Unit 1 : Activity 1 : Talking About Things in the Sky : "Which Way is North?" : What's Up? : Unit Exam

Unit 1: How to Point to a Star
Unit 2: Where on Earth Are You?
Unit 3: Earth's Rotation and the Sun's Apparent Motion
Unit 4: Yearly Changes in the Sky
Unit 5: Seasons and Climate
Unit 6: Sundials
Unit 7: Navigation
Unit 8: Ancient Astronomy
Unit 9: Constellations




Talking About Things in the Sky

The stars and planets in the night sky look like they are all the same distance from us. In fact most objects in the night sky appear as small dots on a dome over our heads. However, these dots of light travel across the sky as the earth rotates so it can be very difficult for one person to tell another person where to look in the sky to see a star. But a system has been worked out that allows us to do this, which we will explore later.

Transparent celestial sphere with Earth visible at the centerFirst, since the stars appear as dots of light on a dome overhead, we can use a convenient concept of the "Celestial Sphere" to describe them. The Celestial Sphere is an imaginary sphere on which all celestial objects and important coordinate lines and points are imagined to be inscribed.

The image to the right shows the celestial sphere projected onto the sky. We say the stars and other objects in the sky have positions on this sphere.

We now need to cover some common conventions, or common ways people describe where they are on Earth.

Picture yourself on a ship in the middle of the ocean. The Horizon circles you in all directions and there is nothing in sight except the sun, a blue sky overhead and the ocean, a deep cool green. You have a magnetic compass in your hand and the needle in the compass lines up with the earth's magnetic field and points North. If you look due North, you are looking in the direction of the North Pole. If you were able to magically walk across the ocean and you kept going, you would eventually reach the North Pole of our planet.

So, the North Point is the point on your horizon closest to the North Celestial Pole. Naturally, this point is due north of you on the horizon approximately where the compass needle is pointing. Please note that there is some error in a magnetic compass which we will talk about later.

Your meridian runs from the North Point through the point directly overhead -- your Zenith-- to the South Point on the horizon. The South Point is of course the point on the horizon due South, or the direction that will lead you to the South Pole of our planet. As a trick to keep in mind, if you stand facing North, your right hand indicates East and your left hand, West.

Drawing of the observer's meridian and horizon


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The Horizon System

Using this Horizon System will be very useful for recording your observations accurately. Even before you learn about other coordinate systems, you should make note on each observation how high above the horizon the observed object was, and in what direction it was. "How high", measured in degrees from 0 degrees (at your local horizon) to 90 degrees (directly overhead at your local zenith), is called altitude.

The animated graphic below demonstrates altitude. The telescope is pointing to the left. The image on the right is the view through the telescope as it changes the altitude (i.e. moves up and down). Click play where indicated.

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To record direction, use north, east, south, west, northeast etc. until you learn to use azimuth. Azimuth is measured in degrees around the horizon (0 degrees when facing due North to 360 degrees after you have rotated in a circle once. North (N) = 0 through E = 90, S = 180, W = 270).

The graphic below is a different view of the same telescope, this time representing a change in azimuth. In this case you are looking down on the top of the telescope and as it rotates on its base, you can see the view change on the right.



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