During some part of the story that you are recording with your observations, you're going to begin to doubt your data for reasons that are impossible to predict here and now. You may come to believe that your observations should not be used, reported, or that they should even be thrown out.
Resist the temptation to throw out observations! Keep all of your observations. Remember, you don't need to report those for which you've lost confidence. It may be that some type of error has occurred and that your observations are wrong. It could also be that something extraordinary has occurred and you've accurately recorded it. When you have reason to question your data, indicate it in your log book but don't throw observations away. As with most things in life, you can learn from your mistakes when observing variable stars.
If you have truly made an error in your observations, try and figure out what you did wrong. Look at your estimates. Think about what you did. Review your log. If you can find your mistake, you can avoid it in the future. Otherwise, you may just make the same mistake again.
A spreadsheet is an excellent tool to analyze your variable-star data. Get to know the capabilities of your spreadsheet and how your observations can best be displayed. Charts and graphs allow you to visualize your data and make it much easier to understand. A column of numbers may look neat and organized but a scatter diagram can tell you an interesting story.
Spreadsheets allow you to display your information in many ways ranging from two-dimensional line graphs to three-dimensional contour maps. Selecting the proper display is important. If you've used spreadsheets before, and if you're familiar with statistics, then you know that you can support or refute almost any position using the same data. Because this is possible, you must be careful. Use methods that are familiar and accepted. If you're not sure what they are, ask. Present you observations with an honest intent to display your observations accurately.
With all of that said, you're probably wondering what you can do with your data and a spreadsheet. You're going to be very pleased with the possibilities! Now is when those lonely, cold nights of observing will result in something that you can show someone. You will be able to quantify you work and produce something that you can touch and see; something more than a number written on a page.
Light curves allow you to plot the changing brightness of a variable star over time. All that is required is to put the date and time (the Julian date) of your observations into one column and place your brightness estimate into another column. Make sure that you match the correct estimate with the proper date and time.
Usually, you'll have several choices regarding the type of chart you wish to use, such as a line, point or bar. In most cases, a point chart is best. This type of chart displays each brightness estimate as a point correctly correlated with the proper date and time. Within spreadsheet programs, these charts are sometimes called XY (scatter) charts.
Since your desire is to show a trend over a time interval, it's important to decide how you want your chart to look. Usually, the time interval is displayed along the horizontal axis (X-axis) and the brightness estimate is displayed along the vertical axis (Y-axis). Also, since you are going to compare observations made at uneven intervals (even a few seconds difference between observations can be important), the XY (scatter) chart is best. This chart plots data in which the independent variable (time) is recorded at uneven intervals. Your brightness estimate is the dependent variable because the actual brightness of the variable star depends upon when you make the estimate.
Light curves allow you to analyze the gross variations in brightness that a variable star is experiencing. You can see how a star varies over the course of seconds, minutes, hours, days, a month, a year or over the course of decades. Now it's time to learn how we can analyze the subtle variations experienced by some variable stars.
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