Angular Distances and the Rotating Sky

Because the sky appears spherical, astronomers measure distances in the sky as angles. For example, from east to west is 180 degrees and from the horizon to zenith is 90 degrees. Notice that the sky rotates a full 360^o in 24 hours, or 15 degrees/hour.

Measuring Positions on the Sky

Astronomers have invented coordinate systems to specify the positions of objects in the sky in the same way that map-makers created a system of latitude and longitude to specify locations on the Earth.This global reference system is called the celestial coordinate system.It is useful for astronomers in communicating the locations of celestial objects between observers at different locations on the Earth.

As you saw in the Celestial Sphere lab, the same object can appear at different places on the sky depending on the location of the observer.To determine where we should look to find a particular object, we need to translate from the celestial coordinates of an object into its local coordinates, centered on the observer.

Azimuth is measured from due north - your meridian - towards due east, along the horizon in degrees, minutes and seconds of arc (0^o to 360^o).

Elevation is measured up from the horizon in degrees, minutes and seconds (0^o to 90^o).

If you are working on the assignment at home, you will need to download and install the version of Stellarium appropriate for your operating system from http://www.stellarium.org. Alternatively, you may use the purely web-based version of Stellarium at the same location.

1. What is today's date?What time is sunset in New York City today?

Stellarium will sometimes place you in a random location not your own.Make sure Stellarium's location is set to New York, NY.If not, use the Location tool on the left-hand menu to change your location setting.

Stellarium sets its date and time from your computer's clock, but we can change this.Click on the icon on the left-hand menu to open the date & time window.Set the time to 1^h30^m after sunset.You now see the sky as it appears at the end of twilight.

You will begin by "facing" to the south.Find a bright star somewhere to the east of due south and click on it with your mouse.

2. Clicking on the star opens up a host of information about the object.The name of the star appears in the upper left corner of the screen.Record the name and Stellarium's configured time (bottom center of the Stellarium window).

Name: _____________________________ was observed at ______________ PM

Now click on .This displays the local coordinate grid.Use the grid to measure the elevation (height above the horizon) and azimuth (east-west position) of your star.

a.Elevation:_________________ degrees

b.Azimuth:_________________ degrees