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semidiameter

One half of the angular width of the sun or moon, as observed on earth. This width is accidentally about the same for the sun and moon at about 16' on the average. The moon is much smaller than the sun, but it is also much closer; its semidiameter is coincidentally about the same as the sun's. The SD of sun and moon both vary with time since their distances from us change with orbital position.

Normally, half the diameter of a circle is called its radius, but because of refraction, the apparent shape of the sun or moon when near the horizon is not a circle but an ellipse. Half of an ellipse is sometimes called its semidiameter. This might have contributed to the origin of this name, but we don't know for sure. The tabulated values in the Nautical Almanac are not related to this; they are geometric factors determined by the distance to the sun.

If you wish to measure the SD of the sun, then you should do so when it is well above the horizon, 45° or more to be safe. This measurement is a good way to check your index correction as we discuss elsewhere in the course.

The term semidiameter has been used since the very early 1800's at least, and generally it seems in 1850 to have referred to the correction more than to an observation, with the assumption that no confusion would occur. From 1920 to 1950, Bowditch referred specifically to semidiameter as the width of the "visible" disk of the sun as the semidiameter, which fuels the confusion, as this implies measurement. The word "visible" has been removed after 1970 or so to leave the term more vague.

It is our conclusion that the term best refers to the exact value we use in all corrections, and indeed could also be observed for higher sights. Thus, the top-to-bottom-width of the sun that you might measure when the sun is low on the horizon would be twice the semidiameter as distorted by refraction. This however is still just a matter of terminology. You would get good sights nevertheless. You would apply the full correct semidiameter, and then the refraction part of the altitude correction would account for that effect and your sun line would still be a good one—at least as good as low sights can be in the first place.

Abbreviation:  SD

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