From our vantage point -Earth - we can see the phases of the moon and planets, and enjoy the shows when they line up in different patterns. As I've been writing this, in late Winter of 2016, Mars and Venus have been drawing closer together every evening, and once a month the Moon passes by them - it's a real celestial treat. Two years ago, Jupiter and Venus did the same dance, the difference being that Jupiter was about 5 times as far away as Mars is this year.
This isn’t just techno geek stuff - being able to see the phases of the planets in their different positions and distances really helps in “getting” the dimensions of the solar system. The planets go through the same phases as the moon, but not quite in the same fashion. Back in the 7th grade, I did my own mock ups (fun to do - try it at home) of the Sun and planets in our garage using a light for the Sun and a globe representing Earth, as I circled around with a neighbor's volleyball. You can do the same thing yourself, and you don't need a garage - any dark room will work.
So. I know that you can watch computer mockups of the solar system, and see gorgeous images of the wonders of the universe. Good - more power to them. What we want here, though, is to understand those ratios and phases, so we can get a feel for the size and structure of the solar system when we’re outside with our feet on the ground looking at the real thing. It’s a lot like getting the feel of a vehicle you are driving. So the images here are maps.
The planets go through the same phases as the moon, but not quite in the same fashion. One thing you should see right away is that the inner planets - Venus and Mercury - can never really be seen in their “full” phases. Since they orbit between us and the Sun, they're full when they are on the other side of the Sun - behind it. Good luck seeing that! Then when they emerge from behind the Sun, they are almost full, but gradually shrink into crescents - just like the Moon - until they come between us and the Sun. And of course, they never rise too high in the sky. Mercury barely gets a little distance from the Sun, and Venus never gets directly overhead, about halfway at best.
The outer planets - Mars, Jupiter, Saturn, Neptune, and Uranus - almost always look close to full, because we on Earth are inside their orbits, and we are relatively close to the Sun. +This is where we can start seeing the ratios between the planets / in the solar system. In the Spring (Winter, etc) of 2015, Venus was about 50 million miles away and Jupiter behind it was about 500 million miles away. (Spring 2017 features Venus and Mars) We had great views right after sunset, when the Sun had set, but was just below the horizon. There was a workable ratio: at that point in time, Jupiter was roughly 10 times as far away as Venus. We were looking past the hub of the solar system (the Sun), and a good distance toward the far side.
Spring 2017 featured Venus and Mars in almost the same positions, with Mars being 2 times as far away as Venus.
Fortunately, the orbits of almost all of the planets show up in the same narrow band in the sky, from our point of view. That means that they are always lining up with one or another, making interesting patterns in the night sky, combining sometimes with the moon and stars.
Beyond that is infinity, sprinkled with stars.
Okay. Those of you who are astronomers, astrologers, or just star gazers who like to look at the night sky know most of this already. From what I've seen, most textbooks and descriptions don't appreciate that the ratios give us a handle that we can use to grasp the dimensions of our celestial neighborhood.
The point is to show that even though the distances might seem hard to visualize, the ratios are pretty simple; on the level of 1/10 or 1/100. Well, and to encourage everyone to get outside at night and look at the planet Earth, the solar system you live in, and the star groups you are part of. The visual images here show the same ratios and relationships that these heavenly bodies have, but they are just maps. Use them for orientation or perspective, get some binoculars or a small telescope, and go outside and look.
Next step: that sprinkling of stars isn't infinite, it's actually our galaxy, with a well defined structure and dimensions. It is the best panorama you can see,* but a lot of it is faint, so you have to stay under a clear dark sky for a while, to let your eyes adjust. Everything you can see with the naked eye is part of the Milky Way, with three exceptions: To the North is the Andromeda Galaxy, which is the most distant "object" we can see, and in the Southern Hemisphere you can see the Magellenic Clouds, which are two small galaxies that might have been captured by the Milky Way. (From what I gather, astronomers aren't sure yet if those little galaxies have been captured by the Milky Way's gravity, or if they will escape and pass on by.)
There are two problems with seeing the whole Milky Way. One is that it is so faint. It takes a long time (about 45 minutes) for your eyes to adjust and let enough light come through. The photos and visual images of galaxies that you see in books or online aren't snapshots - the camera lenses have to be left open for quite a while before enough light accumulates to form good pictures. Your eyes don't work quite the same way, but the effect will be the same - the longer you let them adjust, the more you will see.
Just as bad, there are dust clouds in the Milky Way that block some views, so it looks like separate bands, or streaks, that stretch across the sky. Even though the bands stretch along in the same directions, it's still hard to piece these fragments together and see the whole picture. Take a look at Mellinger’s Milky Way Panorama, or any picture of the galaxy, and you’ll see the dust clouds clearly. They look a lot like clouds here on Earth, especially when they hide the Milky Way’s bright center, just like rain clouds blocking a sunset. Keep in mind that we are inside the Milky Way, which means that it extends behind us, too. When you're looking at the center, the galactic profile is pretty obvious, but the further you look away from the center, the more you're seeing the outer edges. In Mellinger's Milky Way Panorama, the left and right sides of the image should actually curve around to the north and south respectively, and connect behind us.
We also have to contend with 1) light pollution on Earth, and 2) the fact that our views tend to be blocked by city buildings, hills and mountains, trees and such. You need a pretty wide view in order to fit all of the visible pieces of the galaxy into one cohesive whole.
For this view you can't do a mock up in your garage. It helps to see some of the photos of the Milky Way first, though, so you can see how big the clouds are that block a lot of the light. When you look at the Panorama you can see where the dust clouds are. When you're outside you won't need a telescope, or binoculars, but you do need a clear sky and a wide view - from horizon to horizon is best, but work with what you have. The bands that define the galaxy extend all the way to the northern and southern constellations, and you end up looking past the edge away from the center. The center of the galactic lens is in Sagittarius, which is about 30 degrees south of the celestial equator. If you can get to South Africa, South America (Chile or Argentina), or Australia, the center will be directly overhead, with the bands stretching away on opposite sides. It's the best - the most inclusive view you can get.
This is easy to see in the summer. In June, when the Sun sets, the constellation Scorpio will already be up, moving from southeast toward it's zenith in the south. Scorpio is called the scorpion because that is what it looks like, with claws held aloft and a huge curving tail with a barb at the end. Just past the barb is a teapot looking constellation, which is Sagittarius. Right there, between the barb and the teapot, is a bright patch of light and stars which is the Milky Way's center. Remember that the vista extends across the sky - the center is just part of the picture.
The longer you stay, the more your eyes will adjust and you'll see even more, of course. Eventually, the accumulated light will take over, and you will lose track of where Scorpio and Sagittarius are. About then you should see the galaxy stretching from horizon to horizon.
O my brothers and sisters … May you grok the fullness.
It will likely be cold. Bring warm clothes, or a sleeping bag.
* You thought I forgot, huh? Well, the Milky Way is part of the best panorama we can see, but not all of it. When we look at the Milky Way, we can also see three of the four galaxies closest to us.
Milky Way 100,000 - 120,000 light years across
Large Magellanic Cloud 160,000 light years away 14,000 light years across
Small Magellanic Cloud 200,000 light years away 7,000 light years across
Andromeda 2,500,000 light years away 220,000 light years across
There is an even closer companion galaxy in Sagittarius, but it's so small and obscured that it's really hard to see, so I'm not including it here.
The Magellanic Clouds are just a little more and a little less than 1 /10 the diameter of the Milky Way, and they are about as far away as the Milky Way's diameter.
The Andromeda Galaxy is close to twice the size of the Milky Way, and about 11 times as far away as its diameter. That's not far at all, relatively speaking. Our neighborhood is crawling with galaxies!
In the Milky Way Panorama, the Magellanic Clouds are down toward the lower right. The Andromeda galaxy is to the left, visible as a small bright oval a bit below the plane of the Milky Way. We only see its center in the image, but time exposures show that it extends outward several times larger. Standing on Earth and looking at it with unaided eyes, it would appear to be about as big as your hand, held out at arms length. Without a time exposure, it will look the same as it does in the Panorama, though - only the small bright central oval shows up.
Try stretching out your arms so that your left hand cradles the Andromeda galaxy, and your right is holding Sagittarius and the Milky Way's central core. Or, in the Southern Hemisphere, stretch from Sagittarius to the Magellanic Clouds. You now have a a fair grasp of the neighborhood. It is all us.
How's that for depth of field?
Ah, to sit on the edge of the galaxy and dangle our feet in the cold depths of intergalactic space. We can, you know. Well, almost. Since we are inside our own galaxy, we'll have to go with the cool depths of interstellar space. But we can see into the intergalactic realm, at least.
I appreciate a great view as much as anyone, but we don't have to sit on the edge of the Grand Canyon, or view the Hawaiian islands from the summit of Mauna Kea. Watch just about any sunset when you have a clear view of the whole sky. It's inspiring how these views expand our vision, and stretch our minds to try to see everything. Our sight takes in dimensions that seem to outline infinity, and we devour the space with our minds.
It can be a lot of space. The view from just about any mountaintop on a clear day can include hundreds of cubic miles, and objects that appear in the sky add dimensions we never expected. Even the area inside a closed classroom can get our minds enthralled.
I was awed when Mr. Haugland, our seventh grade science teacher, demonstrated how the phases of the moon depended on where it was as it travelled around the Earth. He turned off the overhead classroom lights, closed the blinds over the windows, and turned on a bright lamp on his desk. He had a student - Hans - stand in the center of the classroom and hold a globe of the Earth over his head. The lamp on the desk represented the Sun, Mr. Haugland explained, and Hans' globe was the Earth. He (Mr. H) held a white volleyball up over his head and said it stood for the moon.
"You see," he said, "that the Earth that Hans is holding up is bright on the side where it's lit by the Sun - the light on the desk - and dark on the other side. It's the same for my ball here - the Moon - it's lit on the lamp side and dark on the other side."
He was standing between his desk and the wall on the left side of the classroom, and we could see clearly how both the globe and the ball were half lit and half dark.
"Now, this stays the same while the moon - my volleyball here - moves around the Earth." He started walking along the wall, toward the back of the room. "The ball is always bright on the Sun side and dark on the other side, but when it's close to the Sun you could only see a slice of light, but now as we get about one fourth of the way around the earth, you can see half of the lit side. We call this the first quarter, because it's one fourth of the way around the Earth.It might seem confusing, because it's not saying how much we can see, but how far it's gone around the Earth.
He continued walking to the back of the classroom. "Then when it's halfway around, we can see all of the lit up side. Does anybody know what we call that?"
"Full moon, full moon," called half a dozen voices from the dark.
"Very good, very good." Mr. H's smile lit up too, not quite as bright as the "moon."
He continued along the back wall of the classroom, then followed the right side wall toward the front. Stopping halfway, he held the ball out conspicuously so we could see the light and dark sides distinctly.
"And what do we call this phase?" he asked.
"Third quarter!" Voices in the dark.
"Right," Mr. H verified. He walked back to the front of the room, and the lamp on his desk. "Then, when the moon gets too close to the Sun, we can't see the bright side at all." He crossed between the lamp and our seats, and all we could see was the back, dark side of the moon.
"Then, when the Moon passes the Sun, it starts the cycle all over again, and it's called a New Moon. You probably guessed that already."
One more time, he circled the classroom holding up the volleyball so we could all see how both the "Earth" and the "Moon" were always half light and half dark.
As seen from the Earth, and for us kids in the classroom, when the Moon was on one side or the other of the room it was half full, and when it was at the back of the room (behind the Earth) it was completely lit up - a full moon. The only element missing was the numbers, Mr. H said. The moon was roughly a quarter of a million miles away, and the Sun was approximately 93 million miles away.
Gosh. 93 million miles, right there in a 40 foot square classroom.
That night I got to see the real moon, which was pretty close to the first quarter phase, at a 90 degree angle from a line between the Sun and the Earth. A quarter of a million miles away - I didn't have to imagine that distance, I was looking at it. One million miles was 4 times that, which made the Sun - 93 million miles away - almost 400 times as far away as the Moon (372 times). These were ratios that I could grasp.
Ratios that we can grasp. With that thought in mind, we can expand our views to include the rest of the solar system, and our galaxy, the Milky Way. We don't have to wonder what million of miles look like - they are right there, in our night time sky.