SOLAR ECLIPSES†

 

A Solar Eclipse is the obscuration of the Sun by the Moon. At which lunar phase does a solar eclipse occur? New Moon – because the Moon must be in the same direction in the sky as the Sun. We are quite fortunate that the angular sizes of the Sun and Moon are both nearly equal. There is no science behind this, it just turns out the ratio of diameter-to-distance is about the same for both.

 

There are three varieties of solar eclipses: partial, total, and annular. The total and partial eclipses are directly analogous to the lunar cases, i.e., the Sun is either totally or partially blocked by the Moon. Because the orbits of the Earth and Moon are not exactly circular, the distances and angular sizes of the Sun and Moon vary. Although the change is not much, one can have the occurrence where the angular size of the Moon is less than that of the Sun. So, even if the centers of the two bodies are aligned, the Moon does not completely block out the Sun. This is an annular eclipse.

 

Since the Sun and Moon have almost exactly equal angular sizes, the length of a total eclipse must be rather short, and in fact the maximum duration is only 7.5 minutes. This also means the size and path of the shadow are rather small. So the number of people to have witnessed a total solar eclipse is much smaller than those who have seen a lunar eclipse. [The loss of the Sun was of great concern to ancient civilizations. The Chinese people banged on pots, pans, drums, etc., to scare away the dragon they believed was eating the Sun.]

 

(Photograph was taken by Alan Crawford.)

Compared to lunar eclipses, solar ones have provided much more science.

 

(1) As the Moon gradually blocks out the Sun, a very thin atmospheric layer is seen for a brief second. From the analysis of this layer's spectrum, the element helium (helios) was discovered prior to that on the Earth.

(2) At one time, the study of the corona, the beautiful halo of the Sun, was only possible during total eclipses. Now solar instruments, known as coronagraphs, can replicate an eclipse, so astronomers observe the corona at any time.

(3) Predictions from Albert Einstein's Theory of General Relativity about the bending of starlight by a gravitational body were confirmed in 1919 by comparing photographic positions of the stars with and without the eclipsed Sun in the pictures.

 
 

Viewing a Solar Eclipse

Special precaution must always be taken when viewing a solar eclipse. NEVER LOOK DIRECTLY AT THE SUN WITH THE NAKED EYE.  If precautions are not taken, then serious, permanent eye damage will occur. There are several very safe observing techniques:

 

(1) Put together a pinhole camera by making a tiny circular hole in a sheet of paper or metal. The hole's edge must be sharp – not ragged. The small hole acts as a camera lens. Allow the image to be projected several feet away.

(2) Reflect the solar image with a flat mirror onto a wall or sheet of paper.

(3) Use a safe, appropriate filter for direct viewing of the Sun.

(4) Use the leaves of trees as pinhole cameras. The view of hundreds of crescent Suns on the pavement or sidewalk will be one of the most memorable observations of your life.

 

Try to notice during the partial phases of the eclipse whether the overall brightness of the day seems to be changing.

 

 

The Naked-Eye Sky (copyrighted) by James Sowell, 2013