22 Jul 2012
This week we cover another basic concept in astronomy, the Milky Way. And by the time we are finished you will either want to go outside and observe for yourself, or fire off an email. Or both. Here we go!
Since humans have walked the Earth they have observed a distinct band of light on the starry canopy overhead, one that can be seen all year long.
For the longest time no one knew what it was but it did make its way into countless myths. How could something that majestic and awe-inspiring not be included in the great stories?
We of course call it the Milky Way, derived from the Latin "via lactea," given us by the Romans. But what is it really?
Well, believe it or not, we didn't have a full grasp of the true nature of the band of light until these last centuries.
The great telescopes of the last couple hundred years first helped us here by seeing that the Milky Way consisted of literally countless stars which were just like the thousands of individual stars we see every night.
The fact that it was a band of light all around us in every direction implied at least that it was a huge disk of stars, not a random scattering. But were we in the middle of this disk or off to a side somewhere?
It turns out that some very clever men at the beginning of the 20th century discovered that we are actually not in the center; we are off to one side. The center of the starry disk is some 27,000 light years away, over near the constellation Sagittarius. But even though the center of the galaxy is far more populated with stars, it doesn't look that much brighter over there. Why? Because there is so much interstellar dust from eons of supernova explosions that it blocks much the light of those stars.
It was also discovered through some special laws of physics and the way light behaves that the disk was swirling around this center like water around a drain, and that the swirling disk had arms.
The glorious band of light we call the Milky Way is actually a spiral galaxy, seen edge-on, teeming with hundreds of billions of stars. We see it best during summer because our night skies are aiming towards the star-rich inner sanctum. During winter our night skies are directed towards the scanty outskirts.
Go out and take a look tonight after the Moon sets. Let your eyes adapt for 10-15 minutes then, starting in the southern skies where it is brightest, follow the great white way overhead into the northern skies. It is a stunning, awesome sight.
What's that? Your skies are too light polluted to see it? Try and get out to the desert this summer. In the meantime, you can email International Dark-Sky Association (darksky.org) and ask what you can do to reduce light pollution in your area before we tragically lose the Milky Way for this next generation.
08 Jul 2012
Most of the sky events you read about here take place in the evening when most of us can enjoy them without too much effort. But there is a phenomenon happening next Sunday (July 15) which will force most of us to wake up before the crack of dawn. It will be greatly worth the little effort it takes to drag yourself out of bed and look towards the eastern skies.
On that morning in the four o'clock hour, a satellite, an inner planet, an outer planet, and a star will be gathered in one place near the eastern horizon like some kind of celestial conference. Here are the attendees:
The Moon has finally made its way around our planet to once again meet up with the sun. We often see the crescent Moon when it is in the west, in the evening skies, the crescent facing towards the west. But to see the crescent facing eastwards means it is headed towards, not away from, the sun. In just a few days after this gathering it will pass by the sun and by next week will be a crescent again in the western evening skies.
Jupiter is the bright dot at the top of the quadrangle. The gas giant, the heavyweight champion of the sun's system, plods around the sun ever so slowly, taking 12 earth years to go around our star just once. We, with a faster, inside orbit, are swinging around to catch up with it now. Because of this, over the next months Jupiter will rise earlier and earlier each day until finally it is in a position where we can more conveniently see it in our evening skies.
The dimmer star on the right side of the formation is Aldebaran. This great star only looks weak compared to the other three because it is so inconceivably far away. Located at more than 65 light years away - over 380 trillion miles - it is an "orange giant" almost a hundred times bigger than our sun. It is the reddish "heart" of the bull Taurus. Aldebaran, like Jupiter, will rise earlier and earlier during the night until months from now Taurus' heart will beat in our evening skies.
Our last member, the ever so bright "star," is actually Venus. Remember just weeks ago she travelled by the sun, in front of it, for all to see. Now she is on this side of the sun blessing our dawns as the Morning Star. Because of orbit and proximity, Venus is at its brightest now, just in time for this exquisite encounter.
This is certainly not the last gathering - or "conjunction" - of planets and stars and moons, but it is certainly one of the prettiest we will have for a while. Try and make the effort to get up early next Sunday to see it. Get up, find it, heave a great sigh at its beauty, then get back to bed.
01 Jul 2012
When, on the 4th of July, you are celebrating our nations's independence by watching fireworks, some of us might be celebrating for an additional reason. We are at aphelion! Let me explain what that is and why we should be celebrating.
All objects in our solar system - planets, minor planets, comets, asteroids, the whole kit and boodle - all travel around the sun, not in perfect circles, but in ellipses. What are those?
Ellipses are all over the place. They are often referred to as ovals. Here is an easy way to see a whole array of ellipses. Look at a cup from directly over the top. The rim is a circle. But as soon as you start to tilt the cup, that circle becomes an ellipse. Keep tilting it and the ellipse becomes more and more what we call "eccentric" until you get to the point where you finally look at the rim from edge on.
All those shapes were ellipses and just about every one of them can be seen in the orbits of objects in our solar system. But our planet's orbit, although elliptical, is almost a circle - but not quite.
There is one more very important thing to know about this phenomenon before we celebrate. The sun is not in the middle of any of these elliptical orbits; it is slightly off-center.
For the sake of simplicity, if not for accuracy, draw a simple circle then move its center slightly off to one direction. In your mind, picture a tiny sun there. That drawing is similar to how we travel round our off-centered sun.
Now here is the thing to notice. Something traveling around your drawn path will sometimes be closer to the "sun," sometimes farther. The closest we get to the sun in our orbit is called perihelion, literally "near the sun." The farthest point, on the opposite side, is called aphelion, literally "away from the sun."
Our planet reaches aphelion on the evening of the 4th of July. So why celebrate? Thinking cap time!
The weeks we are farthest from the sun also happen to be when we are tilted most towards it here in the northern hemisphere. Thus, we get the most amount of sunlight when we are farthest from its heat! That means our summers aren't nearly as hot - nor our winters as cold - as they could be. Reason for celebration right there.
But even more to be thankful for is that our aphelion distance next week is only several million miles farther out than our perihelion distance in January. On the big scale of our solar system, that means we orbit in almost a perfect circle.
If our orbit were more elliptical, more oval-like, our close approach to the sun would give us unbearably hot daytime temperatures, possibly lethal on a global scale. Our aphelion, just six months later, would be dangerously farther and Earth would be deadly cold.
So while celebrating our independence next week, take a moment and be grateful as well for our awesome orbit.