Complete Works, Volume Four

...diversity, that variety of days, nights, and seasons happens, so that for the Periœci dwellers on the same parallel of latitude but opposite meridian there are different times for day and night, but the same summer and winter. For the Antœci dwellers on the same meridian but opposite latitude, day and night are the same, but summer and winter occur at different times. Finally, for the Antipodes dwellers on the exact opposite side of the globe, both day and night as well as summer and winter are entirely different.

It must also be added that the inhabitants of all parallels contained within the Tropics are usually called Amphiscii from the Greek for "both-shadowed", as if having shadows on both sides (specifically at midday). This is because when the Sun is to the North, their shadows fall to the South; and when the Sun is to the South, their shadows fall to the North. The inhabitants from the Tropics up to the Polar circles are called Heteroscii from the Greek for "different-shadowed", as they have only one direction for their shadow (as we do toward the North). This relates to what the poet Lucan wrote concerning the Arabs living within the Torrid zone who traveled to the Temperate zone:

You have come, O Arabs, into a world unknown to you,
marveling that the shadows of the groves do not fall to the left.
original: "Ignotum vobis, Arabes, venistis in Orbem, / Vmbras mirati nemorum non ire sinistras." This quote from Lucan's Pharsalia describes how travelers from the southern hemisphere or tropics were surprised by the direction of shadows in northern lands.

Finally, the inhabitants from the Polar circles to the Poles themselves are called Periscii from the Greek for "shadows all around". Because the Sun does not set there for long periods, their shadows are driven in a full circle through every part of the horizon. I say "with the Sun not setting," because for everyone except those exactly at the poles, the Sun does sometimes rise and set. During those times, they can be considered among the Heteroscii.

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CHAPTER XVIII.

Concerning Twilights, which are seen at the Horizon in any position of the Sphere.

Since the motion of the "First Mover," which is demonstrated by the revolution of the celestial Sphere, causes the rising and setting of the Stars relative to the horizon, we must first discuss Twilights and the Refractions observed at the Horizon before speaking consistently about rising and setting.

First of all, Twilight is nothing other than that faint, doubtful, or middle light. It appears in the eastern region before the Sun rises, where it is specifically called Dawn original: Aurora. It also appears after the Sun sets in the western region, where it retains the name of Twilight original: Crepusculum.

The cause of this light is the Atmosphere the region of vapors that clothes the Earth. Because the atmosphere is taller than the solid edge of the Earth, it receives the rays of the Sun while the Sun is still below the horizon in the morning, and keeps them later in the evening. By reflecting these rays down to us, the atmosphere appears bright.

Indeed, if there were no vapors and only the purest air, we would see no light at all before sunrise or after sunset. Instead, the transition would be an immediate jump from total darkness to daylight, and from daylight to total darkness.

It should be noted that the start of morning twilight and the end of evening twilight occur when the Sun is about eighteen degrees below the horizon, measured perpendicularly. Because the closer the Sun is to the horizon, the more of the Atmosphere it illuminates, the Twilight becomes clearer as the Sun approaches.

I will pass over the calculation that the height of the Atmosphere above the Earth is about 40 Italian miles. It seems to be much less in reality. This is because the first and last light may not come directly from the Sun, but through reflections from the lower parts of the atmosphere.

I note instead that Twilight is shortest in a Right Sphere an observer at the equator where the sun rises vertically. Because the Sun rises and falls perpendicularly, those eighteen degrees of depth are crossed as quickly as possible.

In an Oblique Sphere an observer at any latitude between the equator and the poles, Twilight is longer. This is because the path of the Sun between the horizon and the eighteen-degree mark is tilted, making the distance it must travel longer than eighteen degrees. Because this path is even more slanted in summer than in winter, summer twilight is longer. In this City Gassendi likely refers to Paris or Aix-en-Provence, for example, twilight lasts nearly four hours in summer, while it does not even reach two hours in winter.

Furthermore, it can be observed in this City that evening twilight merges directly into morning twilight for eight days before and after the summer Solstice. In the middle of the night, the light never fully fails because the Sun does not sink a full 18 degrees below the horizon. At the Solstice itself, it misses that mark by 20 minutes.

There is no need to point out that this continuous twilight lasts for more days and is brighter at night the more oblique the Sphere becomes. This continues until the Sun travels entirely above the horizon (as it does beyond the Polar circle), creating a continuous day without any twilight at all.

We should not omit that the tilt of the horizon and the inequality of the Sun's paths mean that while twilight is longest at the summer Solstice, it is not actually shortest at the winter Solstice. In this City, the shortest twilight occurs around the first of March and the twelfth of October.

Finally, in a Parallel Sphere at the poles, there is continuous Twilight for about 52 days. This is because it takes that long for the Sun to either descend or ascend through the 18 degrees below the horizon.

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CHAPTER XIX.

Concerning the Refractions of Stellar Rays, which happen most at the Horizon.

Regarding Refractions: it is well known that rays passing obliquely from a thinner medium into a denser one are bent, or "refracted," toward...