29:51 Introductory Astronomy Lab

Spring 2001

Experiment 4

Week of February 19, 2001

Observations of the Sun

This week we will try and make up for some lost time by doing a daytime observation of the Sun in the Tuesday session. Then if it is clear, we will proceed with the planetary observations (Experiment 3) on Wednesday or Thursday night as well.

With regard to planetary observations, the Great Red Spot will transit the center of Jupiter’s disk at 7:29 PM Thursday night; perfect for observing!

The Sun

The Sun is the dominant object in the solar system, and our paradigm for a star. It therefore makes sense for astronomy students to examine this object. Obviously you do not look through a telescope at the Sun as we do with other astronomical objects. There are two safe ways of conducting these observations. The first is to have a guaranteed sunfilter over the aperture of the telescope. The second is to use the telescope to project the image of the Sun on a screen and examine the projected image. This is the approach we will be using.

First, make or install an opaque disk over the aperture of the Celestron 8”, which has a 1.5 inch diameter hole off to one side. The purpose of this diaphragm is to limit the amount of light that actually comes into the telescope structure. Next point the telescope at the Sun. The old-timer’s way of doing this is to look at the shadow the telescope casts on the ground. When it is round, you are pretty close to pointed at the Sun. Finally, project the image of the Sun on a white screen (it could be a piece of white cardboard). Focus the image by pointing the telescope at the limb of the Sun, and then turn the focus knob until it is a sharp line.

Orient yourself. You can move the field of view around on the Sun by moving the right ascension and declination fine controls. You should scan back and forth, up and down, until you have a pretty good feel for the “heliography” of the Sun on the day of the observations.

Observational Goals

There are three “tasks” you should set for yourself in these observations of the Sun. As in all of the observational exercises, accurate drawings and comments, times of the observations, etc should be written down in your notebook.

1. Note the number of sunspots, and (as best you can) accurately record their position on the disk of the Sun. Once again, the four cardinal directions can be determined by using the right ascension and declination fine controls.
2. Make a careful drawing of the appearance of the sunspots. If there are many, make sketches of at least the two or three most prominent groupings.
3. Repeat the observations one or two days later. Note how the sunspots have moved on the surface of the Sun. Note also if the sunspots have changed in appearance.
4. Note the edge of the Sun (the technical term is the limb) and think about the fact that what you are actually viewing is a sphere of incandescent gases.