Which Way is Up?

Seems like a silly question, right? Everyone knows up is... well, up!

That doesn’t work for astronomy, at least, not exactly. After all, for someone in China, up is down for someone in New York. And for someone in Hawai’i, both of those people are pointing sort of sideways. So who gets to say which way is up?

Obviously the answer is not as simple as it may have seemed. In fact, astronomers have given this question a lot of thought. Understanding their answer is the key to being able to do practical astronomy.

Up or Down?

We see that up in one place on Earth is down somewhere else, and sideways in another place. That is because by up we really mean away from the center of the Earth, and, of course, the Earth is shaped like a ball.

Imagine holding a perfectly round ball. How can you tell someone about a particular place on that ball, or a particular direction? You might talk about the top of the ball, the part that is furthest from the center of the Earth, but saying the bottom of the Earth doesn’t make sense. The deepest part of the Earth is the center. Keep going past the center and you are approaching the surface (the top on the side opposite from where you started. There is no bottom. Every point on the surface is somebody’s top.

So How Do You Describe a Place on the Earth?

You can talk about a point on the surface of the ball that is, say, one third of the way around, but there are lots of points one third of the way around in any direction, all located in a circle that goes all the way around. There is really no way to describe a particular place on that circle unless you have some kind of marks or bumps on the ball.

OK, let’s imagine we buried a treasure near a place that we are facing towards that is one third of the way around the Earth. We might tell someone that the treasure may be found twenty feet to the left of one third around the Earth from where we are facing. That seems pretty exact to us. But for the person who follows our directions, which way is left? It’s one direction if she is looking along the ball away from where we are standing, but left is the other way if she is looking back towards us.

This is not really much of a problem if you are standing on Earth holding the ball. But suppose the ball is just out in space somewhere and there is no Earth to tell us which way is higher or lower or left or right. We have the same problem with Earth, which is really like a ball out in space. We can’t exactly say where the top of Earth is. Like before, we may feel we are at the top, but someone on the opposite side of Earth would say we are at the bottom (and upside-down).

Axis and Equator

The Axis

One thing that solves this problem is the fact that Earth is rotating. As soon as you have a ball that is rotating, you have two special points on its surface: the two points that are not moving. Actually, there is a whole line of points that are not moving, the line that connects those two points and that runs from one point through the center of the ball (or of the Earth) to that other point. That line is called the axis of rotation or simply the axis. All the other points of the ball are rotating around the axis.

The Equator

Now that we have an axis, it is easy to talk about some other points. All the points that are an equal distance from those two special points on the surface (the ends of the axis) lie on what is called the equator. (It’s called equator because the distance from any point on it to one end of the axis is equal to the distance of that point to the other end of the axis.)

The Earth rotates. Everybody knows that. What most people don’t know is that this rotation is the reason, and the only reason, that Earth has an axis, an equator and two Poles.

The Poles

The poles are those special points on the ends of the axis, where the axis comes to the Earth’s surface. We have a North Pole and a South Pole. But which one is north and which one is south?

Well, the Earth rotates in a way that if you are anywhere that is moving around the axis (not exactly on a pole) the Sun seems to rise from a certain direction and set in a different direction. If you stand with your right hand toward where the Sun rises and you left hand toward where the Sun sets, then the direction you are facing is called north and the pole in that direction is the North Pole. The other pole, of course, is the South Pole.

North, South, East and West

To really understand more of this, you will have to understand this table. It may look complicated, but... well, it is. However, it is actually possible to understand all of this.

POINTER STAR LOCATIONS
Star Name	J2000 Right Ascension	J2000 Declination
Dubhe	11^h 03^m 43.67152^s	61° 45′ 03.7249″
Merak	11^h 01^m 50.47654^s	56° 22′ 56.7339″
Polaris	2^h 31^m 49.09456^s	89° 15′ 50.7923″

Here we go.

Do you see the Right Ascension heading in the table? What is that? Ascension is pronounced like uh-SEND-shun as in mention or extension. Ascend means to climb up, like ascending a flight of stairs.

How about Declination? Decline means to slope down.

That probably doesn’t help much... yet. But it will.

Notice how the Ascension entries are provided in terms of time: hours (h), minutes (m) and seconds (s). That’s an important clue. The stars seem to be moving all night (and all day also) as the Earth turns. In other words, as time passes the stars are in different positions in the sky. So Ascension has something to do with when a particular star ascends in the sky. When, of course, refers to time. That is why Right Ascension is measured in hours, minutes and seconds. It tells when a particular star appears and starts ascending up the sky... sort-of.

The Declination entries in the table are shown as angles (degrees°, minutes′, and seconds″). They tell how far down you have to look away from straight up to see a star... sort-of.

To really make sense instead of just sort-of making sense we have to know about the Prime Meridian.

But that’s another story.