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One Saros later, on April 8, , this eclipse will recur, except instead of Africa and Asia, it will track across northern Mexico, the central and eastern United States and the Maritime provinces of Canada.

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As a solar eclipse approaches, the mainstream media often will provide a variety of warnings and advisories against looking at the sun with bare eyes, as blindness could ensue. This has given most people the idea that eclipses are dangerous. The sun constantly emits invisible infrared rays that can damage your eyes. Ordinarily, we have no reason to gaze at the sun.

Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the sun onto a white card. The farther away the card, the larger you can focus the image. Look for sunspots. Notice that the sun appears somewhat darker around its limb or edge. This method of solar viewing is safe so long as you remember not to look through the binoculars or telescope when they are pointed toward the sun; put another way, never look directly at the sun when any part of its blindingly bright surface is visible.

Open a sun-facing window and place the covered mirror on the sunlit sill so it reflects a disk of light onto the far wall inside. The farther away from the wall is the better; the image will be only 1 inch across for every 9 feet or 3 centimeters for every 3 meters from the mirror.

Modeling clay works well to hold the mirror in place. Experiment with different-sized holes in the paper.

Again, a large hole makes the image bright, but fuzzy, and a small one makes it dim but sharp. Darken the room as much as possible. What do you see? Is it worth a photograph? You will see scores of partially eclipsed suns projected through pinhole gaps between the leaves. This is caused by diffraction, a property of light. According to Vince Huegele, an optical physicist at the NASA Marshall Space Flight Center, the light rays do not shoot straight by the rim of the gaps, or a pinhole, but bend around the edge. This wave effect creates a pattern of rings that resembles a bull's eye.

Acceptable filters for unaided visual solar observations include aluminized Mylar. Some astronomy dealers carry Mylar filter material specially designed for solar observing. Unacceptable filters include sunglasses, old color film negatives, black-and-white film that contains no silver, photographic neutral-density filters and polarizing filters. Although these materials have very low visible-light transmittance levels, they transmit an unacceptably high level of near-infrared radiation that can cause a thermal retinal burn. The fact that the sun appears dim, or that you feel no discomfort when looking at the sun through these types of filters, is no guarantee that your eyes are safe.

There is one time when you can safely look directly at the sun: during a total eclipse, when the sun's disk is entirely covered. During those few precious seconds or minutes, the magnificent corona shines forth in all its glory surrounding the darkened sun; a marvelous fringe of pearly white light.

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It differs in size, in tints and patterns from eclipse to eclipse. It is always faint and delicate, with a sheen like a pale aurora. It has a variable appearance. Sometimes it has a soft continuous look; at other times, long rays of it shoot out in three or four directions.

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It may stand out from the disk in filmy petals and streamers. As best as we can determine, the earliest record of a solar eclipse occurred over four millennia ago. In China, it was believed that the gradual blotting out of the sun was caused by a dragon who was attempting to devour the sun, and it was the duty of the court astronomers to shoot arrows, beat drums and raise whatever cacophony they could to frighten the dragon away. In the ancient Chinese classic Shujing or Book of Documents is the account of Hsi and Ho, two court astronomers who were caught completely unaware by a solar eclipse, having gotten drunk just before the event began.

In the aftermath, Zhong Kang, the fourth emperor of the Xia dynasty ordered that Hsi and Ho be punished by having their heads chopped off. The eclipse in question was that of Oct. An Assyrian tablet also attests to the event. For cities located at the west of the CET meridian, hour is shifted towards evening sunrise and sunset are later.

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Elsewhere on Earth, people shall know where is their reference meridian. In order to get with a sundial the same hour as on a watch, you need to apply the longitude correction to solar time. But you also need to add the value of the equation of time and if required, another hour for the daylight savings in summer.

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The equation of time is due to the annual variation of apparent speed of the Sun. This variation is coming from two causes: the excentricity of the Earth orbit, and the inclination of the Earth rotation axis on the ecliptic plane.

Lecture 9: Eclipses of the Sun & Moon

This equation is usually provided as a graph or a table, allowing people to fetch the value for the current date. So when it is 12 h of solar time on the sundial, it is in fact 13 h 32 min 20 s on a watch civil time in Europe. You can also use the same method to calculate when the Sun will pass to the South, at a given location. And by inverting the correction, you can calculate the solar hour for any civil hour. Hour lines on a sundial converge towards the same point, where the polar style is attached to the sundial, except when this point is rejected to infinity, as for direct East or West sundial, or polar sundials.


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Below, from left to right, a simple sundial layout of solar time, a sundial with hour lines corrected for the reference meridian, a sundial giving mean time between June and December half analemma curves , and a sundial with complete analemma curves. Depending on the moment in the year, the Sun's altitude is different. The shadow casted on the sundial is therefore longer or shorter and this length allows the calculation of particular moments such as solstices and equinoxes, or even dates or Sun's declination.

Shadow volumes are real three dimensional objects and they are always seen in perspective. Take the shadow from cloud "a". It reaches the ground between the sun and the eye. The shadow is seen below the cloud. The shadow nearest the ground is closer and so appears wider. In contrast, shadows from clouds "b" and "c" slant downwards at exactly the same angle but reach the ground past the eye. To the eye they appear to radiate upwards from the cloud and would, if they were still visible, pass high overhead.

Next time you see sunrays, imagine them for what they really are, miles long columns of sparkling sunlit air highlighted by the darkness of adjacent unlit voids. Let the mind fly around and through them to give them solid form that replaces the flattish way we normally see the sky.