(The github project is here.)

Custom Solar Eclipse Viewer

The idea is to improve your experience of a total eclipse by combining an eclipse viewer (eclipse glasses) with useful localized data and simple tools, all printed on a medium-sized card which you can hang around your neck.

An example is here.

The steps to produce a custom solar eclipse viewer:

• edit a config.ini file (a simple text file) to specify the date of the eclipse, your viewing location, and other settings.
• run the code provided here to read in the config.ini file as input, and produce a PDF file as output.
• print the PDF on card stock (stiff paper). Print as two-sided.
• cut out the outlined viewing hole, and glue appropriate filter material over the hole for safely viewing the partial phases of the eclipse. The filter material should be purchased from a trusted source.
• punch out a round hole near the center of the card, to be used for looking at its shadow.
• punch out a round hole at the bottom of the card, intended for a lanyard to hang around your neck.

Thousand Oaks is a good source for filter material.

Features Of The Viewer

• the eclipse viewer is used to look at the partial phases of the eclipse.
• a small hole near the center of the card creates an image of the Sun in the shadow of the viewer.
• a small hole at the bottom of the card is for a lanyard, such that the card can simply hang around your neck when you aren't using it to view the partial phases.
• QR codes link to a web page you specify (recommended: link to a satellite image of the current cloud cover in your area).
• if clouds are threatening to obscure totality, you can use scales printed on the sides of the viewer to measure their angular separation and motion relative to the Sun.
• a timeline specific to your chosen location includes reminders of what to look for during the countdown to totality (shadow effects, shadow bands, Baily's beads, and the diamond ring effect).
• charts show the progress of the partial phases of the eclipse.
• important data about the eclipse from your location is included: times, length of totality, altitude of the Sun above the horizon, and so on.

Implementation Notes

• the Polynomial class is a neat little thing.
• Code that lets me mix-and-match AffineTransform objects. This was very useful in removing repeated code, in making geometrical operations made up of several sequential operations easy to implement and understand. This is implemented in the custom.solar.eclipse.draw.mix package.

References

• Elements of Solar Eclipses, Jean Meeus, 1989. This was my main reference.
• Explanatory Supplement to the Astronomical Ephemeris, 1961.

Remarks Regarding Meeus' Documentation

On page 27, The times of interior contacts... should reference the absolute value of L2', not L2' itself. This point is important.

On page 26, equation (7) for the parallactic angle, I use the South Celestial Pole when the location is in the southern hemisphere. (His remark about cos q being the same sign as η may amount to the same thing; I'm not sure.)

On page 27, there seems to be a small numerical error in the worked example, for the value of H. The stated value of 1.411188 should be 1.411192, according to his data. This amounts only to a very small angle, on the order of 0.01 arc-seconds. (The source of the difference may be in the value of a constant.)

Meeus doesn't mention that there can be a change of day in the calculation of local circumstances. That is, the correction applied to T0 may push the value out of the range 0..24.