An Introduction to Naked Eye Astronomy: Annual and Daily Motion

At the beginning of the semester, just a few months ago, I paid very little attention to the night sky.  That all changed when I took the awesome Ancient Astronomies course at CU with professor John Stocke.  Although I'm definitely not an expert, I've learned a lot, and I love sharing it with you guys.  However, sometimes it can be tough to tell what exactly you are looking at, where you should look for something, or even how the sky changes throughout the day or year.  Hopefully this post, as well as any others I make, can help you figure things out.*

To further assist you in your understanding, I made a video supplement to this post.  Below is the video embedded within the post, but if for whatever reason that is not working, click HERE to check it out.  I refer to it several times later on in the post, and it just makes things easier to understand.  You can watch it as you read along.

First off, look at the picture below.  In it, we see the Earth encased in a plastic globe with the constellations printed on it.  For our purposes, that plastic globe with the constellations printed on it is called the celestial sphere.  Inside the celestial sphere, you can see a tiny little yellow ball: the Sun.  (I can't actually see it in this picture, but the Moon is probably inside the ball as well.)  Is that how the universe looks then?  Of course not.  We've known for many, many years that the sun is neither that close in size or distance to the Earth.  However, this model is how we can think of the night sky, especially when it comes to naked eye astronomy.  It shows, from where you are on the Earth's surface, what constellations the Sun and the Moon are in, and what constellations you can see from your specific point on the planet.

A lot of the time in naked eye astronomy, we say things that mean one thing while they sound like another.  Like what I just said: "what constellations the Sun and the Moon are in."  If we were to look up at the night sky and see the Moon smack-dab in the middle of a constellation, we realize that the distance between the moon and the stars is actually much greater than what it initially appears.  However, for our intents and purposes and to make things simpler, we just say "the Moon is in the constellation of...."  Just like when we talk about the daily motion of the Sun.  When we say that the Sun sets or rises, we realize that the Earth is the celestial body that is moving.  But from an Earth-bound perspective, it doesn't look that way.

Before we go any further, it's time to meet a very important star: Polaris, the North Star.  As you probably know, Polaris is called the North Star because it is situated directly to the north.  This star sits almost exactly on something that is called the North Celestial Pole.  Look back up at that globe for a second.  See that metal bar that sticks right through the middle of the Earth?  That's the Earth's axis of rotation, or how the Earth spins around.  And see how that pole intersects with the celestial sphere?  Those are the North and South Celestial Poles.

Confused?  I thought you might be.  I know I definitely was.  Stand up, find someplace where you can safely spin around, and start to spin, looking straight out in front of you.  After doing that a few times, look straight up.  Notice how when you looked straight in front of you, everything seemed to move, and move pretty fast too.  Now, when you look up at the ceiling, you can still see everything moving, but there is a single point that doesn't move at its center.  That single point is the equivalent of the North Celestial Pole.

Now, let's look at the video that I made to accompany this post.  We will start with "Part I: Polaris in Boulder".  Part 1 shows how the celestial sphere spins around Polaris over the course of a single night.  This part of the video takes place at 40° N, and is facing north.  "Part II: The Equator" shows the spinning celestial sphere from the point of view of someone standing slightly north of the Equator, just far enough that they can still see Polaris.  They are still facing north.  "Part III: The North Pole" tracks the daily motion of the celestial sphere from the perspective of someone who is lying on their back and looking straight up, situated at the North Pole.  Try and draw comparisons between what you see at the Equator and the North Pole; and what you saw while you were spinning and looking straight in front of you, and straight up.  While you watch all three clips, notice how in each one there are some stars and constellations that never set, with the fewest being at the Equator and the most being at the North Pole.  These stars and constellations are called "circumpolar."

So now it's time to introduce two different types of celestial motion: daily, and annual.  Daily motion can really apply to anything in the night sky, and is what we've just been looking at.  The three parts of the video taking place in Boulder, the Equator, and the North Pole were three views of daily motion at different locations on the planet.  Even with the video to help guide you, it can be difficult to absorb, so this might be a good place to stop and absorb what you have learned.

Excellent.  Now, on to the other main type of motion that we will discuss today: annual motion!  Annual motion applies to the celestial objects that move over shorts amount of time (i.e. not thousands or millions of years) against the backdrop of the celestial sphere: the planets, including the Sun and the Moon.  The motion of the moon amongst the heavens is arguably the most easily observed, as it changes constantly, and you can compare its relative position with that of the sun.  A discussion of the moon and its phases will take up an entire post though, so we can do that later.  For now, it's just important to recognize that the main objects that we see moving through the sky from night to night are the planets.  

There are two such planets easily observable in the night sky right now, and I fear that as I am writing this it might already be too late for one.  Venus and Jupiter are typically the brightest planets in the sky, with the former being quite variable in its brightest and degree of visibility throughout its journey around the sun.  Again, a topic for another time.  If it can still be seen in the night sky, however, look for Venus right after the Sun sets in the west.

I think I've used this picture like four times or something now, but it's the best one I've got.  The Moon is in a waxing crescent phase above Venus as the Evening Star, the bright object between the Moon and the mountains.  We will talk more about Venus and its varied locations in the morning and evening sky some other time.  This picture was taken in early November here in Boulder.  

So what did you hopefully learn in this post?  This post was really just a rudimentary introduction to how the sky moves each night, also called daily motion.  You should remember the term "North Celestial Pole," and remember that that is where Polaris is located.  You also briefly learned about what it means to be a circumpolar stars or constellation.  We just barely touched on annual motion, a topic we will delve into deeper later, as well as the phases of the moon.  In the next Intro to Astronomy post, I will show you how to locate several celestial objects and constellations, including The Big Dipper, Polaris, Orion, and Jupiter.  Until then, clear skies!

*I am writing from a latitude of about 40 degrees North, in Boulder, Colorado.  The information in this post can apply to anyone within that belt around the world.  For example, people in Beijing and Baltimore, both cities around 40 degree North latitude, will see the same thing every night as people in Bursa and Boulder.


Works Cited:

All of the videos here were made using footage from the excellent Stellarium app.  Check it out, it's free!