Total Solar Eclipses

Visualization

The video below is an example from a set of animations that show the path of the Moon's shadow during a total solar eclipse. The small black dot in the center of the frame is the umbra. Inside the area covered by the umbra shadow, the eclipse is total. In the much larger penumbra, the eclipse is only partial. For more about this stuff, go here.

The animations were rendered in POV-Ray, a free 3D raytracing program. I wrote a program that calculates the eclipse circumstances and then generates a POV-Ray scene file for each animation.

POV-Ray is well suited for tracing eclipse shadows because, unlike a lot of commercial 3D animation software (including LightWave), it's a pure raytracer, it uses double precision math, and it represents spheres natively, rather than as a mesh of polygons or surface patches.

The camera was positioned in geosynchronous orbit above the point of maximum eclipse on the Earth's surface. Rather than trying to get a penumbra using an area light, I simulated the umbra and penumbra using two shadow-casting objects, one small and opaque and one large and transparent, with a single directional light source representing the Sun. Although this meant I had to calculate the sizes of the shadow components myself, it rendered much faster and gave me greater control over the appearance of the shadows.

A total solar eclipse happens somewhere on the Earth every one to two years, on average. The last one visible from the continental U.S. was in February 1979, and my dad flew us all to Winnipeg, Manitoba just to see it. On the morning of the eclipse, after driving around looking for a spot near the centerline of the umbra's path, we ended up observing this cosmically mysterious event from the parking lot of a strip mall. But we did see it, and it's still the only one I've witnessed.

My parents went to Hawaii to see the July 1991 eclipse there. The August 2017 solar eclipse will be the first one visible on the mainland in 38 years.

There are several authoritative sources of detailed eclipse information on the Web, including Fred Espenak's NASA Goddard site and Eclipses Online from the UK's HM Nautical Almanac Office. As a point of comparison, both sites link to animations by Andrew Sinclair, former head of HMNAO.

The eclipse calculations in my program are based on Jean Meeus's Astronomical Algorithms, 2nd ed., along with a few ideas from Astronomy on the Personal Computer by Montenbruck and Pfleger and Astronomy with your Personal Computer by Peter Duffet-Smith. The Earth texture map is an altered version of one of NASA's blue marble images from its Visible Earth site.

The animation is intentionally stylized, of course. For an idea of what the Moon's shadow really looks like from space, see this Astronomy Picture of the Day, a photograph of the shadow during the August 11, 1999 total solar eclipse taken from the Mir space station.

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