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.

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.