Blue field entoptic phenomenon
Constellations were identified without regard to distance to each star, but instead as if they were all dots on a dome. Orion is among the most prominent and recognizable constellations. The pole stars are special because they are approximately in line with the Earth's axis of rotation so they appear to stay in one place while the other stars rotate around them through the course of a night or a year. Planets , named for the Greek word for "wanderer," process through the star field a little each day, executing loops with time scales dependent on the length of the planet's year or orbital period around solar system.
Planets, to the naked eye, appear as points of light in the sky with variable brightness.
Night sky - Wikipedia
Planets shine due to sunlight reflecting or scattering from the planets' surface or atmosphere. Thus the relative sun-planet-earth positions determine the planet's brightness. With a telescope or good binoculars, the planets appear as discs demonstrating finite size, and it is possible to observe orbiting moons which cast shadows onto the host planet's surface. Venus is the most prominent planet, often called the "morning star" or "evening star" because it is brighter than the stars and often the only "star" visible near sunrise or sunset, depending on its location in its orbit.
Mercury , Mars , Jupiter and Saturn are also visible to the naked eye. Earth's Moon is a grey disc in the sky with cratering visible to the naked eye. It spans, depending on its exact location, arcminutes - which is about the size of a thumbnail at arm's length, and is readily identified. Over 28 days, the moon goes through a full cycle of lunar phases. People can generally identify phases within a few days by looking at the moon. Unlike stars and most planets, the light reflected from the moon is bright enough to be seen during the day.
Venus can sometimes be seen even after sunrise. Some of the most spectacular moons come during the full moon phase near sunset or sunrise.
The moon on the horizon benefits from the moon illusion which makes it appear larger. Comets come to the night sky only rarely. Comets are illuminated by the sun, and their tails extend away from the sun.
A comet with visible tail is quite unusual - a great comet appears about once a decade. They tend to be visible only shortly before sunrise or after sunset because those are the times they are close enough to the sun to show a tail.
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Clouds obscure the view of other objects in the sky, though varying thicknesses of cloudcover have differing effects. A very thin cirrus cloud in front of the moon might produce a rainbow-colored ring around the moon. Stars and planets are too small or dim to take on this effect, and are instead only dimmed often to the point of invisibility.
Thicker cloudcover obscures celestial objects entirely, making the sky black or reflecting city lights back down. Clouds are often close enough to afford some depth perception, though they are hard to see without moonlight or light pollution. On clear dark nights in unpolluted areas, when the moon is thin or below the horizon, the Milky Way , a band of what looks like white dust, can be seen. The Magellanic Clouds of the southern sky are easily mistaken to be Earth-based clouds hence the name but are in fact collections of stars found outside the Milky Way known as dwarf galaxies.
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Combat helicopter on the dotted background and displa new road,sweeping curve,blue sky,no traffic. The dots may be elongated along the path like tiny worms. The speed of the dots varies in sync with the pulse ; they briefly accelerate at each heart beat. Most people are able to see this phenomenon in the sky, although it is rather weak, and many people do not notice it until asked to pay attention.
The dots are white blood cells moving in the capillaries in front of the retina of the eye. The eye and brain "edit out" the shadow lines of the capillaries, partially by dark adaptation of the photoreceptors lying beneath the capillaries.
The white blood cells, which are much rarer than the red ones and do not absorb blue light, create gaps in the blood column, and these gaps appear as bright dots. The gaps are elongated because a spherical white blood cell is too wide for the capillary. Red blood cells pile up behind the white blood cell, showing up like a dark tail. In a technique known as blue field entoptoscopy , the effect is used to estimate the blood flow in the retinal capillaries. The patient is alternatingly shown blue light and a computer generated picture of moving dots; by adjusting the speed and density of these dots, the patient tries to match the computer generated picture to the perceived entoptic dots.
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