• Choose a month

  • Rapt in Awe

    My Journey through the Astronomical Year

    Think of this as a "companion text" to this, the main web site. Not required reading, butI hope you'll find it interesting and helpful.

Step 8 – Directions in the sky – sometimes east is west!

Knowing the major directions in the sky – north, east, south, and west – can be confusing, but it is easy if you remember these two rules:

The direction the stars appear to move is how we define “west.”
The direction from a star to Polaris is how we define “north.”

Notice that these are new definitions. We are not talking about the cardinal directions – north, south, east, and west – as they appear on the horizon, though these are closely related. These sky directions are a bit different because we are looking at a sphere from the inside – the dome of the sky. They are absolutely essential, however, for talking intelligently and usefully about where things are in the sky in relation to one another.

Terms such as “above” and “below” are relative and not always that helpful when trying to find your way around the sky dome. Instead, learn to think in terms of the cardinal directions, north, south, east, and west.

So face south. This puts east to your left and west to your right.

Hey that was easy! Yes it was.

Now face north. East is now to your right and west to your left.  Wow – there’s nothing to this! Nope. Nothing to it – until you look at the section of sky beneath the North Star.  Now east and west get flipped. Now – in the sky – west is to your right and east to your left!

Remember – this is the special case that applies only to objects that are below the North Star. This chart should help you understand why.

 

West is always the direction the stars appear to move as theyc ircle the north celestial pole, marked approximately by the North Star. (Click image to see a much larger version.)

 

Why the change in direction? Nothing has changed really. Remember Rule 1: The star always appear to rotate to the west.  Since they appear to circle the North Star – the North Celestial Pole really – then beneath it they will appear to move from left (east) to right (west.) It is confusing because the western point of the horizon is still to your left – but you are not dealing with the horizon – you are now dealing with the sky dome.

Now lets look at the second rule. It applies everywhere in the sky, no matter what direction you face. North is always toward the North Star. We’ll illustrate this by looking north.

North is always towards the north celestial pole, marked approximately by the North Star, Polaris.

North is always towards the north celestial pole, marked approximately by the North Star, Polaris. (Click image to see a much larger version.)

Meridian and celestial equator

Let’s examine this sky dome a bit more and draw some imaginary arcs on it. One we’ll call the meridian and the other the celestial equator. Here’s how they’ll appear to us as we look toward the southern horizon.

Facing south at latitude 42° North the celestial equator appears to cross the meridian at a point 48 degrees above the southern horizon. (The  double line marks the meridian. I'm not absolutely sure why it is double in this image, but that's how my version of the free "Stellarium" software represented it. All the image son this post come from Stellarium which can be downloaded here. (Click this image for a much larger version.)

Facing south at latitude 42° North the celestial equator appears to cross the meridian at a point 48 degrees above the southern horizon. (The double line marks the meridian. I’m not absolutely sure why it is double in this image, but that’s how my version of the free “Stellarium” software represented it. All the image son this post come from Stellarium which can be downloaded here. (Click this image for a much larger version.)

Think in  terms of two huge circles.

The meridian is an imaginary circle that runs through the north and south celestial poles. Think of it as starting at the North Pole, running up through the North Star, on over your head, and down to the point that is due south on your horizon.

The second huge circle is a little more difficult to locate precisely. It is a projection of the Earth’s equator onto the sky dome and is called the celestial equator.

The celestial equator runs from the eastern point on your horizon (due east) to the western point (due) west. But it does not cross directly over head. Instead it makes a huge arc and goes through a point on the meridian that is exactly 90 degrees south of the North Star.

Finding the celestial equator in your sky isn’t as difficult as it may seem. Let’s return to that half circle, the meridian. As half a circle, it totals 180 degrees. It starts at the northern horizon and climbs to the North Star. How many degrees is that? It depends on your latitude. Here in Westport, Massachusetts, it is approximately 42 degrees. That’s the first point on our imaginary circle – our meridian.

The next point on this circle will be the point directly overhead. We call it the zenith. That will be 90 degrees above the northern horizon – 48 degrees beyond the North Star.

Now head south from the zenith – the point directly overhead – and you will find that in an additional 42 degrees you have reached the point where the celestial equator crosses the meridian. At this point there are still 48 degrees of meridian left to take you to the south point on the horizon.  This is much easier to see in a simple diagram than it is to imagine from words. But remember this easy rule:

To find the point where the celestial equator crosses the meridian at your location, simply subtract your latitude from 90 degrees. The remainder is the height of your celestial equator above the due south point on your horizon.

Do this for Westport, MA, and you get 90 minus 42 equals 48 degrees.  So face south and use your fist as a measuring stick, estimate how high 48 degrees is. That is the point where the celestial equator crosses. To see the entire equator in your mind, draw an imaginary arc from due east through the meridian at this point to due west. That’s the celestial equator.

Do you get the concept? To test yourself, try to answer these questions without reading on.

If you were standing at the north pole – latitude 90 degrees, where would the celestial equator appear to you?
If you were standing on the equator – latitude 0 degrees – where would the celestial equator appear to you?

Think you have the answers? Ok, read them here.

At the north pole the celestial equator would appear to be a projection of your horizon running from east, through the due south point on the horizon, to west.  At the equator, the celestial equator would run from east to west, passing through your meridian directly over head – at the zenith. Confused? Return to the simple rules:

The direction the stars appear to move is how we define “west.”
The direction from a star to Polaris is how we define “north.”
The zenith is the point directly over head.
The meridian runs from due north to due south passing through the North Star and the zenith.
The celestial equator runs from due east to due west making an arc that crosses the meridian 90 degrees, minus your latitude, above the due south point on your horizon.

Exercise: Test you sense of direction!

Brilliant Vega is the brightest star in the constellation of the Lyre.  See if you can find the other stars as outlined in the image below. Or, find just the two that form a distinctive triangle with it. These may not be visible in light-polluted skies, but you should see them if you look at Vega with binoculars.

Click on this to get a larger image suitable for printing and using inthe exercise below.

Click on this to get a larger image suitable for printing and using inthe exercise below.

Your assignment is this:

1. Click on the image above and when you get to the larger version, print it.Draw arrows from Vega indicating the directions of North and of West.

2. While observing, determine the cardinal directions in relation to Vega and the associated stars, starting with North.

3. What then is the direction from Vega to Sulafat, the star in the lower right corner  – or from Vega to the star in the triangle that is below and to the right in this drawing.

Scroll down for the answer.

>
<
>
<
>
<
>
<
>
<
>
<
>
<
>
<
>

Click to enlarge.

The direction from Vega to Sulafat is roughly southeast. Click to enlarge.

Advertisements

One Response

  1. Greg, This has changed my astonomical life, you have made this so clear thank you very much!!!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: