Observing Projects
At the risk of repeating ourselves, we remind you to never look directly at the Sun, because it can easily cause permanent blindness. You can view the Sun safely without a telescope just by using two pieces of white cardboard. First, use a pin to poke a small hole in one piece of cardboard; this will be your “lens,” and the other piece of cardboard will be your “viewing screen.” Hold the “lens” piece of cardboard so that it is face-on to the Sun and sunlight can pass through the hole. With your other hand, hold the “viewing screen” so that the sunlight from the “lens” falls on it. Adjust the distance between the two pieces of cardboard so that you see a sharp image of the Sun on the “viewing screen.” This image is perfectly safe to view. It is actually possible to see sunspots with this low-tech apparatus.
For a better view, use a telescope with a solar filter that fits on the front of the telescope. A standard solar filter is a piece of glass coated with a thin layer of metal to give it a mirrorlike appearance. This coating reflects almost all the sunlight that falls on it, so that only a tiny, safe amount of sunlight enters the telescope. An Hα filter, which looks like a red piece of glass, keeps the light at a safe level by admitting only a very narrow range of wavelengths. (Filters that fit on the back of the telescope are not recommended. The telescope focuses concentrated sunlight on such a filter, heating it and making it susceptible to cracking—and if the filter cracks when you are looking through it, your eye will be ruined instantly and permanently.)
To use a telescope with a solar filter, first aim the telescope away from the Sun, then put on the filter. Keep the lens cap on the telescope’s secondary wide-angle “finder scope” (if it has one), because the heat of sunlight can fry the finder scope’s optics. Next, aim the telescope toward the Sun, using the telescope’s shadow to judge when you are pointed in the right direction. You can then safely look through the telescope’s eyepiece. When you are done, make sure you point the telescope away from the Sun before removing the filter and storing the telescope.
Note that the amount of solar activity that you can see (sunspots, filaments, flares, prominences, and so on) will depend on where the Sun is in its 11-year sunspot cycle.
Use a telescope with a solar filter to observe the surface of the Sun. Do you see any sunspots? Sketch their appearance. Can you distinguish between the umbrae and penumbrae of the sunspots? Can you see limb darkening? Can you see any granulation?
If you have access to an Hα filter attached to a telescope especially designed for viewing the Sun safely, use this instrument to examine the solar surface. How does the appearance of the Sun differ from that in white light? What do sunspots look like in Hα Can you see any prominences? Can you see any filaments? Are the filaments in the Hα image near any sunspots seen in white light? (Note that the amount of activity that you see will be much greater at some times during the solar cycle than at others.)
Use the Starry Night™ program to examine simulations of various features that appear on the surface of the Sun. Select Favourites > Explorations > Sun to show a simulated view of the visible surface of the Sun as it might appear from a spacecraft. Stop time flow and use the Location Scroller to examine this surface. (a) Which layer of the Sun’s atmosphere is shown in this part of the simulation? (b) List the different features that are visible in this view of the Sun’s surface. (c) Click and hold the Decrease current elevation button in the toolbar to move to a location on the surface of the Sun, from which you can look out into the chromosphere. (The Viewing Location panel will indicate the location on the Sun’s surface.) This simulated view of the chromosphere is at the color of the wavelength of hydrogen light. The opacity of the gas at this wavelength means that you can see the structure of the hot chromosphere that lies above the visible surface. Use the hand tool or cursor keys to change the gaze direction to view different features of the Sun, zooming in when necessary for a closer look at features on the horizon. (d) Provide a detailed description of the various features visible in this simulation of the Sun’s surface. You can see current solar images from both ground and space-based solar telescopes by opening the LiveSky pane if you have an Internet connection on your computer.
Use Starry Night™ to measure the Sun’s rotation. Select Favourites > Explorations > Solar Rotation to display the Sun as seen from about 0.008 AU above its surface, well inside the orbit of Mercury. Use the time controls to stop the Sun’s rotation at a time when a line of longitude on the Sun makes a straight line between the solar poles, preferably a line crossing a recognizable solar feature. Note the date and time. Run Time Forward and adjust the date and time to place this selected meridian in this position again. (a) What is the rotation rate of the Sun as shown in Starry Night™? This demonstration does not show one important feature of the Sun, namely its differential rotation, where the equator of this fluid body rotates faster than the polar regions. (b) To which region of the Sun does your measured rotation rate refer? It is this differential rotation that is thought to generate the magnetic fields and active regions that make the Sun an active star. Occasional emission of high-energy particles from these active regions can disturb Earth’s environment and disrupt electrical transmission systems. Examine this image of the Sun and compare it to real images seen in textbooks, the Internet, or from the links in the Solar Images layer in the LiveSky pane. (c) In particular, how does the distribution of sunspots and active regions on this image compare to the distribution of these regions on the real Sun?
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Collaborative Exercises
Figure 16-20 shows variations in the average latitude of sunspots. Estimate the average latitude of sunspots in the year you were born and estimate the average latitude on your twenty-first birthday. Make rough sketches of the Sun during those years to illustrate your answers.
Create a diagram showing a sketch of how limb darkening on the Sun would look different if the Sun had either a thicker or thinner photosphere. Be sure to include a caption explaining your diagram.
Solar granules, shown in Figure 16-9, are about 1000 km across. What city is about that distance away from where you are right now? What city is that distance from the birthplace of each group member?
Magnetic arches in the corona are shown in Figure 16-26a. How many Earths high are these arches, and how many Earths could fit inside one arch?