University of Michigan - Department of Astronomy

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updated: 04/19/2000


Constellation Observing Outdoors

I yelp astronomy like a sun-dog, and paw the constellations like Ursa Major.
-- Mark Twain

Overview

Introduction

The ancients believed the stars to be fixed upon a great crystalline sphere in heaven. Though we no longer share this view, the celestial sphere concept is still valuable in navigation, and it can be a practical model of one’s universe. After all, everyone knows that in about 24 hours, the stars, sun and moon all rise in the east and set in the west. In this lab, you will locate constellations by their coordinates (both celestial and local), and estimate your terrestrial location from them as well.

Celestial, Terrestrial and Local Coordinate Systems

Three coordinate systems must be defined to predict the height of a star above the horizon: the celestial, based on stellar positions; the terrestrial, using Earth-based positions; and the local system, centered on you, the observer. The three systems are summarized below.

Celestial coordinates Right Ascension is measured east of the vernal equinox in hours, minutes, and seconds of time (0 to 24 hr).
  Declination is measured north or south (+/–) of the celestial equator in degrees, minutes and seconds of arc (–90o to +90o).
     
Terrestrial coordinates Longitude is measured east or west of Greenwich, England in degrees, minutes and seconds of arc (180o W to 180o E).
  Latitude is measured north or south (+/–) of the Earth's equator in degrees, minutes and seconds of arc (–90o to +90o).
     
Local coordinates Azimuth is measured east of the northern horizon, or your meridian, in degrees, minutes and seconds of arc (0o to 360o).
  Elevation is measured up from the horizon in degrees, minutes and seconds of arc (0o to 90o).

Angular Distances and the Hour Angle

Since the sky appears spherical, astronomers measure all distances 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 360o in 24 hours, or 15 o/hr. With your arm extended, the apparent length of your fist with thumb raised (a la Fonzie) is roughly 15 degrees or an hour. Your fist alone (across the knuckles) is about 10 degrees. The full Moon, a half degree, is roughly the width of your thumb held at arm’s length.

To know when an object will cross your meridian (reach its highest elevation, or "transit"), astronomers use the hour angle. This angle measures the difference between the right ascension (R.A.) on your meridian (also known as local sidereal time or LST), and the R.A. of the object of interest. For example, if a star transited 90 minutes ago, its hour angle (H.A.) would be 1h 30m. Similarly, if an object on the equator just rose, its H.A. would be –6h or 18h. One may use

HA = LST – RA , or HA = RA(on your meridian) – RA.


Questions

Orientation

  1. Begin by finding the North Celestial Pole, which is very near Polaris, the North Star. Polaris is a faint blue star at the tip of the Little Dipper. You will see it due north (azimuth = 0o or 360o), and roughly 40–50 degrees above the horizon. Using the protractor device provided, measure the elevation of Polaris. Make a few observations and have your partner(s) do some also. Average the values to get a final answer.

    Average elevation of Polaris =                     degrees.

  2. What is your latitude?                     degrees.

  3. Find a bright celestial object near transit (and not too close to the Pole) and estimate its hour angle. Record this value here, and refer to it at the end of the lab. Be sure to also include the time of the measurement.

    HA of                              =                     hours at          :         

Identifying Constellations

  1. Looking north, sketch the orientation of the Big Dipper and Cassiopeia with respect to the horizon and Polaris.
  2. Following your instructor, identify at least 6 other constellations. List them here. Put a check mark in front of those belonging to the Zodiac.

    1 5
    2 6
    3 7
    4 8
  3. If any planets or the Moon were visible, list them and in which constellations they were found. Are they the ones you labeled as being on the Zodiac?









  4. Pick one constellation and sketch it below in some detail. Use larger dots for brighter stars. Do not refer to a book. Feel free to use binoculars. Label the North Pole, horizon (N, S, E, W), your meridian, zenith, or anything else helpful for orientation.














  5. Estimate the azimuth and elevation of the brightest star in the constellation you sketched above. Mark that star on your drawing.

    Azimuth =                     degrees     Elevation =                     degrees

  6. Estimate this star's declination and right ascension. For the R.A., remember to begin by measuring its hour angle, and note the R.A. on your meridian (LST). The instructor may need to give you the LST.

    Declination =                     degrees                       minutes   R.A. =                     hours                     minutes

  7. Research an interesting object in this constellation (star, nebula, galaxy, etc.). On a separate page, describe it in roughly 100 words. 
  8. Now, go back and measure again the hour angle of the object you observed previously. What is the difference in their values? What is the difference in the times of your observations? What do you conclude about the results?