Friday, March 30, 2012

Friendly Francolin!

Since I got my new smart phone back in January, one thing I've appreciated is the ability to have a camera (of both the still and video varieties) with me at almost all times.  Monday while I was heading up to Hale Pōhaku for dinner I came across this friendly francolin in the parking lot. After it showed no signs of being bothered by my walking less than ten feet away from it, I decided to get some footage, since I can now.

Cute little feller, isn't he? (She? I can't tell from this distance.) Now if I ever see that mother francolin and her four chicks again...

Thursday, March 29, 2012

Southwest-Style Brownies

Today I decided to make some brownies using a package mix I'd bought some time before. It called for a few additional ingredients, an egg, some water, and some oil. I pulled out the carton of Eggbeaters I was using for eggs, which I had got about the same time I got the brownie mix, perhaps a month previously. Out of habit I gave it a sniff as I opened it, and discovered a very strange smell that I couldn't place.

My first thought was that it had gone bad in the time it had been sitting in the fridge, but it didn't smell like you would expect something that'd gone bad to smell. I was trying to find the expiration date on the carton when I found the answer to my conundrum: this wasn't regular Eggbeaters, it was "Southwestern Style" Eggbeaters! Featuring red and green bell peppers, onions, and a mix of spices for a tasty breakfast!

Instantly, I realized that what I'd been smelling had been the vegetables, which my mind couldn't place because it wasn't expecting them (a nice demonstration of how we see what we're looking for – or don't smell what we're not expecting, in this case). Since it was my only source of eggs, however, I went ahead and added a cup to my brownie mix in a quest to see what I would get out of it.

I now have what may be the world's first batch of Southwestern Style brownies (or "Breakfast Brownies" as my roommate Jonathan came up with), which really confuse your tongue. Generally they taste like brownies, but every so often you get a taste of something that definitely isn't chocolate-y, and there's a lingering spiciness left after you eat one. Quite interesting, though probably not something I'll make on purpose again.

Monday, March 26, 2012

Supernova Remnants, and the Pulsars that Light Them

Last week I showed you a picture of what happens to small stars (those less than ten times more massive than our Sun) when they run out of fusible hydrogen in their core. Today, let's take a look at what happens to a big star.

Upon running out of hydrogen to fuse, large stars initially proceed very similarly to their less weighty brethren. They begin to fuse the helium produced by hydrogen fusion into heavier elements such as carbon, oxygen, and neon, but unlike smaller stars they have enough mass and gravitational force in their cores to continue. Neon gets fused into heavier elements such as silicon, magnesium, and calcium, slowly working up the periodic table until the star starts making iron in its core. Iron is an interesting element because its nucleus has the highest binding energy per nucleon of any element. (Though certain isotopes of nickel have virtually the same binding-energy that iron does.) What this means, practically, is that you cannot get any energy from iron by either fusion or fission, either by fusing it into more massive elements or by splitting it apart.

Once a star arrives at iron in its core, it's done. It can't squeeze any more energy out of its core, so all that is left supporting the star against its own great weight is electron degeneracy pressure in a slowly-growing iron-nickel core. Electron degeneracy pressure is a quantum mechanical effect with no real analog in classical mechanics. Simply put, electrons exert pressure because two fermions (a class of particle of which electrons, protons, and neutrons are a member) cannot be in the same place and the same quantum mechanical state at the same time. Squeezing them together causes all the low-energy states to be taken, so the electrons vigorously resist any further compression, which would require large amounts of energy to raise electrons into high-energy states. However, there is a limit to how much pressure electrons can exert; if the mass of the core exceeds the Chandrasekhar limit of about 1.38 solar masses, electron degeneracy pressure catastrophically fails and the core collapses in on itself.

At this point, if the star is less than about 20 solar masses there is only one mechanism that can save the star from collapsing into a black hole: neutron degeneracy pressure. Similar to electron degeneracy pressure but involving neutrons, the star's core collapses into a neutron star, a sphere of neutrons packed as tightly as an atomic nucleus and about the size of a city.

As the core collapses into a neutron star, the star's outer layers fall inward tremendously fast, at speeds up to 23% of the speed of light. The neutron star at the center, however, is already packed as tightly as it possibly can be, so the infalling material rebounds off the core in a colossal shockwave to produce what we see as a supernova.

(There is a lot more going on at the same time, of course – supernovae are incredibly fascinating events where relativity and quantum mechanics are both in play, and I'm giving you merely the barest overview of all the processes happening.)

If the star in question started off heavier than about 20 solar masses, even neutron degeneracy pressure will be unable to support the core and nothing in the universe can prevent it from continuing to collapse into a black hole. However, I'm going to focus on neutron stars in this post because the picture I have for you today contains one. The story behind this particular supernova is ancient and varied, so settle in...

The story starts about a thousand years ago, in Anno Domini 1054, when a new star appeared in the constellation Taurus. As was customary, Chinese and Japanese astronomers noted the appearance of a "guest star" and recorded its location. It was also apparently observed by as least one person in the Arabic world. This star was apparently bright enough to be seen in the daytime for a period of several weeks, after which it slowly faded over a period of about two years and finally disappeared, whereupon it dropped out of history.

In 1731 a mysterious nebula (one of the first discovered telescopically) was discovered just off the tip of one of the horns of Taurus by one John Bevis. In 1758, while searching for the return of Halley's comet, Charles Messier stumbled upon this nebula and initially mistook it for his quarry. After watching it for a few weeks he realized that it wasn't moving, and came up with the brilliant idea to publish a catalog of objects that looked like comets but weren't, so that other amateur comet hunters wouldn't be fooled as he had. Thus, this nebula became the first object on what would become his now-famous list of not-comets: Messier 1.

In 1844, almost a hundred years later, the nebula was sketched for the first time by William Parsons, 3rd Earl of Rosse, whose love of astronomy and independently wealthy nature led him to build the largest telescope in the world at that time ("the Leviathan of Parsontown", 6 feet in diameter). His sketch reminded him of a crab, and so he gave our nebula the whimsical name the Crab Nebula. It remained a popular object of observation with astronomers, both amateur and professional.

In 1921 the American astronomer Carl Lampland noted changes in the Crab Nebula which implied a small size for it. In the same year, another astronomer demonstrated that the nebula was expanding. Several astronomers noticed its proximity to the "guest star" of 1054, but nothing was made of it until 1928 when the venerable Edwin Hubble definitively proposed that the nebula be associated with the star. However, it wasn't until later, when the theory behind supernovae had been worked out, that Nicholas Mayall showed that the Crab Nebula was nothing less than the remains of the supernova that exploded into the sky nearly 900 years earlier.

Although the association of the nebula with a supernova was now clear, it wasn't until the 1960's that neutron stars were first predicted by Franco Pacini. A few short years later, in 1968, a neutron star was detected in the center of the Crab Nebula, which made it both the first neutron star ever known and a shining confirmation of Pacini's hypothesis. It also explained why the nebula was so much brighter than a 900-year-old supernova remnant was expected to be.

The neutron star that lurks at the center of the Crab Nebula – known as the Crab Pulsar – is a fascinating beast by terrestrial standards. It is about 25 kilometers (about 15.5 miles) across, and makes a complete rotation every 33.08471603 milliseconds – 30 times a second! As the neutron star spins, it sends out a constant stream of electromagnetic radiation all across the electromagnetic spectrum (including visible light) from both poles of its extremely powerful magnetic field. The axis of its magnetic field is not the same as its rotational axis (much like the Earth, though a bigger offset), and as it spins around it sends off a powerful beam that appears to "blink" on and off as seen from Earth, much like the beam from a lighthouse. The neutron star is thus known as a pulsar, a portmanteau of pulsating star.

This electromagnetic energy being given off comes from the rotational energy of the Crab Pulsar, which is slowly slowing down by 38 nanoseconds per day. The energy being given off along with the star's powerful magnetic field (thousands of times more powerful than the Earth's) being spun through the Crab Nebula 30 times a second causes it to light up. Electrons are accelerated to nearly half the speed of light and spiral along the magnetic field lines of the pulsar, giving off synchrotron radiation as they do, which create a blueish glow visible in the center of the Crab Nebula in long exposures.

Now, after all this background, I suppose I should show you the picture you no doubt read this post for. I hope you can now better appreciate just how amazing this object is, even if my picture cannot do it justice. Here it is, the Crab Nebula:

The Crab Nebula, Messier 1, in Taurus.
While the Crab Pulsar itself is quite invisible in a small telescope such as the imaging telescope, you can easily see the remnant of that titanic explosion 958 years ago. Within that shell of gas exist fantastic filaments and mysterious structures, brought about by electrons powerfully accelerated to relativistic speeds by the pulsar's magnetic field. Despite being around 6,000 light-years away it is persistently the strongest source of X-rays and gamma rays from outside the solar system. The central neutron star itself is a sphere of ultra-dense matter more massive than the Sun compressed into an area smaller than New York city and spinning over 30 times a second. This matter (commonly called "neutronium") is so dense that a single thimble-full would weigh over 100 million tons. I could go on and on at length about how fascinating this single member of the group of objects know as pulsars are, but you get the idea.

I hope this post has given you a sense of the wonder and excitement I get when I study astronomy and physics. Learning more about the incredibly varied denizens of our universe never fails to amaze and astound me, and I enjoy nothing more than bringing that feeling to others. If this post made you stop and think at all, then I will feel I have succeeded. A hui hou!

Sunday, March 25, 2012


Living on an island composed of five volcanoes, only one of which is officially extinct, it's hardly surprising that we get earthquakes semi-regularly. Usually, however, these earthquakes are too small to be felt, or happen far enough away not to be noticed (by people).

However, this morning at 10:47 there was a magnitude 4.4 earthquake about 13 miles north by northwest of Hilo that I very definitely felt.

It's the red one.
I was sitting at my computer when I felt a huge jolt from the north-west. It was strong enough to move my (wheeled) chair, though not strong enough to have actually caused any damage. It was accompanied by a low, dull sort of booming noise. It all happened so fast, though, that I didn't realize what had happened for a few seconds. Thankfully it wasn't strong enough to cause any real damage, and there haven't been any aftershocks since.

As I think I've said before, life's never dull when you live on an active volcano in the middle of the ocean!

(If you're waiting for my promised post about supernovae, fear not, it's still in the process of being written. I just wanted to write something about the earthquake while it was still fresh.)

Wednesday, March 21, 2012

Planetary Nebulae, and the White-Hot Dwarfs that Light Them

Monday night I was able to get some images with the imager, and conditions were great nearly the entire evening. One of the objects I imaged was Messier 46 (or, rather, I walked a volunteer through imaging it, as I was doing some training). M46 is an open cluster in the constellation Puppis the Poop Deck not too far away from Sirius, the brightest star in the night sky.

Messier 46 in Puppis, with the planetary nebula NGC 2438 visible near the top.

M46 by itself is a fairly unremarkable (if fairly rich) open cluster of perhaps up to 500 stars about 30 light-years across, located about 4,500 light-years from us and receding at a speed of 41.4 km/s. What's interesting is that at first glance it appears to have a planetary nebula (cataloged as NGC 2438, visible above the middle of the cluster) embedded in it. However, the nebula does not appear to share the motion of the cluster, and is just a chance alignment along our line of sight.

The nebula itself is turns out to be pretty interesting. It is located closer to us than M46 at about 2,900 light-years, and has the amazing property that its central white dwarf star is one of the hottest stars known, measuring in at an astounding 75,000 K. (That's about 74,700 °C, or 134,672 °F.)

To put those number in perspective, our Sun's surface is about 5,778 K (or about 9,940 °F), while the very hottest O-type main-sequence stars (ones that are in their hydrogen-fusing phase, about 90% of the lifetime of all stars) might get up to 30,000 K (53,540 °F). Note that all these temperatures refer to the star's surface temperature – even in our fairly low-on-the-temperature-scale Sun the core temperature is over 15 million K (about 27 million °F). That still means that the Earth-sized ultra-dense sphere of electron-degenerate carbon and oxygen at the center of NGC 2438 is over two-and-a-half times hotter than the hottest stars operating under fusion power alone, and nearly thirteen times hotter than our Sun.

Planetary nebulae are so-named not because they have anything to do with planets, but because in the early telescopes through which they were discovered they looked somewhat like them (sort of a faint, fuzzy disc-shape). They come about when a star less than ten times the mass of the Sun runs out of hydrogen in its core to fuse, at which point it begins to work its way up the periodic table fusing heavier and heavier elements in its core while its outer layers puff up and the star becomes a red giant. The outer layers slowly puff off into space to form the nebula, while the core eventually stalls at carbon and oxygen (or maybe neon, if the star has between eight and ten times the Sun's mass) and forms a white dwarf, which is basically just the exposed core of a former star.

At this point no new energy is being released, so the white dwarf enters a long period of cool down. Due to their composition of electron-degenerate matter, temperatures inside the white dwarf are a fairly steady 1,000,000 K or so (about 18 million °F), while the outside cools off as it radiates energy into space, lighting up the outer layers it puffed off earlier and making the planetary nebula visible. Eventually the nebula disperses into the interstellar medium, and the white dwarf is left to gradually get cooler, and cooler, and cooler...

To end this post, I thought I'd mention that the hottest white dwarf found to date had a surface temperature of an incredible 200,000 K (360,000 °F) – almost three times hotter than the white dwarf in NGC 2438, nearly seven times hotter than the hottest O-type stars, and a whopping thirty-five times hotter than the surface of our own star. Pretty amazing, huh? On the flip side, the coolest white dwarfs found have surface temperatures of just about 4,000 K (6,740 °F) – they've already cooled off to less than the Sun's surface temperature.

Check back tomorrow, when I'll show you a picture of what happens when a star with more than ten times the mass of the Sun runs out hydrogen in its core. Hint: it's a lot more explosive!

Edit (3/24/12): As you can probably tell, it's way past tomorrow and I still haven't gotten that post up. That's because it turned out to be much longer than I anticipated when I started it, and then I went to work for two days. I will have it up in the next few days, though.

Tuesday, March 20, 2012

The Great Nebula in Orion.

Last Wednesday I was able to take out the imager at the Vis under good conditions for all of the second time this year.

Since it's still winter in the northern hemisphere, and the Earth's night side is still pointed more or less away from the galactic core, there aren't many of the objects I typically like to image (globular clusters) out at night. This led me to look around for another object to image, and I settled on the Great Nebula in Orion, AKA Messier 42, AKA the Orion Nebula. This nebula is, without a doubt, the most impressive nebula in the sky. It's also the brightest, and one of the few that can be seen with the naked eye (it's the middle "star" in the three stars that make up Orion's sword. No, not his belt, but his sword. I should post a picture...). I decided to devote an entire night to it and ended up getting almost two hours' worth of exposures. (Normally I like to image as many things as possible in a night, and for things like globular clusters that works alright, but the picture I got has taught me to reconsider that position when imaging nebulae...)

But enough words! You want to see the picture, and I want to show it to you:

The Great Nebula in Orion, Messier 42.

Gorgeous, no?

The Orion nebula is the closest star-forming region (or "stellar nursery") to Earth, at 1,344 \(\pm\) 20 light-years distant. It is probably the most photographed celestial object in history \(-\) in fact, it was the very first nebula ever to be imaged, on September 30, 1880 by Henry Draper (who was a pretty amazing character, one of the pioneers of astrophotography, and the person in charge of the U.S. expedition to observe the transit of Venus in 1874).

The Orion nebula is a large ball or bubble of gas (which fluoresces red) and dust (which silhouettes as black, or reflects light as blue) about 24 light-years across. Several extremely young and massive stars much hotter and more luminous than our Sun have formed near the center and are steadily blowing a hole in the side of the bubble of gas and dust in which they are embedded. The four brightest are known as the Trapezium and are a little too bright to see in this picture (they're located in the center where it's brightest). This allows us to see into the central cavity.

Interestingly, it's quite possible that the Orion nebula doesn't look anywhere near as impressive from the other side. From that perspective it might appear as a mostly featureless dark nebula, with perhaps some small emission nebulae around the edges. This is because the stars in the Trapezium appear to be blowing open the side of the bubble asymmetrically, and we happen to be on the side where it's open. It does make you wonder about the other dark nebulae we can see in the sky, and whether they might not be visions of such cosmic grandeur from the other side...

One last note: on the left side of the image, you might be able to make out two faint straight lines. Those are satellite tracks that appeared in the luminance images while I was imaging. They're pretty faint, so you may or may not be able to see them.

Monday, March 19, 2012

Superluminal Science

Well, it appears that the possibility of superluminal neutrinos took another hit recently, as an independent group of scientists reported that they detected no neutrinos moving faster than light using the same beam of neutrinos that the scientists from the OPERA detector used (they were the ones who initially reported the anomalous findings).

Along with the report I mentioned a few weeks earlier about how some problems had been detected in the timing apparatus used to measure the time of flight for the neutrinos, and it's looking like relativity theory was correct after all. There will still be some more tests carried out, but barring any major upsets this is probably the last we'll hear about the proceedings.

I'd just like to finish this post by emphasizing how this was a wonderful example of the scientific method in action. Scientists (or pseudo-scientists) depressingly often try to make sweeping claims on too little evidence. The public hears the initial claim, but rarely do they hear when those same claims are quietly retracted. But the scientists who reported the initially anomalous findings did a tip-top job of remaining objective and open to other possibilities. They didn't try to make any sweeping claims, merely reported that their results (which were conducted over several years and many, many test runs) didn't fit with accepted theory. The fact that some of their equipment wasn't quite working perfectly was easy to overlook, given the enormous complexity of the whole system. So overall it's been an exciting little episode in physics.

Wednesday, March 14, 2012

Another Sun Panorama

Just another picture of the Sun, this one taken last Saturday when I was finally able to get it between the clouds after three days of shut-out. Or rather, when I was finally able to get the six different pictures that I stitched together to make it. This version is only 1200 by 1200 pixels, but the full picture is 4500 by 4500!

Just like my picture from last year with the planets, this picture was made using the 11-inch solar telescope we have at the Vis – though unlike last year's, this picture was made by directly connecting my camera to the telescope, rather than shooting through an eyepiece. This caused the Sun to overflow the field of view of my camera, which allowed me to take multiple photos and combine them into this panorama.

It's a bit disappointing in this picture, but that sunspot group near the bottom was one of the largest I've ever seen when I first set eyes on it on Wednesday. By Saturday as you can see it had diminished somewhat – although it's still larger than all the inner planets, and probably larger than the ice giants Uranus and Neptune too.

As a side note, while combining the pictures that make up the final image, I discovered an incredibly useful and easy way to do so. Simply setting the mode of a picture on top to "difference" allowed me to easily see how much, well, difference there was between two photos. The difference mode subtracts one photo from the other pixel by pixel. If the pixels are identical in color to each other, you'll get black. If not, you'll get something that depends on how different they are.

For something fairly featureless like the Sun in white light, it's then a simple matter of moving the upper photo around to minimize the difference as much as possible (which in practical terms, means maximizing the blackness, since the closer the two photos are to each other, the more they cancel each other out and make black). Once you have the maximum amount of black you can set the mode back to normal, and marvel at how closely the photos merge together (although I still spent the better part of an hour manually smoothing and blending the edges). I'm sure this is probably standard panorama-making procedure that people learn about in Photography 101, but I'm really happy to have stumbled upon it. Can't wait to try it on my next set of Moon panorama photos!

Friday, March 9, 2012

Stella Nostra

Today I'd like to share with you a picture of the Sun that I took last week using the imager at the Vis while testing out one of our new solar filters that we got for the upcoming transit of Venus in June (which you will hear much, much more about as we get closer to it).

Our Star.
There's not too much detail to see, but you can make out a sunspot (probably about the size of Earth) in the upper-middle. When I came back to work this week, the Sun had a humongous sunspot complex on it that was probably about the size of Jupiter, which just goes to show how fast it can change. In fact, I would bet that it was that sunspot group that was responsible for the massive solar flare that hit Earth this morning. I'll try to get a picture of it if I can, but it's been very cloudy up here the last few days. It has a suspicious tendency to cloud over whenever I grab my camera...

If you're wondering about the title, that's simply Latin for "Our Star".

Monday, March 5, 2012

Globular Cluster Photo Series (Part 16): NGC 3201

The globular cluster I have for you today is the first one I've imaged not found on Charles Messier's famous list. Instead, it bears the New General Catalogue (NGC) number 3201. The New General Catalogue was [and remains] one of the most comprehensive lists of deep-sky objects visible with typical amateur equipment ever, even though it was originally compiled in the 1880's. It contains a core group of 7,840 objects, and was later revised to include an additional 5,386.

Anyway, NGC 3201 is a lovely cluster far enough south in the constellation Vela the Sail that Messier wouldn't have been able to see it from his location in Paris. It's about 15,000 light-years away and perhaps 80 light-years across, which combines to give it the impressive visual size of 18.6 arcminutes, nearly two-thirds the width of the full Moon and twice as wide as M97 from yesterday.

In the northern hemisphere, most of the brightest objects are in the Messier catalog; thus, almost by definition, anything without a Messier number is not going to be as bright or as impressive as something with one (there are some exceptions both ways, but it's a general rule of thumb). In the southern hemisphere, this is not the case simply because Messier couldn't see down there.

This was well demonstrated the night I got the data for M79 which I showed in the previous post, and NGC 3201, which turned out to be the more impressive one. Here's the picture for comparison:

NGC 3201 in Vela.

M79 turned out to be another fairly small nondescript cluster (although its possible extra-galactic origins make it pretty cool in other ways). NGC 3201 was rather impressive by comparison. I was even able to make it out faintly by eye in one of our 14-inch telescopes, which was neat. Come to think of it, this is probably the southernmost globular cluster I've imaged to date.

Sunday, March 4, 2012

Globular Cluster Photo Series (Part 15): M79

Today I have another Messier globular cluster to show you, Messier 79. This one is located just below Orion's feet in the constellation Lepus, the Hare. M79 is fairly small on the sky, just 8.7 arcminutes across (less than a third the width of the full Moon). This is mainly due to its large distance of about 42,000 light years from us, as the cluster itself is roughly average in size at about 118 light-years across.

Messier 79 in Lepus.

There are two very interesting things about Messier 79, and they may be related to each other. First, M79 is in an unusual position for a globular cluster: it's almost directly opposite from the galactic core in the sky. By far the majority of globular clusters are located somewhere in the same hemisphere as the core (and hence are visible during the summer); M79 is one of the very few that's not.

Secondly, M79 may not be native to the Milky Way. It is possible that it (along with three other smaller, fainter globular clusters) were originally satellites of another dwarf galaxy that is in the process of being absorbed into the Milky Way. A candidate galaxy was discovered back in 2003 and is known as the Canis Major Dwarf Galaxy based on the constellation in which it was first discovered in. I say "candidate" because it is still not entirely certain if this galaxy actually exists as an independent galaxy or is simply an over-dense part of the Milky Way. This is because it lies behind the galactic disk from our vantage point, making it impossible to see in visible light and difficult to analyze.

If the Canis Major Dwarf Galaxy does exist it would be the closest external galaxy to us, being only about 25,000 light-years away, and actually closer to us than to the Milky Way's core. It seems to correspond with a structure known as the Monoceros ring, an incredibly long stream of tidally disrupted stars that stretches over 200,000 light-years long and wraps around the Milky Way three times. If M79 was originally a satellite of the CMDG it would help to explain its strange position (although it's not impossible for a true Milky Way globular to have such a position; it's just rather unlikely).

Anyway, enough about this unusual globualr cluster; tune in tomorrow to see our very first non-Messier one!

Friday, March 2, 2012

Leonine Galactic Triplets

Monday night I was able to take out the imaging telescope for the first time this year, after spots of bad weather and poor conditions the last few weeks. Fortunately, conditions were pretty good and I was able to get several images, including the following image of the charming collection of galaxies known as the Leo Triplet.

The two galaxies near the top were spotted by Charles Messier in 1780, and bear the Messier numbers 65 (left) and 66 (right) respectively. The bottom galaxy was faint enough to elude his small telescope and was discovered instead by Sir William Herschel (who also discovered the planet Uranus) in 1784. It has the NGC number 3628. These three galaxies form what is believed to be a physical group at a distance of about 35 million light-years.

Each of the three galaxies is roughly the size of our own Milky Way galaxy, and it seems likely that some or all of them have had close gravitational interactions in the past. M66 in particular appears to show signs of gravitational disturbance, visible in the slightly distorted shape of the top spiral arm in this image. It's a bit harder to tell with the other two, lying nearly edge-on to us as they do, although you can kind of make out that the dust lane running across NGC 3628 is bent, tell-tale evidence of past (and ongoing) interaction.

All three galaxies show pretty nice dust lanes, really, whether they're edge-on or more face-on to us.

Thursday, March 1, 2012

Happy Leap Day!

Happy February 29th, everyone! As this is the first leap year since I started my blog I thought I would give a quick overview of the Gregorian calendar system that brings it about.

Any such discussion should begin, however, with the introduction of the Julian system which the Gregorian system replaced. The Julian calendar was a modification of the Roman calendar put in place by Julius Caesar, taking effect in 45 BC. It was an attempt to keep the calendar in sync with the seasons, which is necessitated by the fact that the Earth does not take an integer number of days to orbit the Sun. The sidereal year, the amount of time it takes for the Sun to return to the same apparent location in the sky relative to the background stars, is about 365.25636 days. While this is not too complicated, we also have to account for the fact that the direction of the Earth's rotational axis is not constant with time, but rather wobbles very, very, slowly like a top. This cycle takes a long time to complete \(-\) about 24,000 years \(-\) but it does produce noticeable results, such as the fact that the time between successive equinoxes or solstices (called a tropical or solar year) is about 365.24237 days \(-\) about 20 minutes shorter than a sidereal year. It's also more important for purposes of agriculture, because the seasons are tied to the solar year rather than the sidereal year.

Anyway, because the year is not an integer number of days, any attempt to use an integer number of days in a calendar will lead to a slow drifting of seasons. Given enough time (and at \(\sim{}\)1 day every 4 years, it wouldn't take too much time) the seasons would drift slowly backwards through the year. Spring, summer, fall, and winter would all begin earlier and earlier. While this might not sound too bad in theory, it can play havoc if you're a farmer wanting to use the calendar to know when you should be planting or harvesting your crops \(-\) and in a pre-industrial society, that could mean the difference between surviving through the winter or starving to death.

Prior to Julius Caesar the Romans tried to fix this situation by the random inclusion of extra (intercalary) months. And I do mean random, the Roman officials in charge for the year were in charge of determining if they needed an extra month, and since these officials served for "a year", there could be quite the temptation to increase the length of one's rule or deny a rival extra time in office. Julius Caesar decided to come up with a system that would be independent and self-correcting after a stint in Egypt, where a system of adding 1 day every 4 years had been proposed, but not carried through. His mathematical advisors came up with the same idea, and in 46 BC he mandated the adoption of the Julian calendar.

Now, the Julian calendar was quite the improvement over the previous chaos, to be sure. It was good enough, in fact, that some countries continued to use it up into the 20th century. It kept the dates of equinoxes and seasons much better than any previous attempts, and only gained 1 day every 128 years. Given the length the Roman empire lasted after the adoption of the calendar, the few days gained weren't that big a deal. However, by the Middle Ages, the seasons were definitely out of sync. By AD 1582 the equinoxes were happening about 10 days earlier than they should (March 11 instead of March 21). Pope Gregory XIII decided to fix this state of affairs, and asked the finest mathematical minds in Europe to come up with a better system, which eventually came to bear his name.

The difference between the Julian and Gregorian system isn't very big. Under the Julian system, the calendar gains about 3 days every 400 years, i.e., in 400 years there are 100 leap days in the Julian system. The Gregorian system fixes this by taking away 3 of those leap days, for a total of 97 leap days ever 400 years. The way this was accomplished was by decreeing that years ending in 00 would not be leap years (like they ordinarily would) unless they were also divisible by 400. So the 2000 was actually rather special: it wouldn't be a leap year except for the fact that it is divisible by 400. Likewise, the year 2100 will not be a leap year, even though it is halfway between two leap years.

Such a simple, tiny change, yet what a big difference it makes! In the Julian system, over a period of 4 centuries there are \[100\times366+300\times365=146,100\,\text{days}\] while in the Gregorian system there are \[97\times366+303\times365=146,097\,\text{days}\]
That doesn't seem like a big difference, yet the Julian system is accurate to 1 day every 128 years, while the Gregorian system is accurate to 1 day every 7,700 years, over 60 times as accurate. Pretty neat, huh? Happy Leap Day!