From the September 2003 issue

Making a light curve from your observations

Investigate the structure of Saturn's rings as they pass in front of a star.
By | Published: September 2, 2003
Cassini wallpaper: Rings in UV
If you have access to a digital stopwatch and a short-wave radio or a time and weather receiver, you can make a light curve of the upcoming Saturn occultation. A light curve shows how the star varies in brightness over time, which is directly related to the ring’s structure.

One of the most accurate means of creating a light curve involves dictating your observations into a portable cassette recorder while recording the continuous time beats from the WWV, the time station out of Fort Collins, Colorado. Just tune your short-wave radio to 2.5, 5.0, 10.0, or 15.0 MHz; Canadian observers can tune into CHU from Ottawa at 3.33, 7.34, and 14.67 MHz.

The stopwatch comes in handy when the WWV or CHU signals fade (as they sometimes do). When this happens, start your stopwatch and keep track of the time until the signal returns.

Determine the longitude and latitude of your observing site to within 10 or 20 seconds (less than half a mile), as well as your elevation above sea level to within 10 feet. This information can be determined by carefully measuring your location on a U.S. Geological Survey topographical map of your area. These maps can be found at the local library or a map store.

It is most important that you respond quickly to brightness changes and other events by calling out your observations almost as they occur. You should be able to note an event within half a second of its occurrence, which is about as efficient as the eye-brain-vocal relay system gets.

As the star passes behind the rings, call out your observations as clearly and in as few words as possible. To keep the nonessential verbiage down and the reporting efficiency up, develop a code that allows you to gauge unexpected brightness fluctuations.

For example, you might say “peak” when the star seems to be very bright in the rings. If at some point it seems even brighter than the previous peak, say “peak 1.” As the star fades you might say “F.” If it fades further still, say “F1,” then “F2,” etc. When it becomes semivisible say “weak,” and when the star vanishes, say “zip.”

Once the star reappears, say “B” for brightening, and for subsequent brightenings say “B1,” “B2,” etc. If the star appears to flicker in brightness, say “flick.” If you decide your observation is in error, say “cancel” and then quickly revise it.

As soon as possible, plot your observations on regular graph paper. Divide the horizontal axis into equal time increments spanning the occultation and divide the vertical axis into varying brightness scales that correspond to your observations. As the tape plays back, note each observation as a dot in the appropriate time position. When you have plotted all your observations, you will see at once where the tenuous and dense regions of the rings are, as well as the gaps. Your graph will essentially represent a cross section of the ring structure.