Venus

Young Astronomers Blog, Volume 28, Number 13.

Often when you look up into the night sky you will find a bright object low in the horizon in the morning to the east or in the evening to the west. This is not the Moon; it is the planet Venus. Venus is the third brightest object in the sky after the Sun and Moon and can reach an apparent magnitude much brighter than the brightest nighttime star Sirius.

Venus, named after the Roman goddess of love and beauty, was visible to folks in ancient times and was known as one of the “wanderers” that slowly moved against the background of stars. For a time, the ancients thought Venus was two planets, a morning star, and an evening star. Galileo was the first to observe Venus through a telescope in 1610 when he discovered that Venus went through phases like the Moon.

Venus is moving around the Sun faster than the Earth and completes almost exactly 13 orbits over an eight-year cycle. Venus orbits the Sun once every 224.7 days, while it takes longer, 243 days, to rotate one time in the opposite direction of the other planets in our solar system. When we compare the orbit of Venus to that of the Earth at 365 ¼ days, we find that Venus traces out five 583.94 day “synodic” periods every eight years relative to the Earth. When this pattern is traced out, it can appear as five “petals.”

When Venus is behind the Sun as viewed from the Earth, it is at what we call superior conjunction. At this point, it is not visible in the night sky. As it moves counterclockwise relative to the Earth, it begins to appear in the west before sunset. Initially it is seen as a full disk and somewhat dim. After around 220 days, it reaches a peak at its greatest eastern elongation and is visible as a half disk. Then, it sinks lower in the sky, and becomes more and more of a crescent, but larger. Finally, after 72 days, it reaches a point between the Sun and Earth called inferior conjunction. As it continues to move counterclockwise relative to the Earth, it begins to rise in the east before sunrise and again appears large and as a crescent. It reaches a peak at its greatest western elongation appearing again as a half disk. It then sinks lower in the sky becoming dimmer as it moves farther from the Earth and once again approaches superior conjunction.

The orbit of Venus is tilted slightly by 3.4^o relative to the plane of Earth’s orbit. As such Venus does not pass directly behind the Sun during most superior conjunctions or directly in front of the Sun during most inferior conjunctions. When Venus passes in front of the Sun, a transit occurs, and this happens only twice a century. Unfortunately, for the 21^st century, we have missed it. They occurred in 2004 and 2012. We will have to wait again until 2117 and 2125.

Venus reaches it brightest when viewed from the earth halfway between its greatest eastern elongation and inferior conjunction. It does the same halfway between inferior conjunction and greatest western elongation. This year, 2020, it happens to be the brightest since 2012 – remember that eight-year cycle?

Venus is often called the Earth’s sister planet. It is similar in size and not too far away as planets go. It is also a bright and beautiful object in the night sky. However, this is where the similarity ends. The atmosphere of Venus is mostly carbon dioxide, which creates a greenhouse effect trapping the Sun’s heat. Clouds of sulfuric acid hang out at around 30 miles above the surface. The surface of Venus is over 850 degrees Fahrenheit with a pressure 90 times that of the Earth at sea level. The surface is covered with volcanoes that are thought to have resurfaced the planet a few hundred million years ago. It is also possible that Venus snows metal that covers its mountains.

Although Venus rotates very slowly and the winds at the surface are mild, high level clouds move around the planet every four (Earth) days at over 200 miles per hour. This is described as a “super-rotation.”

Venus was a mystery for many years until spacecraft began to visit the planet in the 1960s. The United States achieved the first flyby of another planet when Mariner 2 flew by Venus in 1962. The Soviet Union attempted several landings on Venus before succeeding with Venera 7 in 1970. They followed with additional landings including Venera 9, the first to send images from the surface. Several other Soviet spacecraft operated on the surface, each for just an hour or two before succumbing to the heat and pressure. The United States placed the Pioneer Venus Orbiter into orbit in 1978. In 1990, the U.S. Magellan spacecraft mapped most of the Venusian surface over a four-year mission. The European Space Agency put the Venus Express into orbit in 2006. The Japanese Space Agency launched the spacecraft Akatsuki (“Dawn”) in 2010. After a problem with the main engine, Akatsuki reach orbit in late 2015.

Just recently, scientists using data from the Akatsuki mission, found that the super-rotation of Venus’ upper atmosphere could be explained by thermal tides, which result from solar heating. The “tides” maintain the angular momentum of the rotating atmosphere, which otherwise would slow down due to the slower moving surface.