Sunday, April 30, 2006

Epimetheus, Saturn, and Titan from Cassini

Epimetheus glides in front of Saturn's rings with Titan far in the background.

Phil Plait's Bad Astronomy has pointed out another beautiful Cassini raw image. Think Titan is out of focus? Nope, it's just a thick atmosphere of nitrogen with a touch of methane.

Browse the Cassini's mission raw image archive here.

I made an animation out of another set of Cassini images here.

Friday, April 28, 2006

Aurora from space

Check out this awesome movie of the aurora as seen from the ISS: Movie

Also see Astronaut Don Pettit's longer science movie with real-time looks at the aurora. Don't miss the Manicouagan, Quebec impact crater visible by moonlight about 3/4 the way into the movie.

You can see how restricted the aurorae are to a particular height in the atmosphere (starting at about 100km in the ionosphere and extending up to 300-500km) and the color variation with height (the red emission of oxygen requires an excited atom to remain unjostled by other atoms for a time period before it will emit a particular wavelength).

Astronaut Don Pettit discusses the aurora from space further in this article.

Monday, April 24, 2006

Robins singing at night and street lights: the light pollution problem

This weekend I traveled to Yerkes Observatory in Williams Bay, Wisconsin, on a trip organized by the Ryerson Astronomical Society. It was fantastically clear all night long. And quiet.

Oddly, at least to my city ears, I heard no birds all night long. The first birds I heard chirping occurred well after the start of nautical twilight at 4:52AM--in fact, it wasn't until the start of civil twilight thirty minutes later that I really began to hear a lot of robins and other birds.

Now, in Chicago, I have heard from friends time and time again, that if they stay up too late, they begin to hear robins singing late at night and can't get to sleep. We are talking 2 or 3 AM, not the expected nautical or civil twilight. What's going on?

I imagine, although haven't studied it, that birds are primed to begin singing by having a biochemical reaction when exposed to a dark period of a certain length followed by light. Plants use this method to produce flowers during certain seasons, and biologically it's easier to have a too-long cycle that is reset daily than to accurately time the cycle. So, for robins, during the night their cycle is reset by the first light they encounter. Normally, this is the dawn. But in many cities, it is unshielded streetlights. And they chirp away.

This is a direct result of light pollution caused by unshielded streetlights in the city of Chicago. Fully shielded lights (also known as full-cutoff) do not emit any light above horizontal--as a result they don't light trees above the streets and keep the birds from singing. You can't say the results from Yerkes are a result of no lights: Yerkes isn't completely isolated--it's in the town of Williams Bay, which has streetlights. Fully shielded streetlights.

When someone asks me if light pollution has a real effect on people other than astronomers, I believe I will mention my friends that can't get to sleep because of the robins.

Wednesday, April 19, 2006

Messenger departing Earth

The Messenger spacecraft on its way to Mercury flew back past Earth on August 2nd, 2005, and produced this wonderful movie while departing Earth. In 24 hours it moved from about 5 Earth diameters distance to past the Moon's orbit (which is about 30 Earth diameters away). You can see the spacecraft's long low-energy trajectory here. Click on the image above to get the movie.
Why I am posting this? Phil Plait's probably going to mention it, and the movie has been sitting on my "post about" pile for a while.

P.S. Here's the original source for the movie:

The Sodium doublet

Steven previously mentioned his favorite absorption line in the sun's spectrum as sodium. And what's not to like: Strong, conspicuous, identical to the emission you see from low-pressure sodium lights, the sodium doublet is to the Fraunhofer lines like a hot dog is to ballpark food.

With this image I experimented with a vertical blur to remove the variations in intensity along the length of the slit due to dust, irregular coatings, and a small piece of crud in the digital camera. The spectrum is also sharpened, so you can't use this image to find astrophysical data.

The color is weird--I am always surprised how redward the sodium doublet is from where I think it should be in the yellow, but in reality it's more orange than yellow. And yellow is visually such a small portion of the spectrum that there isn't much to it.

Other spectrum posts:
Terrestrial Oxygen
The Solar Spectrum: Magnesium Green

Wednesday, April 12, 2006

Airplane radiation graph with a geiger counter, MDW to LAX

A recent flight to Los Angeles had great results. Halfway through the flight the pilot announced he was dropping from the 38,000ft cruising altitude to 30,000ft to avoid persistent turbulence (in my notes I wrote 36,000ft but it can be difficult to hear what the guy was saying, and I think the data shows a better match to 30,000ft). The data were taken once a minute.

I also added to the graph a yellow line showing the background rate in my office which is 7.5 microrads/hr. So flying at 38,000ft gives you a rate about 40 times greater than here in my quiet little office in Chicago.

I've previously done this on trips to Los Angeles: Dean W. Armstrong: Geiger Counter

In the past I've also looked at potassium salt, Pentax Takumar lenses, dinosaur bones, and have a live report of radiation rates from my office.

Tuesday, April 11, 2006

Direct measurements of 2003 UB313

Mike Brown, Emily Schaller, Henry Roe, David Rabinowitz, and Chad Trujillo released their Hubble measurements of the disk of 2003 UB313 and found it to be just 34 milliarcseconds in diameter--that's roughly a millionth of a degree on the sky (the moon is half a degree in diameter). That makes it 2400km +-100km in actual size, and makes it a brightly reflective object with an albedo of 0.86+-0.07 (86% reflectivity). Freshly fallen snow on Earth is at most 90% reflective, so the surface of 2003 UB313 has got to be covered in new ice of some sort--Brown et al. suggest methane ice, like Pluto.
Previous measurements using infrared had it at 3000km+-400km.

Search this blog for previous posts about 2003 UB313.

a delay in posting

I had the good fortune of being quite busy over the weekend: I was a guest of Hugh M. Hefner at the occasion of his 80th birthday, and it was quite a bash, even by the standards I've associated with other Mansion events I've been to. But this meant I've haven't any blog posts to put up nor drafts to finish.

Tonight I gave a talk on interplanetary dust to the Ryerson Astronomical Society--a topic I had been interested in since seeing the zodiacal light again during my spring break geology field course, but didn't know much about. I believe the talk was well received. The material was compelling enough that I expect some posts to come from it.

P.S. Official publically-released videos and photos at
Peggy and I are in the last video with the Girls Next Door, halfway through the video.

Thursday, April 06, 2006

The Moon in the northern sky

While on the couch in my apartment in the late-evening last night the light of the just-past First Quarter Moon fell upon my eyes. It told me a few things--that the Sun would be roughly where the Moon was now in a quarter-year (three months, or early July), and since my window faces northwest the moon was in the northern half of the sky. The "roughly" part had to be included because the Moon's orbit around the Earth is not precisely in the plane of the solar system (also known as the ecliptic). Instead it's tilted off about 5 degrees from the ecliptic, and it's why we don't have solar and lunar eclipses every month. Instead, we only get them roughly every six months: when the intersection of the plane of the moon's orbit and the ecliptic (also known as the nodes) line up with the line between the Earth and the Sun. We just had a solar eclipse just a week ago , so I know that the Moon should be nearly the full +5 or -5 degrees off the ecliptic.

I created a snapshot of the view from the couch with Stellarium and it shows the moon at +25 degrees declination, or a few degrees above the ecliptic, but not fully +5. Why? It's just past first-quarter, so it's heading back down to the ecliptic. When I plugged into Stellarium the previous night, you could see the Moon was at +27.5 declination, some +4 degrees above the ecliptic. Add the two weeks we are past the vernal equinox (where we'd expect the ecliptic to be near +23.5 declination), and it puts the Moon where we see it.

So, come early July, I can be lounging on my sofa in the late afternoon and the Sun should be streaming into my apartment and onto my face (apart from any leaves on the trees blocking it).

Tuesday, April 04, 2006

Forrest Mims vs. Eric Pianka

The online science world is abuzz over Forrest Mims's claim that a UT Austin Bio prof said scientists should kill 90% of the world's population. Some Intelligent Designers have jumped on the bandwagon and called the DHS. A reasonable statement of the whole affair is at Panda's Thumb.
If Forrest Mims sounds familiar, he's the guy I mentioned in transmitting audio over an LED's light.
When crazy claims like these appear, you always have to ask yourself, what's more likely: A whole academy conspires to turn off the video camera, a scientist calls for the destruction of mankind, and only one person makes a stink? Or... did someone mishear what the scientist was saying.

Monday, April 03, 2006

Tempest continued

Michael Milligan gave me a great site for the NOAA GOES weather satellite images and merged a few for a look at the low-pressure system that gave us a windy day Friday. What's great about the GOES site is you can get full-resolution 16-bit TIFFs from the site. I took the East Coast selection, cropped it, resized, edited the histogram, scaled it to account for its equatorial perspective, and redid the tempest:

Reasonably (640x480) sized image of the above.