FirstLight Astronomy Club

33°29.6'N / 117°06.8'W / 1190 ft.

Moon Dance

Just a couple Saturdays ago, on the 19th, you may have witnessed the sublime site of Venus nearly touching a young crescent Moon in the western skies after sunset.

It was a gem of an experience. But if you went out the next evening to see it again, you may have noticed that things were not exactly the same, that the Moon was actually a little higher in the sky and had a slightly fuller crescent.

What's all this then? What you experienced is the movement of the Moon around our planet, movement that gives us those phases.

We are all familiar with the Crescent Moon and Full Moon and Quarter Moon (often called Half Moon). But few of us actually sit down and wonder why these things are.

We all learned in grade school that the Moon goes around the Earth. Many of us mistake its movement through the night sky, side by side with the stars, as the Moon orbiting around the Earth. But that movement is an illusion; it's just our planet spinning below the sky, the same fata morgana that "moves" the stars and the sun through the heavens.

The Moon really moves the other way, from west to east. But it moves slowly, very slowly, taking a whole month to complete one circuit. Test it! Go out tonight and notice exactly where the Moon is at a certain time. Go out tomorrow night at the same time and see that it has moved about a handspan eastward.

Now this journey is why we have all those phases of the Moon. Here's how. Imagine the Moon in the part of the sky right near the Sun. The lit side of course is facing the Sun, its dark side is facing us. We cannot see the moon because of this. We call it New Moon because a new cycle has begun.

Now let's fast forward two weeks, halfway through its cycle when the moon is on the opposite side of the sky, opposite the sun. Now it is fully lit from our point of view. It is a Full Moon.

The phases we are accustomed to result in the lighting of the Moon as it goes from New to Full and back again.

For example: In the next days after New, as the Moon passes by the Sun, it is at such an angle from us that we can just begin to see its lit side, but just barely. That is the crescent Moon, a thin one to be sure.

As the Moon moves around us it appears to be getting more fully lit. It is "growing" or, as we say, waxing.

After a week of a waxing crescent, it is now half lit from our point of view and directly above at sunset. Some call this half Moon. It is a quarter of the way around us, so astronomers call it First Quarter Moon.

It has lost now its crescent look. For the next week, until Full Moon, it will appear more football-like. This is called a gibbous Moon - a waxing gibbous to be more exact.

The Moon will continue to "fill out" as it goes to the other side of the Earth.

Once it is full, growing time is over. Now it is swinging around back towards the sun again, and its lighted part will appear to be getting smaller - it is now a waning moon.

After two weeks of waning from Full to Gibbous to Third Quarter to Crescent, the Moon will begin the cycle all over again.

Did you notice that during the full circuit that the lit part was facing the setting sun as it was waxing, and that it faced the soon-to-be-rising sun as it was waning?

None of these facts were lost to, well, probably every people group that has gone before us. Knowing the Moon and its phases was a great timekeeper and calendar source for a thousand generations of people.

Maybe for you, too! With just a little practice you can start predicting when what phases will be present. After a while you can put that watch away, you'll be able to estimate the time of the night just by looking at the Moon, just like our ancestors did.

Until next time, clear skies!

The Perfect Galaxy? The Milky Way

If you could give away one galaxy in our universe to as many people as you could, how many people would receive one before you ran out? Thousands of people? Hundreds of thousands?

It turns out that everybody on this entire planet could have his or her own galaxy. Wait! Make that many galaxies. Amazingly, there are enough galaxies out there that every man, woman, and child on planet Earth could call more than 20 galaxies their own. There are at least 100 billion of them out there, each with tens to hundreds of billions of stars.

Needless to say, the universe is big and chock-full of galaxies.

We have an idea of how big the visible universe is and approximately how many galaxies are out there, but no one can really comprehend the enormity of it all.

So at the risk of drowning you - and myself - in the vastness of space, let's just talk today about how galaxies are arranged and why the location of our own galaxy, the Milky Way, is so critical to life on Earth.

At the very beginning, over 13 billion years ago, just after the creation of the universe, what "stuff" there was in those early days began to clump under the influence of its own gravity.

Now back then there was nothing much more than hydrogen and helium, as far as the "normal" stuff goes. The other stuff would include dark matter, but that's for another day.

This hydrogen and helium could clump together to form the first stars. Those first fields of stars would be like wannabe galaxies, things we might describe as misshapen starry blotches in the cosmos.

Through mechanisms still not well understood, these first mini-collections of stars could attract each other into enormous collections of stars - the first recognizable galaxies.

All this clumping and coming together meant that a lot of the universe became uninhabited, that is, vast voids opened up between those clumps. The universe would start to look like a map of our country – far-reaching stretches of emptiness lined through with roads of small burgs and large metropolises.

Some of those monstrous galaxy factories could churn out 1000's of galaxies in a particular area of the universe, a collection we humans would later call galaxy “clusters.”

We ourselves belong to a huge cluster of clusters called - to no one's surprise - the Local Supercluster.

Now this would be nothing much more than some interesting space trivia, except for the fact that our place in this supercluster is supercritical to life on this planet. Why?

We do not want to be in the middle of those clusters, believe me. Those seemingly innocent galaxies are not just sitting around doing nothing. They are close. They are interacting. They are gravitationally attracted to each other. They are tearing each other apart.

But aren’t they really far, far away from each other? Not really.

The average distance between the stars within those galaxies is enormous, to be sure. If an average star were the size of a grapefruit, the next nearest grapefruit star would be on the other side of our country. There is puh-lenty of space between them.
But if the average galaxy were grapefruit-sized, the next nearest grapefruit galaxy would just be tens of feet away. That’s very close and leads to a lot of intergalactic harassment.

We should be thankful that the Milky Way galaxy, our home, is sitting out here in the distant suburbs of our cluster. Here we are practically alone. Here we can retain our incredibly critical spiral shape. There are no other nearby biggies that will rip us up into galactic shreds. And life on this little planet can flourish.

Again, even when we aren’t looking for it, we inevitably find another way our planet is perfect for life. Earth is placed perfectly in a perfect type of galaxy in a perfect location on the outskirts of a perfect cluster. What a wonderful world!

Until next time, clear skies!
Temecula Valley High School / Temecula, CA · Some images © Gemini Observatory/AURA Contact Me