- #31
Baluncore
Science Advisor
- 16,693
- 10,358
This is my analysis and summary of the situation.
It is impossible to see on a 3D model of the Earth, where the 99% of the population, simultaneously illuminated by sunlight or twilight, will be.
That is because the Sun illuminates an extra 0.25° over the horizon due to its angular diameter. The 18° definition of astronomical twilight adds to that, giving 18.25° over the horizon. The part of the sphere that is then deemed to be illuminated is not 180°, but is 18.25° + 180° + 18.25° = 216.5°.
That leaves a dark circle with a diameter of only 360° - 216.5° = 143.5°. That circle, with radius = 71.75°, is centred over the Pacific Ocean above the tropic of Capricorn. The dark circle does not include the populous islands of Japan, Indonesia, or the Philippines, they will probably be in twilight.
What point is on the tropic of Capricorn, at an arc distance of 71.75° from the Japanese coast near Tokyo? Google Earth, Measure, intersects at latitude 23.5°S, longitude 175.464°W, as the approximate centre of the dark circle.
That view of the dark circle looks about right, with Indonesia, Philippines, Japan and California being over the horizon. Only Australia, New Zealand, Papua - New Guinea, Hawaii, many small Pacific islands, and Antarctica are then in the dark. The rest of the world, with 99% of the population, is enlightened.
It is impossible to see on a 3D model of the Earth, where the 99% of the population, simultaneously illuminated by sunlight or twilight, will be.
That is because the Sun illuminates an extra 0.25° over the horizon due to its angular diameter. The 18° definition of astronomical twilight adds to that, giving 18.25° over the horizon. The part of the sphere that is then deemed to be illuminated is not 180°, but is 18.25° + 180° + 18.25° = 216.5°.
That leaves a dark circle with a diameter of only 360° - 216.5° = 143.5°. That circle, with radius = 71.75°, is centred over the Pacific Ocean above the tropic of Capricorn. The dark circle does not include the populous islands of Japan, Indonesia, or the Philippines, they will probably be in twilight.
What point is on the tropic of Capricorn, at an arc distance of 71.75° from the Japanese coast near Tokyo? Google Earth, Measure, intersects at latitude 23.5°S, longitude 175.464°W, as the approximate centre of the dark circle.
That view of the dark circle looks about right, with Indonesia, Philippines, Japan and California being over the horizon. Only Australia, New Zealand, Papua - New Guinea, Hawaii, many small Pacific islands, and Antarctica are then in the dark. The rest of the world, with 99% of the population, is enlightened.