Amateur astronomers usually consider themselves creatures of the night, since that is when the stars come out. But one star, our most important, most impressive star, the Sun, is visible at a much more convenient hour. The Sun bathes the Earth in life-giving light and heat as we orbit a mere eight light-minutes away. By comparison, light from the nearest star beyond the Sun takes more than four years to reach Earth. Astronomers can learn much about the distant nighttime stars by studying the characteristics and behavior of our own, daytime star. For amateur astronomers, viewing the Sun with a telescope is both interesting and fun!
Rather than appearing as a point of light as all other stars do, the Sun displays a disk half a degree in diameter, large enough to reveal fine detail on its visible surface. But with the Sun being so close and its energy so intense, it must be observed cautiously to prevent it from damaging both our equipment and our eyes. The Sun should never be viewed directly without first exercising precaution (except during the short span of totality during a total solar eclipse), so it is critical to know how to look at the Sun before you try.
Extreme care is necessary when viewing the Sun. The intensity of its light, when focused by even the smallest lens, is strong enough to ignite paper. The retina of an unprotected eye will be instantly destroyed, causing permanent blindness! Never look directly at the Sun without a proper solar-protection filter.
Amateur astronomers usually use one of two methods to view the Sun safely. The first and simplest uses a telescope or binoculars to project the Sun?s image onto a white screen. Move the screen closer or farther from the telescope to adjust both image size and brightness. Always try to tilt the screen slightly so that it is not in direct sunlight, but rather in shade, to increase image contrast.
A second way to look at the Sun is with a solar filter. Proper solar filters are designed to fit over the front of a telescope or binoculars. By dimming the Sun?s rays before they enter the instrument, the dangerously high levels of solar radiation and heat are reduced, preventing permanent damage to both observer and optics.
Never place the filter between your eyes and the eyepiece, since it will be quickly destroyed by the magnified solar energy. Many less-expensive telescopes once came with solar filters that screwed into an eyepiece (a few still do!). They are extremely unsafe, sitting right at the focus point of the light rays. The tremendous heat produced there can crack the filter, instantly frying your eye. If you have this kind of solar filter, discard it immediately.
Projection is wonderful for showing the Sun to a group of people all at once, but usually fails to reveal the fine level of surface detail visible with a filter. Filters provide a more detailed view, though they cost more and allow only one person at a time to view.
Another safety warning: Never look through the finder scope when aiming a telescope at the Sun. In fact, you should cover the front of it with an opaque material just to be safe. Crosshairs exposed to sunlight can melt in just a few seconds, and burns or blindness can result from unintentional exposure of your eyes to light passing through the finder.
Instead, keep an eye on the telescope's shadow on the ground as you move the tube back and forth, up and down. When the tube's shadow is shortest, the telescope should be pointed at the Sun.
What Can You See?
Both viewing methods show the Sun's photosphere, the visible layer of the Sun that produces sunlight. Scattered across the photosphere are dark markings called sunspots. Scopes as small as a 60mm refractor will reveal them. A close look shows that larger sunspots have a darker, central area, called the umbra, surrounded by a lighter region called the penumbra. Single spots can form, but usually spots appear in groups and clusters.
Try keeping a daily sunspot log, noting their number, sizes, shapes, and grouping patterns with pencil diagrams. Track their migration across the Sun's face as it rotates on its axis once every 3-1/2 weeks.
Sunspots are not permanent features on the photosphere, but instead, change in shape and size from day to day. Galileo was first to notice that spots move across the Sun. From his observations in the early 17th century, he inferred that the Sun rotates about once a month. It is now known that the Sun's equator takes 25 days to turn once on its axis, while the poles require 36 days.
The number of sunspots is always changing, increasing, then decreasing over an 11-year period known as the sunspot cycle. During peak activity ? solar maximum ? there may be dozens of sunspots visible at the same time, while at solar minimum, there may be none at all. We are nearing peak solar activity right now, with the crest in the current cycle forecast for the spring of 2000.
The exact cause of sunspots remains a mystery, but astronomers know that they are associated with irregularities in the Sun's magnetic field. These irregularities lower the temperature of the Sun in their immediate area by as much as 1,500 degrees Celsius, forming sunspots. Appearance to the contrary, sunspots are not really dark. They only appear dark in contrast against their hotter, brighter surroundings. In reality, they are hotter than the surfaces of many stars.
Other Surface Features to Look For
Finally, when using a filter to Sun-watch, look carefully along the solar edge, or limb. Notice how it appears slightly dimmer than the center of the disk? This effect, called limb darkening, is caused by our looking through a thicker layer of the Sun's atmosphere toward the edges than toward the center. With sharp optics and a good eye, you might see some small, brighter areas along the limb. These are called faculae, and mark elevated regions of hot gases. Some observers may also notice that the Sun's surface looks "grainy," an effect called granulation. Each granule is a continent-sized cell of heated gas rising from the core of the Sun.
The Sun has a lot to offer those who want to enjoy the science and hobby of astronomy during daylight, too. And with solar max now upon us, it is a great time to meet the star of our sky show.