The Yugoslavian meteorologist Milutin Milankovitch is known for coming up with the idea of orbital forcing, also known as Milankovitch cycles. Orbital forcing is a fancy term for certain changes in Earth’s orbit, which are precession, obliquity, and eccentricity.
I’ve written about all three of these before, but here’s a brief overview:
Precession is the motion of Earth’s axis like a spinning top. Imagine the Earth’s day-by-day rotating motion as that of a top. What do tops also do? They wobble.
Although I’ve written about obliquity, I haven’t used the term before. It’s a fancy word for how the tilt of the Earth’s axis doesn’t stay at the same angle. It changes a bit over a very long period of time, ranging between 22° and 24°.
Eccentricity refers to the changing shape of Earth’s orbit. You might know that it’s not a perfect circle—it’s an ellipse, which I’ll talk about in more depth later. What you might not know is that how elliptical it is—that is, how far it is from being a perfect circle—also changes a bit over time.
These motions are all very well established in science today. We know that they each have an effect on Earth’s climate, and together they cause the ice ages. But Milutin Milankovitch, the scientist who first came up with the idea, had a lot going against him.
Although, Ice Age is actually a pretty good example of what happens during a real-life ice age. I haven’t seen enough of the movies to really talk about how accurate they are, but I know there’s a lot of ice.
And a lot of breaking of ice.
These movies take place during a time when much of the northern and southern regions of the Earth were covered in glaciers. The world looked a lot like the satellite image up above. Whether mammoths and smilodons (sabre-toothed cats) actually lived then is another question entirely.
For the record, dinosaurs were definitely not still alive back then. Even deep under the ice. The quote from the third movie basically sums it up:
“I thought those guys were extinct!”
“Then that is one angry fossil…”
Yeah, they were extinct. But fiction can do whatever it wants.
But why do ice ages happen…and why isn’t the world covered in glaciers now?
As a born Californian, I never saw seasons this dramatically until I went to college in Flagstaff, Arizona.
I remember, in my first year here, when I was taking a walk around campus with a few friends. We passed over a riverbed where water was gently trickling along. Green grass and brush lined the banks. The sight absolutely captivated me. I had never seen anything like it, even in the springtime.
Then winter hit in all its blizzarding glory. At night, the temperature dropped below freezing. Snow fell in flurries that contrasted beautifully with the night. By morning, snow banks over a foot high lined the footpaths. To say nothing of the state of my winter jacket!
Summer in Flagstaff is hot. And I mean hot. It’s sweltering. Everyone crowds under the nearest tree. I experienced two days of it during orientation, and I never want to be here in the summer again.
Flagstaff’s autumn isn’t quite like the red and golden season depicted above. It pours. The rain sweeps down from the skies in torrents, soaking you through to the bone within minutes of being outside. Don’t think you’re safe under an umbrella. Better get some waterproof slacks to cover up those jeans, or you’ll be freezing in your classes all day.
I remember learning that my good blogging friend, the Momma, experiences the opposite seasons. When it’s pouring over here, it’s all green and sunny in Australia. When it snows here, she’s getting summer—I only hope it’s not as sweltering as it is in Flagstaff!
But why? Why should Australia have seasons that are opposite those in America?
Yes, they are named after mythical beasts and ancient queens, but for scientific purposes, all that matters are the regions they denote.This way, astronomers can easily find obscure, faint objects in the sky.
And telescopes can be easily programmed to find the same objects for those with less experience.
Keep in mind, though, that constellations only appear to fall in the same horizontal plane over Earth’s surface. Some of these stars, even in the same constellation, are light-years apart from one another.
So, in that case, the brighter stars must be closer to us and the dimmer stars farther away, right?
Wrong.
I know—not what you were expecting, was it? Well, like I mentioned in my last post, the brightest star in the sky is Sirius. It’s very far away from us. Proxima Centauri, on the other hand, is the nearest star to the sun…but it’s pretty dim, comparatively.
Fun Fact: Proxima Centauri is suspected to be part of a triple-star system also consisting of Alpha Centauri A and Alpha Centauri B. All three stars are gravitationally interlocked.
Proxima Centauri is the nearest of the three, making it the nearest star to the sun by a small margin.
The first thing we need to realize about stars is that they are not just pinpricks of light in the night sky. Shooting stars are about as close to being stars as are sea stars (more recent name for what were formerly “starfish.”)
I’ll talk more about stars and the way they produce heat in later posts—but for now, let’s just say that the hydrogen and helium gases that make up a star is so compressed and so excited that their atoms literally tear themselves apart.
That’s why stars are bright. They produce a ton of energy.
Anyway…because astronomers love labeling things in space, we have a way to describe the brightness of stars. Keep in mind that this only describes their apparent brightness—how they look to the human eye. Like I said, a star’s apparent brightness really has no bearing on its real brightness.
We use the magnitude scale.
This is a system that first appeared in the writings of the ancient astronomer Claudius Ptolemy. It’s likely the system actually originated earlier. Most astronomers actually attribute it to Hipparchus, a Greek astronomer.
According to the magnitude scale, stars are divided into six classes. First-class stars were the best and the brightest, just like the treatment you receive in the first class section of an airplane. And, naturally, sixth-class stars were the dimmest.
Well, actually, they were known as “first-magnitude” through “sixth-magnitude.” But I thought the class explanation made the whole backwards lay of the scale a bit simpler to understand.
I looked up first-magnitude stars on the Internet, and whadya know—I found a chart detailing the names and brightness of each one. It even cooperatively copied and pasted for me. So here you go!
Okay, so I’m guessing first-magnitude can also refer to stars whose brightness goes off the charts of Hipparchus’s system. That would explain the negative numbers. See, the fact is, there’s just too much variation in brightness in the sky to neatly group all stars under six classes.
In the modern day, astronomers use a scale numbered from -30 to 30. On this scale, the sun is -27. The full moon is at -13. When Venus is at its brightest, it measures at -5. And Sirius, the brightest star in the sky, measures at -1.47.
This is the apparent visual magnitude scale. Naturally, there’s an abbreviation for that, since I imagine it would get tedious to write out that the naked eye limit is about 6 apparent visual magnitude. We shorten it to “mv.”
So Polaris, the north star, measures at about 3 mv. The dimmest the naked eye can make out—the naked eye limit—is about 6 mv. The limit for the Hubble Space Telescope is 30 mv.
Naturally, there are dimmer stars than that, and the scale goes beyond 30 for the dim stars. But we won’t really need to talk about anything beyond 30. And there’s honestly nothing brighter than -30. You don’t get brighter than our own nearby sun.
Knowing the brightness of stars—at least their apparent brightness—helps us locate them in the sky. So, we already know how constellations help map out the sky. But how on earth can we keep all the stars straight? There’s trillions and trillions of them.
Easy. All we need to do is rank the stars by brightness within their constellations. We don’t need to worry about naming all of them. A lot of the brighter stars have Arabic names, such as Betelgeuse, Sirius, and Aldebaran. But there’s no way we can name them all.
And now you know why the stars of Orion have those peculiar little squiggles next to them in this diagram. Those are Greek letters. The stars are designated from brightest to dimmest in Greek.
So, according to the diagram above, Betelgeuse is ⍺, or alpha—meaning it’s the brightest star. Close behind is Rigel, β—or beta.
Even if you’re not familiar with the Greek alphabet—I had to learn it in sixth grade, but I never memorized the symbols—you can easily figure it out by ranking the size of the stars. This star chart has a magnitude key at the bottom that abides by the magnitude scale.
Now we know how to identify the apparent brightness of stars. But how do we refer to them by name?
Again, easy. Start with the Greek letter—you can spell it out or you can use the symbols. I thought I’d include a chart of them all for you here.
Keep in mind, when using the symbols for star designations, use the lowercase version.
After the Greek letter comes the possessive form of the constellation name. If that sounds complicated—especially given that they’re mostly in Latin—never fear, just look it up. For example, Canis Major becomes Canis Majoris.
So what would we call the brightest star of Canis Major? Alpha Canis Majoris, or ⍺ Canis Majoris.
Next up in my astronomy posts is the magnitude and intensity of stars. It’s going to be a little bit mathy, so feel free to skip it—I’m planning on releasing a post on the celestial sphere on the same day, and that’s going to be a lot more of just visual stuff.
The ecliptic, as astronomers call it, is the apparent path of the sun against the background of the stars in the sky.
It’s useful because it tells us how to find the planets in the sky. They can be hard to spot if you don’t know where to look, but they will always be somewhere along one imaginary line that arcs across the sky—the ecliptic.
This pattern never changes. The planets don’t follow the ecliptic exactly, but it’s useful for getting an idea of where they should be.
But why does it work—and what exactly does it mean, when it’s obvious we can’t see the sun among the stars of the night sky?
We have a bit of a shorter post today—I thought precession warranted its own post, before I go on to talking about the ecliptic.
Precession refers to the way Earth wobbles around on its axis, a bit like a top. This motion is caused by the sun and moon’s gravity tugging on the planet, and is key to understanding how many ancient cultures viewed the sky.
When you look up at the night sky on a clear, dark night, it seems as if the stars are glittering like bright thumbtacks on a great canvas above you. (You can get a similar effect–with less light pollution–from a planetarium like the one above!)
In reality, space is not like a canvas, and stars are not like thumbtacks. It would be more accurate to describe us Earthlings as floating in a vast, cosmic ocean.
Astronomers know this. But still, it’s helpful to map the sky in exactly the way it appears to us: as a sphere around the Earth. And so we use a model called the celestial sphere.
Telescopes operate solely based on the celestial sphere: the mechanism that aims the telescope doesn’t need to know anything about how far away an object actually is in the cosmic sea, just where it is in the sky.
That makes the celestial sphere a useful reference tool. Researchers need to communicate with telescope operators and say, “Let’s look over there now.”
And so, everything is mapped on a spherical model that pretends the night sky is a finite globe, inside which the Earth hovers like a bubble.
Can you find a horse in this image of the night sky?
Yeah, me neither. I’m lost. I see the Great Square of Pegasus because I know what to look for, but I still don’t see a horse.
Okay, so now I see half a horse. Where’s the rest?
Your guess is as good as mine, guys. Truth is, constellations very rarely look like what they’re named after. Constellations are more like relics of our ancient past than actual descriptors of what we see up there in the sky. But they do serve a purpose, even if that horse up there is missing his back legs.
I really find myself wondering if whoever made up Pegasus was concerned with animal rights… No animals were harmed in the making of this sky map…
Okay, yeah, never mind.
Where was I? Ah, that’s right. The purpose of the constellations, and the reason why it really doesn’t matter if they don’t look like their names say they do.
Cosmos at Your Doorstep 