Day Lab Part 3: The Rotational Velocity of the Sun

In our third and final experiment, we set out to calculate the rotational velocity of the Sun. Our method for doing so was much more complicated and technology intensive than our previous two experiments. We used lab equipment and Excel to analyze Doppler shifts on opposite sides of the Sun by comparing sodium absorption lines and telluric lines. The telluric lines should remain constant because they are present in Earth's atmosphere. The sodium lines, however, will change because they move as the Sun moves.

In order to collect our data, we used a heliostat on the deck and mirrors within the classroom to focus light from the sun into the spectrograph. We then used a program to take photos of the spectrum at different points on the edges of the Sun. We decided that Top and Bottom were the best points for our analysis. Using Excel, we were able to calculate the length of the lines in terms of pixels and then see how much they shifted by. We first looked at the overall data that we collected, which includes the small telluric lines on the right side and then two sodium lines.

We then zoomed in to specific sections and normalized our curves in order to get usable data.

Data from the left sodium line

Data from the right sodium line

Data from the telluric line

As you can see, all of our curves fit quite nicely which implies that our data is relatively accurate (relatively being a generous term in our case). For example, our telluric line only had an offset of -0.2229314.

In order to get the rotational velocity from these two graphs, we took the quadratic values of the trend lines for our data for both the left and right sodium lines as well as their minimum pixel values. An already set up equation then used this data to calculate that the rotational velocity of the sun is 1.642988 km/s.