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  • Writer's pictureEvan Boxer-Cook

Equinoctial Blockage

Updated: Oct 5, 2023

The effect of the equinox on the equatorial ring dial

Typically, when a sundial does not display the time, the explanation is that there is either not adequate sunlight, or that the dial is somehow misaligned.

In the case of the universal equatorial ring dial, however, there exists a fundamental, predictable gap in functionality that occurs on the days surrounding the equinox.

In the dial's typical function, a direct line of sight between the sun, nodus, and inner edge of the hour ring is required.

On most days of the year this is of little consequence, as the sun's daily path north or south of the equator allows its rays to pass over or under the equatorial ring of the dial, through the nodus, and onto the hour ring.

However, because the sun's path at the equinox coincides with the equator, the slope of its arc follows the equatorial ring throughout the day, blocking its light from reaching the nodus (which is positioned in the center of the dial at the equinox).

During this period, the shadow of one half of the hour ring covers the opposite half, rendering any reading imprecise at best and completely impossible at noon (the shadow of the hour ring crosses the meridian ring's shadow at noon, compounding the existing inaccuracy of the noon reading).

On the days surrounding the equinox, only brief glimpses of the nodus beam can be seen as one settles the dial into its northern orientation. While twisting the dial may illuminate the points just before or after the actual time, the status of the dial as a celestial model prevents it from showing the exact hour.

Because the represented macrocosm rules the dial, certain elements are at the mercy of their greater celestial selves. Because the conceptual celestial equator (like the axis and meridian) is represented by a physical element within the dial, and the dial itself must be oriented to match the greater celestial sphere, certain limitations necessarily result from significant alignments.

This limitation doesn't need to be seen as negative, however counterintuitive. It only further credits the dial as an educational model of the celestial sphere, intuitively communicating what exactly the equinox means for the sun. In this case, the dial's blind spot is just as fascinating and valuable as its typical function.

Evan Boxer-Cook


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1 Comment

Elisa Boxer
Elisa Boxer
Sep 24, 2023

This is such a wonderful post and video. I love how you explain everything, and also how you used the bates sundial for reference.

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