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( Wandering star)
In astronomy, the naked-eye planets are the five planets of our solar system that can be discerned with the naked eye without much difficulty. Hence, they were the only planets known to the ancients prior to the invention of the telescope. They are Mercury, Venus, Mars, Jupiter, and Saturn, the five planets closest to Earth. The name planet comes from the Greek term p?a??t??, planetes, meaning "wanderer", as ancient astronomers noted how certain lights moved across the sky in relation to the other stars. They called these objects asteres planetai, or wandering stars. In some cultures, the Sun and Moon have also been counted as planets, to make the seven classical planets. Mercury and Venus are only visible in twilight hours as their orbits are interior to the Earth's orbit. Venus is the most prominent planet, being the third brightest object in the sky after the Sun and the Moon. Mercury is more difficult to see due to its unwavering proximity to the Sun. Lengthy twilight and an extremely low angle at maximum elongations make optical filters necessary to see Mercury from extreme northerly locations like Scandinavia or Alaska.[1] Mars is at its brightest when it is in opposition to the Earth, which occurs approximately every two years. Jupiter and Saturn are the largest of the five planets, but are farther from the sun, and therefore receive less sunlight. Nonetheless, Jupiter is often the next brightest object in the sky after Venus. Saturn's luminosity is often enhanced by its rings, which reflect light back toward the Earth to varying degrees depending on their inclination to the ecliptic; however, the rings themselves are not visible to the naked eye from the Earth. Uranus and the asteroid Vesta are visible to the naked eye in principle on very clear nights, but unlike the true naked-eye planets they are always less luminous than several thousands of stars, and as such, do not stand out enough for their existences to be noticed without the aid of a telescope. In some cultures, the five naked-eye planets are grouped with the Sun and the Moon, called luminaries (meaning "shining object") to form a group of seven heavenly objects (or seven lights of the Earth, starry planets, traditional planets, etc.). Because their existence was known by many cultures throughout the world, since before recorded history, no discoverer is credited for them. Most Western cultures refer to them by their Latin names in honor of the Roman pantheon and their number probably led to the naming of the seven-day week.
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