29:51 Introductory Astronomy Lab

Spring 2001

Experiment 1

Week of January 22, 2001

 

What’s up for this week?  This week we will meet in the daytime period at Tuesday at 12:30 PM. By way of preparation, select a notebook in which you will record all of your observations, calculations (when we do them), etc.  This notebook will also be used to keep your astronomical sketches.  Keeping this notebook is the main requirement of the course.  I will collect and comment on these regularly.

 

This week we will do three  things.

  1. I will talk about the relationship between the celestial sphere, celestial coordinates (Right Ascension and Declination), and the “horizon system”, that is the coordinate system in which we measure the position of things in the sky, i.e. specifying the azimuth and elevation angle.  Bring your SC1 chart!  I will really pound on you about starting to identify constellations in the sky. 
  2. We will make a simple scientific instrument that could have been used in the reign of Amenhotep II.  This is a string with which you can measure the angular distance between two astronomical objects, such as a bright star, and the Moon, etc. Details follow. 
  3. I want you to begin measuring the positions of the planets Venus, Jupiter, Saturn, Mars (when the spirit moves you; it is best observed before dawn these days) and the Moon.  How you are to do this is described below. 

 

String for angular measurement.  I will give you lengths of string.  If you go to room 665, there is a mark on the outer wall which indicates 8 degrees.  If you stand on the far side of the room (inner wall), this mark subtends 8 degrees.  Hold your string at arms length and hold your fingers at the points corresponding to the tips of the mark.  Either make knots here or put beads on the string at these two points.  Put another knot halfway between.  You now have knots or beads at intervals of 4 degrees.  Divide these to provide knots at 2 degree intervals.  Finally, copy this whole sequence over, so you have  the string capable of measuring a total angle of  24 – 30 degrees.  Make a notation in your notebook to the effect that you have made this string, and say what the resolution and total angular span is. 

 

Calibration of the string.  You now would like to test this string out on the sky, and make sure it is giving you the right answer.  Use your SC1 chart to pick some pairs of fairly bright stars that you can see in the night sky (like Rigel and Betelgeuse in Orion).  Use your string to measure the angular distances between them.  Then measure the distances on your SC1 chart, using a compass or ruler.  Compare the agreement between your measurements, and the “right answer”. 

 

Measurements of the positions of the planets.  This will be an ongoing project throughout the semester.  By the end of the semester, you will have directly experienced the phenomenon of the motion of  planets in the sky.  The basic datum for a given planet or the Moon will be the date, time, and Right Ascension and Declination of the object.  How do you do this?  I will leave it to you to get the date and time right.  Here is how you measure the Right Ascension and Declination. 

 

Finally, draw arcs of the proper angular radius centered on each of the reference stars.  Where the arcs intersect correspond to the position of the object.  For two reference stars, there will generally be an ambiguity because the arcs will intersect at two points.  You must use naked eye observations to resolve the ambiguity.  In the case of three or more reference stars, there should be no such ambiguity. 

 

            I would recommend Xeroxing parts of your SC1 for each of the planets and plotting the positions of the planets as the semester goes on.  The position of the Moon can be plotted directly on the SC1.  I want you to follow its orbital motion during the course of the month.  By the end of one month, you will already have an interesting result about the nature of  the Moon’s orbit.