Lab 4: Navigating the Night Sky
Part I: Observing the sky from spaceship Earth
We typically observe the cosmos from the surface of the Earth. This presents certain advantages, but also some difficulties. The advantage of course is that Earth is where we live, and can breathe. It’s pretty nice that most visible light sails right through the atmosphere, so we can peer into deep space while enjoying the comforts of home! The disadvantage is that since the Earth is really big (at least relative to humans), it blocks part of the view, even if you’re in a wide-open place with no buildings or hills in the way. People just aren’t tall enough to see “over the edge” of the Earth.
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Since we’re going to be learning how to determine latitude on Earth from the stars, let’s review the latitude/longitude coordinate system used to locate cities and other features on the Earth. Latitude is measured in degrees, from 0 degrees at the equator to 90 degrees north at the North Pole and 90 degrees south at the South Pole. Longitude is also measured in degrees, starting from 0 degrees at Greenwich, England and increasing to 180 degrees going east or west. For example, New York City has a latitude of 41 degrees north and 71 degrees west (being 71 degrees west of Greenwich).
We are going to use Google Earth to refresh/learn about the latitude/longitude coor- dinate system here on Earth. Follow the directions below to setup Google Earth to best answer the questions. Google Earth: https://www.google.com/earth/
1) After clicking on the link, it should take you to the intro page for Google Earth. Click “Launch Earth”
2) Once you go through the small Google Earth tour (or you can skip it), zoom out until you can see the entire Earth.
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https://www.google.com/earth/
3) On the left you should see a side bar. Click on the “Map Style” icon.
4) To see the latitude and longitude lines on Earth, scroll to the bottom of that menu page and turn on “Gridlines”. Once done, you can close the side menu by clicking the icon again.
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1. Use Google Earth to locate San Francisco and two other cities or regions of your choice. Pick at least one in the southern hemisphere. Estimate their latitudes and longitudes using the grid lines. Complete the sketches below for these locations by adding a small stick figure and a line representing the horizon at their location. Assume that the globe is rotated in such as way that your stick figure appears on the edge of the circle regardless of longitude. The horizon has already been drawn in orange on the diagram for San Francisco as an example.
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Part II: Locating stars (and more!) in the sky
The coordinate system that astronomers use to communicate about the location of an object in the sky uses two quantities: altitude and azimuth. It’s also referred to as the alt/az system.
The azimuth of a star is similar to its cardinal direction, but can be more precise. It is measured in degrees, from 0 degrees due north, to 90 degrees east, and increasing to 360 as it goes around. The circle below is a horizon seen from above. The cardinal direction north (N) is marked.
1. Add the cardinal directions east, south, and west (E, S, W) and then label each one with its azimuth value. Then add the cardinal directions NE, SE, SW, and NW, along with their azimuth values.
The altitude of a star (or any other object, like a planet, or a galaxy) is a measure of how far above the horizon it appears. A star that is on the horizon has an altitude of 0 degrees. A star that is as high in the sky as possible (straight overhead) is said to be at the zenith.
2. The drawing below depicts an observer somewhere on Earth. Imagine they are standing in the middle of a field and the visible portion of the sky is shown as a hemisphere (think of them inside of a snow globe). The zenith and the cardinal directions N, E, S, and W have been labeled for you. Label the horizon, and add the azimuth values to the cardinal directions. What is the altitude of a star at the zenith?
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To check your understanding of the altitude and azimuth coordinate system, watch the following video:
The drawing below depicts a star at an arbitrary location in the sky. Notice the dotted line indicating an imaginary line dropping straight down to the horizon from the star.
Beware!: In the video, the cardinal directions are different than what we have setup here. We have North to the right, while the video has North behind the observer!
3. Judging from this drawing, what is the approximate altitude of the star?
4. What is the approximate azimuth of the star?
Talking about star directions. Azimuth is useful for precisely describing a star’s position in the sky. But it’s more common in everyday communications to use cardinal directions. We will use both methods. Regardless of its altitude, a star (or planet or galaxy or any other object) is said to be “in the east” if it lies anywhere above the cardinal direction east (and closer to the east than to the northeast or southeast). Similarly, a star is said to be “in the southwest” if it’s anywhere above the cardinal direction southwest (and closer to the southwest than to south or west). A star is said to be “due north” if it lies directly above the cardinal direction north (or very nearly so). Even a star at altitude 70 degrees or more is “due north” if a line dropped straight down to the horizon would intersect the horizon at the northern cardinal point.
5. What is the cardinal direction to the star in the drawing above?
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Before moving on to Part III, watch the following video to refresh your understanding of the Earth’s coordinates and the Celestial Sphere:
Part III: The Direction and Altitude of Polaris
1. The small circle below represents the Earth. The north and south poles are labeled (NP, SP). Add a large circle centered on the Earth to represent the Celestial Sphere (use the whole page!). Then draw in the Earth’s spin axis, extending it up and down until it touches the Celestial Sphere. These are the North and South Celestial Poles; label them NCP and SCP. Finally, add Polaris to your sketch and label it. It should be where the NCP is! (Try to find it on your Star Wheel too).
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2. The Altitude of Polaris at the North Pole
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3. The Altitude of Polaris at the Equator
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4. The Altitude of Polaris at San Francisco
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5. Take a look at the results on the last three pages. What is the relationship between an observer’s latitude and the altitude at which Polaris appears in the observers sky?
Now suppose you take a trip to a place that you never thought to locate on a map. One night you are looking at the stars and recognize the Big Dipper. You use it to locate Polaris, as shown here:
Using your hands, you find that Polaris is about 7 fists above the horizon.
6. a) What do you conclude about where on Earth you are? (hint: we learned in Lab 2 how our hand can measure different angles in the sky)
b) Is this enough information to pin point an exact location on Earth? If not, what might you be missing?
7. Which direction is North? Indicate it by marking an “N” somewhere on the horizon!
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We’ve established how useful Polaris is, finally, let’s practice locating it. Find the Big Dipper on your Star Wheel, and notice the dashed line that extends from the two stars farthest from the Dipper’s handle all the way to Polaris. These two stars, Merak and Dubhe, are called the “pointer stars” because they can be used to point the way to Polaris. Notice that Polaris is a long ways away: nearly 30 degrees, or about 5 times the distance between the two pointer stars!
8. Add Polaris to the sketch below. Use the separation between the pointer stars to help you place Polaris correctly.
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