I Miss My Serotonin

The winter solstice comes tonight. The shortest day and the longest night of the year. I, for one, am ready for daylight to make a comeback.

Did you know that the word solstice come from the Latin for sun (sol) and standing still (sistere)? The solstice is when the Sun stands still.

Well...that isn't exactly what happens, but I can see how the ancients might have seen it that way. The Sun actually moves a lot in our sky. In addition to tracing out a path across the sky from sunrise to sunset, the height of the Sun at midday also changes throughout the year. The winter solstice, in the general sense, marks the day when the Sun is the lowest in the sky during midday. However, in astronomical terms, the winter solstice is the exact moment when Earth's axis is tilting farthest away from the sun.

From Astronomy 101, you may remember that our axis is tilted 23.5 degrees from vertical with respect to the plane of the Solar System. You may also be familiar with the North Star, right? Well the North Star got it's name for a reason - it's where our axis appears to point if you were to extend a line straight out into space from the north pole. And that line always points there, regardless of the season. This is because while the Earth rotates on its axis and orbits around the Sun, the tilt of our axis doesn't change. So for the winter solstice, the Earth is at the exact spot in its orbit where the our axis tilts the farthest away from the Sun and for the summer solstice, the axis is tilting towards the Sun.


So from now until June 21st, the Sun will only rise to a higher and higher point during its daily travels. However, right now it's killing me by only getting up to a little over 16.5 degrees above the horizon. Here in Cambridge, I am at 52 degrees latitude. For comparison, back in the states, New York City is around 40 degrees latitude and will see the sun reach 25.7 degrees at its highest. Those 9-ish degrees in latitude may not seem like much, but they translate to 1.5 less hours of daylight for me versus you New Yorkers out there. Given England's penchant for grey skies, I want all the Sun I can get. I may have to overcompensate and move the equator after my tenure here so I can benefit from the Sun wandering back and forth across the zenith. So while the Sun never exactly stands still in our sky, on the solstice it appears to stop at the high or low extreme of its midday elevation. I can see where the ancients where going with that.



These two figures show the range of elevations for the sun at 50 degrees latitude (think England) and 0 degrees latitude (on the Equator). Actually, the illustration for the equator above helps to correct a common misconception. The Sun does not shine directly overhead every day at noon on the equator - it only does so on the vernal and autumnal equinoxes. The equinoxes are the half-way points between the solstices in the spring and fall where the hours of day and night are equal. Don't forget that the equinoxes and solstices are switched below the equator in the southern hemisphere!

Another myth that I would like to dispel is that the Earth is closest to the Sun on the summer solstice and consequently farthest from the Sun this time of year. To that I say: first, props for remembering that Earth's orbit is elliptical - I'm glad you are paying attention, but second, that is unfortunately an incorrect conclusion. Turns out the Earth's distance from the Sun makes less of a difference than the tilt of its axis. When the Earth is at perihelion (its closest point to the sun, peri for close, helio for sun) the heat from the Sun's radiation doesn't differ drastically enough as compared to at apehelion (its farthest point). However, when the northern hemisphere is tilted 23.5 degrees away from the Sun, less radiation strikes the atmosphere in the north and therefore causes the cold temperatures we have come to call winter. The opposite being true for summer.

Lastly, I feel I should warn you about a dangerous trend regarding the winter solstice. It precesses, along with Earth's axis. In fact, one day in the future, Christmas will be in July and Independence Day will be in December...so to speak. My point being that the solstices will be opposite to how they are now, with the shortest day being June 21st and the longest day being December 21st. That will make for quite the change in holiday traditions! Of course, I'm scaring you for no reason, because this won't happen for tens of thousands of years. Breathe easy and go make a snowman.

(N.B. For you those of you who prefer an exact science, the solstice actually occurs on December 22nd at approximately 06:09 UT, also known as Greenwich Mean Time. That's AM here in England. East coasters are 5 hours behind at 1:09 am- West coasters are 8 hours behind at 10:09 pm, back on the 21st.)

(image credits: http://www.physicalgeography.net/fundamentals/6h.html)

Mars Returns

A follow up post to yesterday's, but with a bit more detail about what's happening with Mars and why - for those of you looking to expand your science literacy (and your minds).

The biannual date between Mars and Earth happens because both planets take a different amount of time to orbit the Sun. Earth takes 365+ days and Mars takes approximately 687 days. Therefore, Earth moves faster in it's orbit and gradually "laps" Mars in the race around the Sun. The approach and subsequent passing of Mars is what occurs every two years. However, not every meeting generates such great views of the Red Planet.

As mentioned in the previous post, both planets have elliptical orbits and as such, this complicates the occurrence of these close encounters. The diagram below best illustrates what's going on here. It show how two elliptical orbits have a point where their paths come closest together and where they are farthest apart. Also shown in the diagram, to an exaggerated effect, is that Mars' orbit is more eccentric than Earth's, i.e. it's orbit is more elliptical and in fact over five times more so. If you combine this with the different speeds of each planet in their orbit, they won't always be nearest to each other at the same point in space each time Earth is about to lap Mars.


You may or may not recall a huge fuss about Mars from 2003. This was because Earth lapped Mars that year very close to the point where their orbits are the closest they ever get, approximately 34.6 million miles. This happens roughly every 15-17 years as the lapping point precesses around the Sun over time; it takes 15-17 years to occur back at the same point in space. Each biannual meeting of the planets since a closest encounter like in 2003 will produce less and less spectacular viewing opportunities of the Red Planet until the lapping point comes around again. However, while the planets are still within a certain range of each other, the viewing can still be quite exciting. After this we will have to wait another nine years before the fun begins anew. Compared to 2003, Mars will be 20 million miles farther away this year, at a distance of 55 million miles as Earth overtakes it. But surprisingly, 20 million miles makes less of a difference than you might think. There is still a good show to be had.

(photo credit: Our Dark Skies, http://www.ourdarkskies.com)

Close Encounters of the Mars Kind

At 11:45pm tonight, Mars and Earth will be the closest they've been since October 2005. This planetary rendezvous is due to the elliptical orbits of both planets around the Sun and the point at which these two ellipses are nearest to each other. It happens once every two years for Mars and Earth and it's tonight!

So wrap yourself up in your warmest winter coat(s) and get your bum outside to check it out. No excuses for even you city dwellers as Mars easily outshines any urban light pollution (except for maybe in Times Square) and is well above the horizon. It can be best seen looking south in the middle of the night, moving westward till dawn. And with its red-orange color and steady glow, it is also larger than any star you're used to seeing in the sky - in fact, it is 1/100th the size of the full moon. That may not sound like much, but I assure you it's enough to grab your attention. For reference the full moon takes up half a degree of the sky and a typical star in the night sky is on average less than 1/1000th of the the full moon. So Mars is over 10 times bigger than the stars you are used to seeing! And to put it all in perspective, as many of you may know from seeing Apollo 13, the size of the full moon is comparable to the width of your thumb at arms length. You could say then that Mars is 1/100th the width of your thumb, but that's besides the point.

It's big, it's bright, it's red, and it's awesome.

(photo credits: NASA and the Hubble Space Telescope)

New York City Star Power

The Washington Post did a feature article in the Science section this weekend about the guy I want to be when I grow up. I've probably already talked the ears off of most of you about him because he is so stellar.

Yes, I'm not ashamed to go there.

His name is Neil de Grasse Tyson and he is the head of the Hayden Planetarium. Now I know when most of you think of a planetarium, you perhaps have flashes of elementary school field trips to a dark, grungy auditorium with tilt back seats where you fell asleep while some boring old guy lectured you on the constellations. Let me assure you, the Hayden smashes all those stereotypes to bits...to bits.

It is housed in the Rose Center for Earth and Space, a wing of the American Museum of Natural History in New York City. If you are anywhere within the radius of a reasonable day trip, this is a must. And it's 'must visit' status is in large part due to Dr. Tyson's overhaul of the Hayden back in 2000. Not only is the building itself fabulous - a cavernous glass atrium housing a massive spherical planetarium known as the Hayden sphere - the museum also offers an endless series of fantastic public programs for every age. And if neither of those get you, Dr. Tyson will. I could wax on and on about it, but instead I'm just going to challenge you to prove me wrong.

Read all about Dr. Tyson and his mad astrophysics skills here through the first link and start planning your visit to the Hayden Planetarium.

And remember all you just read when you come to visit me in New York and I show you my awesome office at the Hayden, down the hall from Dr. Tyson.

22 Down...

How many more to go? I've gotten to the bottom of the troublesome 22 sources just uncovered and that's enough for tonight. But as a famous southern belle once said: tomorrow is another day. And I'm sure it will be filled with digging into the details of the other 1,886 sources. For now though, I need to get out of this office!!!

Ich bin uberfragt!

Oh, what have I done?!?...While innocently chugging along at work today, I started uncovering error after error. Error might be too strong a word, but there are a myriad of inconsistencies in my database rearing their ugly little heads. Not cool, not cool at all. It's like when I first created the database I was on crack or something! (No offense to any crackheads out there). I am seriously baffled by this and now unsure how to proceed. I only found the mistakes I did by looking further into 22 of my 1908 radio sources. Who's to say that this problem doesn't pervade the whole database? Does this mean I should go over all 1908 entries with a fine tooth comb? Or is that a waste of time? Do I instead just wait until other potential problems rise to the surface? But if that happens enough times in the near future, then wouldn't it have just been better and more time efficient to review the full database now?

Too many questions swirling around my already full postgraduate student head.

Was the Wait Worth It?

A piece of the 10% has finally been produced! It is an overlay of histograms to show the degree to which NVSS sources are polarized, broken down into groups of total brightness. The y-axis is the fraction of total sources (1 = all sources ) while the x-axis is the percentage of polarization (1 = 100%).

It may not look like much, but it actually tells me that only the dimmest NVSS radio sources are likely to be highly polarized. Which is thankfully a useful result. Yippy skippy.

To see this, note the striking difference between the neon green histogram and the black histogram. The neon green represents the dimmest group of radio sources (less than 10 mJy) and the black represents the brightest (over 1000 mJy). The units of brightness (mJy for milliJansky) are the standard when talking about radio emission. A Jansky is the basic unit, named after Mr. Jansky, naturally, but most of my sources are ten to a thousand times dimmer - hence the milli (or 10^-3 for those of you who partial to scientifc notation). Anyway, the plot clearly shows that the dimmer sources are more likely to be more polarized, whereas very few of the brighter sources are more than 10% polarized.

Now onto further investigations...