As seen from Earth, both Mercury and Venus have phases like our moon. That's because they circle the sun inside Earth's orbit. Planets that orbit between Earth and the sun are known as inner or inferior planets.
The phases of Venus were first observed by Galileo Galilei in the early 1600s. He observed that the planet presented a complete series of phases very similar to those of the Moon, seeing at one point as a fully illuminated disk until it came to be seen like a thin sickle.
Inferior planets can never be at "opposition," which is when the planet and the sun are on opposite sides of Earth. But inferior planets can be at "conjunction," which is when a planet, the sun and Earth are all in a straight line. Conjunction can happen once when the planet is on the opposite side of the sun from Earth and again when it's on the same side of the sun as Earth. When a planet is on the opposite side of the sun from Earth, we say it is at "superior conjunction." As the planet moves out from behind the sun and gets closer to Earth, we see less and less of the lit side. We see phases, similar to our moon's phases.
Venus will pass very close to the Sun in the sky toward its own inferior conjunction on March 25 2017. Watch its crescent get thinner and thinner as the planet's size appears larger and larger, because it is getting closer to Earth.
This marks the end of Venus's apparition in the evening sky and its transition to become a morning object over the next few weeks. At closest approach, Venus will appear at a separation of only 8° from the Sun, making it totally unobservable for several weeks while it is lost in the Sun's glare.
Venus will also pass perigee – the time when it is closest to the Earth – at around the same time, since it will lie on exactly the same side of the Sun as the Earth in the Solar System. It will move to within a distance of 0.28 AU from the Earth, making it appear with its largest angular size. If it could be observed, it would measure 59.4 arcsec in diameter, whilst appearing completely unilluminated.
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. It has the longest rotation period (243 days) of any planet in the Solar System and rotates in the opposite direction to most other planets. It has no natural satellite. It is named after the Roman goddess of love and beauty. It is the second-brightest natural object in the night sky after the Moon, reaching an apparent magnitude of −4.6, bright enough to cast shadows. Because Venus orbits within Earth's orbit it is an inferior planet and never appears to venture far from the Sun; its maximum angular distance from the Sun (elongation) is 47.8°.
Venus is a terrestrial planet and is sometimes called Earth's "sister planet" because of their similar size, mass, proximity to the Sun, and bulk composition. It is radically different from Earth in other respects. It has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the planet's surface is 92 times that of Earth, or roughly the pressure found 900 m (3,000 ft) underwater on Earth. Venus is by far the hottest planet in the Solar System, with a mean surface temperature of 735 K (462 °C; 863 °F), even though Mercury is closer to the Sun. Venus is shrouded by an opaque layer of highly reflective clouds of sulfuric acid, preventing its surface from being seen from space in visible light. It may have had water oceans in the past, but these would have vaporized as the temperature rose due to a runaway greenhouse effect. The water has probably photodissociated, and the free hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field. Venus's surface is a dry desertscape interspersed with slab-like rocks and is periodically resurfaced by volcanism.
As one of the brightest objects in the sky, Venus has been a major fixture in human culture for as long as records have existed. It has been made sacred to gods of many cultures, and has been a prime inspiration for writers and poets as the "morning star" and "evening star". Venus was the first planet to have its motions plotted across the sky, as early as the second millennium BC, and was a prime target for early interplanetary exploration as the closest planet to Earth. It was the first planet beyond Earth visited by a spacecraft (Mariner 2) in 1962, and the first to be successfully landed on (by Venera 7) in 1970. Venus's thick clouds render observation of its surface impossible in visible light, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more complex missions, but they are hindered by Venus's hostile surface conditions.
Desde la Tierra, Mercurio y Venus tienen fases como nuestra luna. Eso es porque circundan el sol dentro de la órbita de la Tierra. Los planetas que orbitan entre la Tierra y el Sol son conocidos como planetas interiores o inferiores.
Las fases de Venus fueron observadas por primera vez por Galileo Galilei a principios de 1600. Observó que el planeta presentaba una serie completa de fases muy semejantes a las de la Luna, viéndose en un momento como un disco completamente iluminado hasta que llegó a ser una hoz fina.
Los planetas inferiores nunca pueden estar en "oposición", que es cuando el planeta y el sol están en lados opuestos de la Tierra. Pero los planetas inferiores pueden estar en "conjunción", que es cuando un planeta, el Sol y la Tierra están en línea recta. La conjunción puede ocurrir una vez cuando el planeta está en el lado opuesto del sol de la Tierra y de nuevo cuando está en el mismo lado del sol que la Tierra. Cuando un planeta está en el lado opuesto del sol de la Tierra, decimos que está en "conjunción superior". A medida que el planeta sale de detrás del sol y se acerca a la Tierra, vemos cada vez menos el lado iluminado. Vemos fases parecidas a las fases de nuestra luna.
Venus pasará muy cerca del Sol en el cielo hacia su propia conjunción inferior el 25 de marzo de 2017. Se va haciendo cada vez más grande porque se está acercando a la Tierra, pero cada vez con más fase.
Esto marca el final de la aparición de Venus en el cielo nocturno y su transición para convertirse en un objeto matutino durante las próximas semanas. En el acercamiento más cercano, Venus aparecerá en una separación de solamente 8 ° del sol, haciéndolo totalmente inobservable por varias semanas mientras que se pierde en el fulgor del sol.
Venus también pasará el perigeo - el momento en que está más cercano a la Tierra - en aproximadamente el mismo momento, ya que se situará exactamente en el mismo lado del Sol que la Tierra en el Sistema Solar. Se moverá a una distancia de 0,28 UA de la Tierra, haciendo que aparezca con su mayor tamaño angular. Si se pudiera observar, mediría 59,4 arcsec de diámetro, mientras que aparecía completamente sin iluminar.
Amateur Astronomy Picture of the Day (AAPOD2), 23/apr/2017