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Sun’s rotation period
Student’s Guide – Intermediate Level
CESAR’s Science Case
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Sun's Rotation Period: A Student's Guide to Measuring Solar Rotation, Summaries of Astronomy
In this science case we will look at Sun images and try to determine how fast does it rotate and what is its rotation period. To do so we will track sunspots in ...
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2022/2023
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Sun’s rotation period
Student’s Guide – Intermediate Level
CESAR’s Science Case
Introduction
Like the Earth, the Sun moves. Sun’s main movement is a rotation over its axis. In this science case we will look at Sun images and try to determine how fast does it rotate and what is its rotation period. To do so we will track sunspots in time-spaced images.
Material
What will you need? = The “Sun’s rotation period” Student’s Guide. = CESAR’s Booklet. = Computer with Web Browser and Internet Connection. = CESAR web tools. = Calculator (physical or online such as wolframalpha.com) and paper and pen. = Or a spreadsheet program such as Open Office, Google Docs, Excel or Numbers.
Background
The Earth constantly rotates about it’s axis, this rotation is the reason for day and night. Like the Earth, the Sun rotates too, and it’s important to understand this movement. The features in Sun’s surface rotate together with the Sun, so if we manage to measure how fast this features rotate, we will know how fast the Sun rotates. The rotation period is the time that takes to complete one rotation, if you know how fast an object rotates its easy to obtain the rotation period.
Image 1: Sun picture aligned to a grid
Step 4 - Calculating speeds
Now that we’ve got the positions in degrees, just make the difference between the final position and the starting position, the result is the distance (in degrees) that the sunspot travelled. Now, we are ready to get the speed, the formula it’s easy: 𝑠𝑝𝑒𝑒𝑑 = 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒^ 𝑡𝑖𝑚𝑒 The distance, measured in degrees, is just the difference in longitude between two days. Then we need the time interval, that is just the difference in time between the two dates of the images expressed in days. To express the date / time difference in days you can use this formula 𝑑𝑎𝑦𝑠 𝑑𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒 +
Once you have the time difference in days and the distance in degrees use the speed formula and you will obtain the speed of that sunspot in degrees/day.
Step 5 - Rotation Period
We now know how fast the sunspot moves, but our goal is to obtain the rotation period, for doing so we can make use of this simple formula. 360º rotation period (days) = speed (degrees/day) Using the formula, you should finally obtain Sun’s rotation period.
Conclusions
In this laboratory you have studied Sun’s rotation. You’ve calculated Sun’s rotation period. For doing so you tracked the movement of a sunspot in time-spaced images and calculated its speed by measuring how much it moved between the images. Do your results make sense? Do your results agree with what you expected? When the results agree with the prediction, both are usually correct. If you do have obtain a consistent value, lets go one step further and try to use the results. Let’s say we locate a big solar flare at the left-edge of the Sun, this solar flare may cause solar wind which could produce a geomagnetic storm if it reaches Earth. Using your calculations of Sun’s rotation period, estimate how long would it take to the solar flare to be pointed at Earth.