sun, it's more of an ellipse, but then that isn't even true. Because the earth/moon orbit the sun together, that gives the earth an even more irregular motion. Then of course the earth's axis of rotation is precessing, which is also affected by the sun and moon. Then, lest we forget the other planets in the solar system, they also affect earth's orbit and its precession.
One last fact: the change in the precession of the earth is called nutation and seems to be mostly affected by the sun and the moon, but wikipedia also lists ocean currents, wind, and even the motions of the earth's molten core as affecting this motion.
I'm just glad that my watch fairly accurately tells me what time of the day it is despite all these perturbing factors.
For these thoughts, I'd like to acknowledge some helpful conversations with Prof. Johnson, Jackie, and whoever I was working with in class that day, I believe David, Dan, and Mee.
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