Venus is always brighter than the brightest stars, with its apparent magnitude ranging from −3.8 to −4.6. This is bright enough to be seen even in the middle of the day, and the planet can be easy to see when the Sun is low on the horizon. As an inferior planet, it always lies within about 47° of the Sun.
Venus 'overtakes' the Earth every 584 days as it orbits the Sun. As it does so, it goes from being the 'Evening star', visible after sunset, to being the 'Morning star', visible before sunrise. While Mercury, the other inferior planet, reaches a maximum elongation of only 28° and is often difficult to discern in twilight, Venus is hard to miss when it is at its brightest. Its greater maximum elongation means it is visible in dark skies long after sunset. As the brightest point-like object in the sky, Venus is a commonly misreported 'unidentified flying object'. U.S. President Jimmy Carter reported having seen a UFO in 1969, which later analysis suggested was probably the planet, and countless other people have mistaken Venus for something more exotic.
As it moves around its orbit, Venus displays phases in a telescopic view like those of the Moon: In the phases of Venus the planet presents a small "full" image when it is on the opposite side of the Sun. It shows a larger "quarter phase" when it is at its maximum elongations from the Sun. Venus is at its brightest in the night sky and presents a much larger "thin crescent" in telescopic views as it comes around to the near side between the Earth and the Sun. Venus is at its largest and presents its "new passes" when it is between the Earth and the Sun. Since it has an atmosphere it can be seen in a telescope by the halo of light refracted around the planet.
Venus's orbit is slightly inclined relative to the Earth's orbit; thus, when the planet passes between the Earth and the Sun, it usually does not cross the face of the Sun. However, transits of Venus do occur in pairs separated by eight years, at intervals of about 121.5 years, when the planet's inferior conjunction coincides with its presence in the plane of the Earth's orbit. The most recent transit was in June 2004; the next will be in June 2012. The preceding pair of transits occurred in December of 1874 and 1882; the following pair will occur in December of 2117 and 2125, 243 years later. Historically, transits of Venus were important, because they allowed astronomers to directly determine the size of the astronomical unit, and hence the size of the solar system. Captain Cook's exploration of the east coast of Australia came after he had sailed to Tahiti in 1768 to observe a transit of Venus.
A long-standing mystery of Venus observations is the so-called Ashen light—an apparent weak illumination of the dark side of the planet, seen when the planet is in the crescent phase. The first claimed observation of ashen light was made as long ago as 1643, but the existence of the illumination has never been reliably confirmed. Observers have speculated that it may result from electrical activity in the Venusian atmosphere, but it may be illusory, resulting from the physiological effect of observing a very bright crescent-shaped object.
- ↑ 1.0 1.1 1.2 Espenak, Fred (1996). "NASA Reference Publication 1349; Venus: Twelve year planetary ephemeris, 1995–2006". Twelve Year Planetary Ephemeris Directory. NASA. Archived from the original on 2012-07-17. Retrieved on 2006-06-20.
- ↑ Cite error: Invalid
<ref>tag; no text was provided for refs named
- ↑ Krystek, Lee. "Natural Identified Flying Objects". The Unngatural Museum. Retrieved on 2006-06-20.
- ↑ Espenak, Fred (2004). "Transits of Venus, Six Millennium Catalog: 2000 BCE to 4000 CE". Transits of the Sun. NASA. Retrieved on 2009-05-14.
- ↑ Hornsby, T. (1771). "The quantity of the Sun's parallax, as deduced from the observations of the transit of Venus on June 3, 1769". Philosophical Transactions of the Royal Society 61: 574–579. doi:10.1098/rstl.1771.0054, http://gallica.bnf.fr/ark:/12148/bpt6k55866b/f617.chemindefer.
- ↑ Richard Woolley (1969). "Captain Cook and the Transit of Venus of 1769". Notes and Records of the Royal Society of London 24 (1): 19–32. doi:10.1098/rsnr.1969.0004, http://links.jstor.org/sici?sici=0035-9149%28196906%2924%3A1%3C19%3ACCATTO%3E2.0.CO%3B2-K.
- ↑ Baum, R. M. (2000). "The enigmatic ashen light of Venus: an overview". Journal of the British Astronomical Association 110: 325. Bibcode: 2000JBAA..110..325B, http://adsabs.harvard.edu/abs/2000JBAA..110..325B. Retrieved on 16 January 2009.