Observations from Earth

Since Galileo’s discovery of Venus’s phases, the planet has been studied in detail, using Earth-based telescopes, radar, and other instruments. Over the centuries telescopic observers, including Gian Domenico Cassini of France and William Herschel of England, have reported a variety of faint markings on its disk. Some of these markings may have corresponded to the cloud features observed in modern times in ultraviolet light, while others may have been illusory.

Important early telescopic observations of Venus were conducted in the 1700s during the planet’s solar transits (see eclipse: Transits of Mercury and Venus). In a solar transit an object passes directly between the Sun and Earth and is silhouetted briefly against the Sun’s disk. Transits of Venus are rare events, occurring in pairs eight years apart with more than a century between pairs. They were extremely important events to 18th-century astronomy, since they provided the most accurate method known at that time for determining the distance between Earth and the Sun. (This distance, known as the astronomical unit, is one of the fundamental units of astronomy.) Observations of the 1761 transit were only partially successful but did result in the first suggestion, by the Russian scientist Mikhail V. Lomonosov, that Venus has an atmosphere. The second transit of the pair, in 1769, was observed with somewhat greater success. Transits must be viewed from many points on Earth to yield accurate distances, and the transits of 1761 and, particularly, 1769 prompted the launching of many scientific expeditions to remote parts of the globe. Among these was the first of the three voyages of exploration by the British naval officer James Cook, who, with scientists from the Royal Society, observed the 1769 transit from Tahiti. The transit observations of the 1700s not only gave an improved value for the astronomical unit but also provided the impetus for many unrelated but important discoveries concerning Earth’s geography. By the time the subsequent pair of transits occurred, in 1874 and 1882, the nascent field of celestial photography had advanced enough to allow scientists to record on glass plates what they saw through their telescopes. No transits took place in the 20th century; the next pair were widely observed and imaged in 2004 and 2012. The next transits of Venus will occur in 2117 and 2125.

In the modern era Venus has also been observed at wavelengths outside the visible spectrum. The cloud features were discovered with certainty in 1927–28 in ultraviolet photographs. The first studies of the infrared spectrum of Venus, in 1932, showed that its atmosphere is composed primarily of carbon dioxide. Subsequent infrared observations revealed further details about the composition of both the atmosphere and the clouds. Observations in the microwave portion of the spectrum, beginning in earnest in the late 1950s and early ’60s, provided the first evidence of the extremely high surface temperatures on the planet and prompted the study of the greenhouse effect as a means of producing these temperatures.

After finding that Venus is completely enshrouded by clouds, astronomers turned to other techniques to study its surface. Foremost among these has been radar (see radio and radar astronomy). If equipped with an appropriate transmitter, a large radio telescope can be used as a radar system to bounce a radio signal off a planet and detect its return. Because radio wavelengths penetrate the thick Venusian atmosphere, the technique is an effective means of probing the planet’s surface.

Earth-based radar observations have been conducted primarily from Arecibo Observatory in the mountains of Puerto Rico, the Goldstone tracking station complex in the desert of southern California, and Haystack Observatory in Massachusetts. The first successful radar observations of Venus took place at Goldstone and Haystack in 1961 and revealed the planet’s slow rotation. Subsequent observations determined the rotation properties more precisely and began to unveil some of the major features on the planet’s surface. The first features to be observed were dubbed Alpha, Beta, and Maxwell, the last after James Clerk Maxwell, the Scottish physicist who first derived some of the basic equations that describe the propagation of electromagnetic radiation. These three features are among the brightest on the planet in radar images, and their names have been preserved to the present as Alpha Regio, Beta Regio, and Maxwell Montes.

By the mid-1980s Earth-based radar technology had advanced such that images from Arecibo were revealing surface features as small as a few kilometres in size. Nevertheless, because Venus always presents nearly the same face toward Earth when the planets are at their closest, much of the surface went virtually unobserved from Earth.